研究者総覧

小原 敬士 (オハラ ケイシ)

  • 大学院理工学研究科 環境機能科学専攻 教授
Last Updated :2020/11/14

研究者情報

学位

  • 博士(理学)(京都大学)

ホームページURL

J-Global ID

研究キーワード

  • スピンダイナミクス   近赤外   電子スピン共鳴   抗酸化   フリーラジカル   光化学   一重項酸素   時間分解ESR   活性酸素   生体分子   薬剤反応性   ミセル   近赤外発光   スポット計測   反応速度   化学物理   懸濁系   化学反応   スーパーオキシド   スピンプローブ   プロトン移動   天然抗酸化剤   ビタミンE   時間分解分光   

研究分野

  • ナノテク・材料 / 基礎物理化学
  • ライフサイエンス / 薬系分析、物理化学
  • ライフサイエンス / 食品科学
  • ナノテク・材料 / 機能物性化学

経歴

  • 2015年05月 - 現在  愛媛大学大学院理工学研究科教授
  • 2007年04月 - 2015年04月  愛媛大学大学院理工学研究科准教授
  • 2006年04月 - 2007年03月  愛媛大学大学院理工学研究科特任講師
  • 1996年04月 - 2006年03月  愛媛大学理学部助手
  • 1996年01月 - 1996年04月  京都大学大学院理学研究科日本学術振興会特別研究員

学歴

  • 1993年04月 - 1996年03月   京都大学   大学院理学研究科博士後期課程   化学専攻
  • 1987年04月 - 1989年03月   大阪大学   大学院理学研究科博士前期課程   無機及び物理化学専攻
  • 1983年04月 - 1987年03月   大阪大学   理学部   化学科

所属学協会

  • 光化学協会   日本酸化ストレス学会   電子スピンサイエンス学会   日本分光学会   日本化学会   

研究活動情報

論文

  • Kosuke Oki, Masayoshi Takase, Shigeki Mori, Akitoshi Shiotari, Yoshiaki Sugimoto, Keishi Ohara, Tetsuo Okujima, Hidemitsu Uno
    Journal of the American Chemical Society 140 33 10430 - 10434 2018年08月 [査読有り]
     研究論文(学術雑誌) 
    Copyright © 2018 American Chemical Society. A core-expanded, pyrrole-fused azacoronene analogue containing two unusual N-doped heptagons was obtained from commercially available octafluoronaphthalene and 3,4-diethylpyrrole in two steps as a heteroatom-doped nonplanar nanographene. Full fusion with the formation of the tetraazadipleiadiene framework and the longitudinally twisted structure was unambiguously confirmed by single-crystal X-ray diffraction analysis. The edge-to-edge dihedral angle along the acene moiety was 63°. This electron-rich π-system showed four reversible oxidation peaks. Despite the nonplanar structure, the Hückel aromaticity owing to a peripheral π-conjugation in the dicationic state was concluded from the bond-length alternation and nucleus-independent chemical shift (NICS) and anisotropy of the induced current density (ACID) calculations.
  • Shin-ichi Nagaoka, Yuki Bandoh, Umpei Nagashima, Keishi Ohara
    JOURNAL OF PHYSICAL CHEMISTRY A 121 42 8069 - 8079 2017年10月 [査読有り]
     研究論文(学術雑誌) 
    Singlet-oxygen (O-1(2)) quenching, free-radical scavenging, and excited-state intramolecular proton-transfer (ESIPT) activities of hydroxyflavones, anthocyanidins, and 1-hydroxyanthraquinones were studied by means of laser, stopped-flow, and steady-state spectroscopies. In hydroxyflavones and anthocyanidins, the O-1(2) quenching activity positively correlates to the free radical scavenging activity. The reason for this correlation can be understood by considering that an early step of each reaction involves electron transfer from the unfused phenyl ring (B-ring), which is singly bonded to the bicyclic chromen or chromenylium moiety (A- and C-rings). Substitution of an electron-donating OH group at B-ring enhances the electron transfer leading to activation of the O-1(2) quenching and free-radical scavenging. In 3-hydroxyflavones, the OH substitution at B-ring reduces the activity of ESIPT within C-ring, which can be explained in terms of the nodal-plane model. As a result, the 102 quenching and free-radical scavenging activities negatively correlate to the ESIPT activity. A catechol structure at B-ring is another factor that enhances the free-radical scavenging in hydroxyflavones. In contrast to these hydroxyflavones, 1-hydroxyanthraquinones having an electron-donating OH substituent adjacent to the O-H-O = C moiety susceptible to ESIPT do not show a simple correlation between their O-1(2) quenching and ESIPT activities, because the OH substitution modulates these reactions.
  • Yamamoto Ryoma, Yamamoto Takashi, Ohara Keishi, Naito Toshio
    MAGNETOCHEMISTRY 3 2 2017年06月 [査読有り]
  • Noma Hiroki, Ohara Keishi, Naito Toshio
    INORGANICS 4 2 2016年06月 [査読有り]
  • Tetsuo Okujima, Chie Ando, Saurabh Agrawal, Hiroki Matsumoto, Shigeki Mori, Keishi Ohara, Ichiro Hisaki, Takahiro Nakae, Masayoshi Takase, Hidemitsu Uno, Nagao Kobayashi
    JOURNAL OF THE AMERICAN CHEMICAL SOCIETY 138 24 7540 - 7543 2016年06月 [査読有り]
     研究論文(学術雑誌) 
    An acenaphthylene-fused cyclo[10]pyrrole 1b was selectively synthesized via an oxidative coupling reaction of the corresponding 2,2'-bipyrrole with the appropriate dianion template, croconate anion. The structure of 1b as the isolated largest cyclo[n]pyrrole was elucidated by X-ray crystallographic analysis. The absorption spectrum exhibited a markedly red-shifted, intensified L band at 1982 nm, which was interpreted by application of Michl's perimeter and Gouterman's 4-orbital models, supported by magnetic circular dichroism (MCD) data and theoretical calculations.
  • Shin-ichi Nagaoka, Hikaru Endo, Keishi Ohara, Umpei Nagashima
    JOURNAL OF PHYSICAL CHEMISTRY B 119 6 2525 - 2532 2015年02月 [査読有り]
     研究論文(学術雑誌) 
    Excited-state intramolecular proton-transfer (ESIPT) and singlet-oxygen (O-1(2)) quenching activities of intramolecularly hydrogen-bonded 1-(acylamino)anthraquinones have been studied by means of static and laser spectroscopies. The ESIPT shows a substituent effect, which can be explained in terms of the nodal-plane model. The ESIPT activity positively and linearly correlates with their O-1(2) quenching activity. The reason for this correlation can be understood by considering ESIPT-induced distortion of their ground-state potential surface and their encounter complex formation with O-1(2). Intramolecularly hydrogen-bonded hydroxyanthraquinones found in aloe also show a similar positive and linear correlation, which can be understood in the same way.
  • Hiroki Noma, Keishi Ohara, Toshio Naito
    CHEMISTRY LETTERS 43 8 1230 - 1232 2014年08月 [査読有り]
     研究論文(学術雑誌) 
    [Cu(dmit)(2)](2-) formally consists of a magnetic Cu2+ ion (S = 1/2) and closed-shell pi-conjugated dmit(2-) ligands. However, it has been determined by electron spin resonance and molecular orbital calculations that the majority (91-100%) of the spin delocalizes over the ligands in the ground state, while the spin significantly (40-90%) localizes on Cu under UV irradiation. Considering the results of X-ray structural analyses under UV irradiation, the spin distribution varies depending on the wavelengths irradiated and retaining the square-planar coordination geometries.
  • Toshio Naito, Tomoaki Karasudani, Naoki Nagayama, Keishi Ohara, Kensuke Konishi, Shigeki Mori, Takahiro Takano, Yukihiro Takahashi, Tamotsu Inabe, Shota Kinose, Sadafumi Nishihara, Katsuya Inoue
    EUROPEAN JOURNAL OF INORGANIC CHEMISTRY 24 4000 - 4009 2014年08月 [査読有り]
     研究論文(学術雑誌) 
    The simple molecular salt NMQ[Ni(dmit)(2)] (NMQ = N-methylquinolinium, dmit = 1,3-dithiol-2-thione-4,5-dithiolate) functions as a diamagnetic insulator with an activation energy E-a(dark) of 0.20 eV. However, at 300 K, it exhibits ca. 40 times higher conductivity (sigma(UV)) under UV irradiation [(375 +/- 5) nm, 15.7 mW cm(-2)] than it does under dark conditions (sigma(dark)). The ratio sigma(UV)/sigma(dark) rapidly increases with decreasing temperature and reaches ca. 880 at 200 K. From the temperature dependence of sigma(UV), the activation energy under irradiation E-a(UV) is 0.12 eV. These observations cannot be explained as the result of sample heating during UV irradiation. Rather, the X-ray photoelectron spectra of the sulfur and nickel atoms, the calculated band structure, and the UV/Vis spectra of the salt can all be explained consistently as follows: charge transfer between the Ni(dmit)(2) moieties upon exposure to 375 nm UV light induces melting of the charge-ordered state and produces the unusually large photoconductivity of NMQ[Ni(dmit)(2)].
  • Tetsuo Okujima, Yoichi Shida, Keishi Ohara, Yuya Tomimori, Motoyoshi Nishioka, Shigeki Mori, Takahiro Nakae, Hidemitsu Uno
    JOURNAL OF PORPHYRINS AND PHTHALOCYANINES 18 8-9 752 - 761 2014年08月 [査読有り]
     研究論文(学術雑誌) 
    A series of O-chelated BODIPYs fused with aromatic rings such as benzene and acenaphthylene at beta,beta-positions was synthesized as a near-infrared dye. The photophysical properties were examined by UV-vis-NIR absorption and fluorescence measurement. Acenaphthylene-fused O-BODIPYs showed a intense absorption at 750-840 nm with the epsilon of 10(5) M-1.cm(-1). and a fluorescene emission at 770-850 nm with the high Phi value of 0.06-0.43.
  • Tomoko Saiki, Shigeki Mori, Keishi Ohara, Toshio Naito
    Chemistry Letters 43 7 1119 - 1121 2014年07月 [査読有り]
     研究論文(学術雑誌) 
    β-DiCC[Ni(dmit)2] (dmit: 1,3-dithiole-2-thione-4,5- dithiolato and DiCC: 3,3'-dihexyloxacarbocyanine) is a chargetransfer salt with redox activity. When voltages are applied, it brings about a reversible, hysteretic, solid-state-electrolytic reaction exhibiting a capacitor-like electric response. Based on the electron spin resonance spectra, the resistivity behavior, and the tight-binding band calculation, it is indicated that one can finely and reversibly control the redox equilibrium between DiCC+ and [Ni(dmit)2]- molecules by applying voltages. © 2014 The Chemical Society of Japan.
  • Yoshimi Sueishi, Masashi Hori, Misa Ishikawa, Keiko Matsu-Ura, Erisa Kamogawa, Yusaku Honda, Masakazu Kita, Keishi Ohara
    JOURNAL OF CLINICAL BIOCHEMISTRY AND NUTRITION 54 2 67 - 74 2014年03月 [査読有り]
     研究論文(学術雑誌) 
    Scavenging rate constants of eight hydrophilic antioxidants, including caffeic acid, chlorogenic acid, genistein, glutathione, N-acetylcysteine, rutin, trolox, and uric acid against multiple ROS, namely superoxide anion, hydroxyl radical, singlet oxygen, and alkoxyl radical were determined with the electron spin resonance method. Direct flash photolysis measurement of the second-order rate constant in the reaction of alkoxyl radical plus the spin trap 5,5-dimethyl-pyrroline N-oxide made it possible to evaluate scavenging rate constants in antioxidants. The magnitudes of scavenging rate constants were notably dependent on the character of each ROS and the overall rate constants were highest in hydroxyl radical scavenging and the lowest in superoxide anion. The highest scavenging rate constant against superoxide anion was obtained by chlorogenic acid (2.9 x 10(5) M-1 s(-1)) and the lowest was by N-acetylcysteine (5.0 x 10(2) M-1 s(-1)) For singlet oxygen, the highest was by glutathione (9.4 x 10(8) M-1 s(-1)) and the lowest was by uric acid (2.3 x 10(6) M-1 s(-1)). All other numbers are listed and illustrated. Redox potential measurements of the antioxidants indicated that the antioxidants are likely to react with superoxide anion and singlet oxygen through electron transfer processes.
  • Tetsuo Okujima, Yoichi Shida, Keishi Ohara, Yuya Tomimori, Motoyoshi Nishioka, Shigeki Mori, Takahiro Nakae, Hidemitsu Uno
    J. Porphyrins Phthalocyanines 18 8-9 752 - 761 2014年 [査読有り]
     研究論文(学術雑誌) 
    A series of O-chelated BODIPYs fused with aromatic rings such as benzene and acenaphthylene at β,β-positions was synthesized as a near-infrared dye. The photophysical properties were examined by UV-vis-NIR absorption and fluorescence measurement. Acenaphthylene-fused O-BODIPYs showed a intense absorption at 750–840 nm with the ε of 105 M-1.cm-1. and a fluorescence emission at 770–850 nm with the high Φ value of 0.06–0.43.


    Read More: http://www.worldscientific.com/doi/abs/10.1142/S1088424614500503
  • Hisako Sato, Kenji Tamura, Keishi Ohara, Shin-ichi Nagaoka
    NEW JOURNAL OF CHEMISTRY 38 1 132 - 139 2014年01月 [査読有り]
     研究論文(学術雑誌) 
    Mono- and multi-layered films comprised of amphiphilic cationic iridium(III) complexes hybridized with the exfoliated nanosheets of synthetic saponite were prepared by the modified Langmuir-Blodgett method. Three iridium(III) complexes with different emission maxima (lambda(max)) were used as incorporated complexes: [Ir(dfppy)(2)(dc9bpy)](+) (lambda(max) = 500 nm) (dfppyH = 2-(4',6'-difluorophenyOpyridine; dc9bpy = 4,4'-dinonY1-2,2'-bipyridine) [Ir(ppy)(2)(dc9bpy)](+) (lambda(max) = 550 nm) (ppyH = 2-phenylpyridine) and [Ir(piq)(2)(dc9bpy)](+) (lambda(max) = 590 nm) (piqH = 1-phenylisoquinoline) for blue, yellow and red emissions, respectively. Six triple-layered films with different layer sequences were fabricated by layer-by-layer deposition onto quartz substrates. Stationary emission spectra were recorded on the prepared films under vacuum and at various oxygen pressures Notably the change in the spectral shape at surrounding oxygen pressure depended remarkably on the layer sequence. Quenching by oxygen molecules was analyzed by applying a two-site model to the Stern-Volmer plots. The present nanometer-thick films were regarded as a benchmark for an optical device emitting different visible lights in response to oxygen pressure.
  • Tetsuo Okujima, Chie Ando, John Mack, Shigeki Mori, Ichiro Hisaki, Takahiro Nakae, Hiroko Yamada, Keishi Ohara, Nagao Kobayashi, Hidemitsu Uno
    CHEMISTRY-A EUROPEAN JOURNAL 19 41 13970 - 13978 2013年10月 [査読有り]
     研究論文(学術雑誌) 
    An acenaphthylene-fused cyclo[8]pyrrole was synthesized by using an oxidative coupling reaction of the corresponding 2,2-bipyrrole. Two conformational isomers 1a and 1b were isolated, and their molecular structures were elucidated by X-ray crystallographic studies. The less-polar and lower-symmetry 1b isomer can be converted into the 1a isomer through a thermal ring flip. Application of the perimeter model developed by Michl to magnetic circular dichroism spectroscopic data and theoretical calculations demonstrate that there is a marked redshift of the near-IR absorption maxima relative to cyclo[8]isoindole because there is a significant stabilization of the LUMO due to the differing effects of a fused ring expansion with acenaphthylene and benzene moieties on the frontier molecular orbitals.
  • Keishi Ohara, Kohji Doi, Yuka Niizaki, Shin-ichi Nagaoka
    JOURNAL OF PHOTOCHEMISTRY AND PHOTOBIOLOGY A-CHEMISTRY 249 1 1 - 8 2012年12月 [査読有り]
     研究論文(学術雑誌) 
    A kinetic study on singlet oxygen (O-1(2)) quenching by 4-hydroxycinnamic acid derivatives (ferulic acid (FA) and caffeic acid (CA)) has been performed by measuring time-evolutions of O-1(2) phosphorescence at 1274 nm in ethanol, in an ethanol/acetone mixed solvent and in an ethanol/H2O mixed solvent under acidic, neutral and basic conditions. In organic solvents, the estimated second-order rate constants of O-1(2) quenching (k(Q)) were distributed between 10(5) and 10(6) M-1 s(-1): for example, in ethanol, 2.2 x 10(5) M-1 s(-1) for FA and 5.2 x 10(5) M-1 s(-1) for CA. On the other hand, the k(Q) values in ethanol/H2O solutions were enlarged from those in organic solvents, and varied largely with pH from 10(6) to over 10(8) M-1 s(-1). In the case of FA, the O-1(2) quenching activity decreased in the order of FA(2-) (at pH > 10)>> FA(-) (pH 5-9) > FA (pH < 4). The result suggests that deprotonation due to the acid-base equilibrium affects the O-1(2) quenching activity. For both of FA and CA, the phenolate anion under basic conditions (pH 10-12) showed the largest activity probably because of occurrence of the favored chemical quenching through the electron transfer from the phenolate anion to O-1(2). These behaviors would affect several antioxidant assays in vivo and in vitro targeting O-1(2). (C) 2012 Elsevier B.V. All rights reserved.
  • Kazuya Morimoto, Takahiro Nakae, Keishi Ohara, Kenji Tamura, Shin-ichi Nagaoka, Hisako Sato
    NEW JOURNAL OF CHEMISTRY 36 12 2467 - 2471 2012年12月 [査読有り]
     研究論文(学術雑誌) 
    Langmuir-Blodgett (LB) films were prepared by hybridizing a floating monolayer of an amphiphilic cationic iridium(m) complex with clay particles in a subphase. Two kinds of iridium(III) complexes were used: [Ir(dfppy)(2)(dc9bpy)](+) (denoted by DFPPY: dfppyH = 2-(4',6'-difluoro-phenyl)pyridine; dc9bpy = 4,4'-dinonyl-2,2'-bipyridine) with the maximum emission wavelength (lambda(max)) at 500 nm and [Ir(ppy)(2)(dc9bpy)](+) (denoted by PPY: ppyH = 2-phenylpyridine) with lambda(max) at 550 nm. Stationary emission spectra were recorded on the following films under vacuum: {DFPPY or PPY/clay}(n) (n = 1-3), {DFPPY/clay/PPY/clay} and {PPY/clay/DFPPY/clay}. The intensity of emission from {PPY/clay}(n) or {DFPPY/clay}(n) increased nearly in proportion to the layer number (n). Both DFPPY and PPY emitted simultaneously from {DFPPY/clay/PPY/clay}, indicating the low contribution of energy transfer. Contrarily PPY emitted exclusively from {PPY/clay/DFPPY/clay} as a result of the efficient quenching of excited DFPPY in the lower layer by PPY in the upper layer. The introduction of oxygen gas resulted in the decrease of emission for all films. Notably {DFPPY/clay/PPY/clay} exhibited a dual emitting character under an oxygen atmosphere, that is, the broad emission maximum was observed around 530 nm at lower oxygen pressure (0 < P-O2 < 4 kPa), while the emission peak shifted to 550 nm at higher oxygen pressure (P-O2 > 4 kPa).
  • Toshio Naito, Tomoaki Karasudani, Keishi Ohara, Takahiro Takano, Yukihiro Takahashi, Tamotsu Inabe, Ko Furukawa, Toshikazu Nakamura
    ADVANCED MATERIALS 24 46 6153 - 6157 2012年12月 [査読有り]
     研究論文(学術雑誌) 
    An organic insulating crystal reversibly becomes a magnetic conductor under UV irradiation. The rapid and qualitative change in the physical properties is wavelength selective and explained by charge transfer between donor and photochemically active acceptor molecules. The photochemical redox reaction in the crystal produces a partially filled band and localized spins simultaneously.
  • Toshio Naito, Tomoaki Karasudani, Shigeki Mori, Keishi Ohara, Kensuke Konishi, Takahiro Takano, Yukihiro Takahashi, Tamotsu Inabe, Sadafumi Nishihara, Katsuya Inoue
    JOURNAL OF THE AMERICAN CHEMICAL SOCIETY 134 45 18656 - 18666 2012年11月 [査読有り]
     研究論文(学術雑誌) 
    UV irradiation reversibly switches a new insulating and nonmagnetic molecular crystal, BPY[Ni(dmit)(2)](2) (BPY = N,N'-ethylene-2,2'-bipyridinium; Ni(dmit)(2) = bis(1,3-dithiole-2-thione-4,5-dithiolato)nickelate(III)), into a magnetic conductor. This is possible because the bipyridyl derivative cations (BPY2+) trigger a photochemical redox reaction in the crystal to produce a change of similar to 10% in the filling of the Ni(dmit)(2) valence band, leaving localized spins on the BPY themselves. In the dark, almost all of the BPY molecules are closed shell cations, and most of the Ni(dmit)(2). radical anions form spin singlet pairs; thus, this material is a diamagnetic semiconductor. Under UV irradiation, a photocurrent is observed, which enhances the conductivity by 1 order of magnitude Electron-spin resonance. measurements indicate that the 'UV:. irradiation reversibly generates. carriers and localized spins on the Ni(dmit)(2). and the BPY, respectively: This high photoconductivity can be explained by charge transfer (CT) transitions between Ni(dmit)(2) and BPY in the UV region. In other words, the photoconduction and "photomagnetism" can be described as reversible optical control of the electronic states between an ionic salt (BPY2+/[Ni(dmit)(2)](-), nonmagnetic insulator) and a CT complex (BPY2(1-delta)+/[Ni(dmit)(2)]((1-delta)-) (delta approximate to 0.1), magnetic conductor) in the solid state.
  • Shin-Ichi Nagaoka, Keishi Ohara, Miki Takei, Mari Nakamura, Mariko Mishima, Umpei Nagashima
    Journal of Photochemistry and Photobiology A: Chemistry 240 75 - 75 2012年07月 [査読有り]
     研究論文(学術雑誌)
  • Shin Ichi Nagaoka, Akiko Fujii, Megumi Hino, Mai Takemoto, Misaki Yasuda, Mariko Mishima, Keishi Ohara, Akane Masumoto, Hidemitsu Uno, Umpei Nagashima
    Journal of Physical Chemistry B 116 2338  2012年02月 [査読有り]
     研究論文(学術雑誌)
  • Shin-ichi Nagaoka, Keishi Ohara, Miki Takei, Mari Nakamura, Mariko Mishima, Umpei Nagashima
    JOURNAL OF PHOTOCHEMISTRY AND PHOTOBIOLOGY A-CHEMISTRY 225 1 106 - 112 2011年12月 [査読有り]
     研究論文(学術雑誌) 
    The UV protection and singlet-oxygen ((1)O(2)) quenching activity of intramolecularly hydrogen-bonded hydroxyanthraquinone derivatives found in aloe have been studied by means of laser spectroscopy. The UV protective activity provided by excited-state intramolecular proton-transfer (ESIFT) in these molecules correlates with their (1)O(2) quenching activity, and the UV protective molecules have high (1)O(2)-quenching function. The reason for this correlation can be understood by considering ESIPT-induced distortion of ground-state potential surfaces in encounter complexes with (1)O(2). (C) 2011 Elsevier B.V. All rights reserved.
  • Keishi Ohara, Akihiro Ikeda, Shin-ichi Nagaoka
    JOURNAL OF PHOTOCHEMISTRY AND PHOTOBIOLOGY A-CHEMISTRY 218 1 93 - 100 2011年02月 [査読有り]
     研究論文(学術雑誌) 
    The quenching kinetics of superoxide (O(2)(-center dot)) in water/oil (w/o) interface region of Aerosol OT reversed micelle has been investigated with a transient absorption method. Photodecomposition of alpha-tocopherol by 266 nm laser light in the presence of O(2) produced O(2)(-center dot) together with the tocopheroxyl radical (Toc(center dot)). The subsequent reaction between Toc(center dot) and O(2)(-center dot) occurred in the time region of 1-150 mu s after the photoexcitation. There. O(2)(-center dot) kinetics could be traced by monitoring the Toc(center dot) absorption. In an Aerosol OT reversed micelle solution containing vitamin C (VC), VC would be located in water droplets inside the reversed micelle and would react with both Toc(center dot) and O(2)(-center dot) at the w/o interface around the reversed micelle. The Toc(center dot) decay in the time region of 1-150 mu s was accelerated by addition of VC, suggesting that the O(2)(-center dot) quenching by VC progressed as well as the Tor quenching by VC. From a numerical simulation with treating the simultaneous rate equations for Toc(center dot) concentration by Runge-Kutta method, the rate constant for the O(2)(-center dot) quenching by VC could be estimated to be 6.9 x 10(7) M(-1) s(-1). This value is much larger than the corresponding value reported in homogeneous water solution, suggesting that the O(2)(-center dot) quenching by VC indeed occurred at the w/o interface of the reversed micelle. (C) 2010 Elsevier B.V. All rights reserved.
  • Hisako Sato, Kenji Tamura, Keishi Ohara, Shin-ichi Nagaoka, Akihiko Yamagishi
    NEW JOURNAL OF CHEMISTRY 35 2 394 - 399 2011年 [査読有り]
     研究論文(学術雑誌) 
    The Langmuir-Blodgett films were prepared by hybridizing the clay minerals (synthetic saponite, synthetic hectorite and sodium montmorillonite)with the floating film of a cationic Ir(III) complex, [Ir(ppy)(2)(dc18bpy)](+) (ppy = 2-phenylpyridine; dc18bpy = 4,4'-dioctadecyl-2,2'-bipyridine). When the single-layered hybrid LB film was irradiated at 430 nm, it emitted light in the visible region, whose intensity responded reversibly to the pressure change of oxygen.
  • Tetsuo Okujima, Guangnan Jin, Naoki Matsumoto, John Mack, Shigeki Mori, Keishi Ohara, Daiki Kuzuhara, Chie Ando, Noboru Ono, Hiroko Yamada, Hidemitsu Uno, Nagao Kobayashi
    ANGEWANDTE CHEMIE-INTERNATIONAL EDITION 50 25 5698 - 5702 2011年 [査読有り]
     研究論文(学術雑誌) 
    Retro: Cyclo[8]isoindole, which has a saddle-shaped geometry (see picture, 2), was synthesized by oxidative coupling of a bicyclo[2.2.2]octadiene(BCOD)- fused 2,2′-bipyrrole followed by a retro-Diels-Alder reaction of BCOD-fused cyclo[8]pyrrole (1). Key trends in the optical spectra of ring-annelated cyclo[8]pyrroles are identified based on magnetic circular dichroism spectra and theorectical calculations. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
  • YAMADA Hiroko, OHKUBO Kei, KUZUHARA Daiki, TAKAHASHI Tetsuro, SANDANAYAKA Atula S. D, OKUJIMA Tetsuo, OHARA Keishi, ITO Osamu, UNO Hidemitsu, ONO Noboru, FUKUZUMI Shunichi
    Journal of Physical Chemistry B 114 45 14717 - 14728 2010年11月 [査読有り]
     研究論文(学術雑誌) 
    Free base and zinc porphyrins are linked with fullerene (C60) through β,β′-pyrrolic positions rather than meso-positions by Diels-Alder reaction of monoanthraporphyrins (H2P-mA and ZnP-mA) and C60 to afford a π-expanded free base porphyrin-fullerene dyad (H2P-C60) and its zinc porphyrin-fullerene dyad (ZnP-C60) in 86 and 51 % yield, respectively. The X-ray crystallographic analysis of ZnP-C60 showed two comma-shaped swirls-like structure in a unit cell. The intramolecular center-to-center and edge-to-edge distances between the porphyrin and fullerene moieties were 11.5 and 2.5 Å, respectively. The porphyrins and fullerenes were packed in layer-by-layer structure and the porphyrins lied down in parallel, The intermolecular center-to-center distances between the neighboring fullerenes were 10.252, 10.028, and 10.129 Å, which were less than typical van der Waals distance and the π-π interaction spread two-dimensionally. The energy of the charge-separated (CS) state of ZnP-C60 (1.11 eV) determined by differential-pulse voltammetry measurements in benzonitrile is significantly lower than those of zinc porphyrin-C60 dyads linked at meso positions because of the low oxidation potential of the <-expanded ZnP moiety. The CS energy of ZnP-C60 in a nonpolar solvent such as toluene (1.40 V) is lower than the triplet excited state of C60 (3C60*), enabling us to attain a longed lived triplet CS state (8.1 μs) in toluene, detected by nanosecond laser flash photolysis experiments. The distance between unpaired electrons in the triplet CS state was determined by the EPR spectrum to be 9.1 Å, which agreed with the distance between a zinc atom of the porphyrin and a carbon edge of C 60 in the crystal structure of ZnP-C60. The rate constants of photoinduced electron transfer and back electron transfer of H 2P-C60 and ZnP-C60, which were determined by femtosecond and nanosecond laser flash photolysis measurements, were analyzed in light of the Marcus theory of electron transfer. © 2010 American Chemical Society.
  • Keishi Ohara, Ryotaro Mitsumori, Mitsuharu Takebe, Daiki Kuzuhara, Hiroko Yamada, Shin-ichi Nagaoka
    JOURNAL OF PHOTOCHEMISTRY AND PHOTOBIOLOGY A-CHEMISTRY 215 1 52 - 58 2010年09月 [査読有り]
     研究論文(学術雑誌) 
    A synthesized vitamin K model compound, NQ-6 (2-hexyloxy-1,4-naphthoquinone) showed weak fluorescence around 440 nm in ethanol. Addition of antioxidants such as vitamin E to a NQ-6 solution suppressed the NQ-6 emission quantitatively. A kinetic study on the quenching of the NQ-6 emission by hydrogen-donor type antioxidants (three tocopherol analogues, catechin, and 2,6-di-tert-butyl-4-methylphenol) in ethanol was performed. The quenching rate constant obtained from the Stern-Volmer plots for the steady-state fluorescence intensity was consistent with the second-order rate constant of each antioxidant for the free-radical scavenging. The NQ-6 emission is thought to be the delayed fluorescence caused by the thermal population to the excited singlet state from the triplet state. Thus, the fluorescence quenching occurred through a hydrogen atom transfer reaction from an antioxidant to NQ-6 in the excited triplet state ((3)NQ-6*). The second-order rate constant for the reaction between (3)NQ-6* and a-tocopherol was estimated to be 1.2 x 10(10) M(-1) s(-1) from the quenching parameter and the (3)NQ-6* lifetime in ethanol (1.2 mu s) measured with the transient absorption. From the high reactivity of (3)NQ-6* to the antioxidants and the amphiphilic property of NQ-6 as a vitamin K model, NQ-6 is applicable to the quantitative antioxidant assay as a new fluorescence probe. (C) 2010 Elsevier B.V. All rights reserved.
  • Keishi Ohara, Takashi Origuchi, Shin-ichi Nagaoka
    BULLETIN OF THE CHEMICAL SOCIETY OF JAPAN 83 3 246 - 253 2010年03月 [査読有り]
     研究論文(学術雑誌) 
    The singlet oxygen (O-1(2)) behavior in emulsions dispersing natural vitamin E (VE) (alpha-, beta-, gamma-, and delta-tocopherols) was investigated by measuring time-profiles of O-1(2) phosphorescence. The O-1(2) rise and decay dynamics noticeably reflected the existence of VE oil-droplets. The decay rate constant (k(d)) in the a-tocopherol emulsion decreased with increase of VE concentration ([VE]), and became roughly constant in [VE] > 5 x 10(-4)M. This k(d) behavior correlated to the oil-droplet diameter measured by dynamic light scattering. The droplet diameter increased rapidly from 270 to 430 nm with increase of [VE], and reached the limit for dispersing in [VE] >= 5 x 10(-4) M. This result means that the O-1(2) surroundings became more hydrophobic with increase of droplet size. On the other hand, in the emulsions dispersing tocopherols other than alpha-tocopherol, the k(d) values were roughly constant, indicating that the oil-droplets did not contribute to O-1(2) quenching. The droplet diameter in the delta-tocopherol emulsion was around 212-243 nm and was much smaller than that in the alpha-tocopherol emulsion. The peculiar results observed in the alpha-tocopherol emulsion should be due to the large hydrophobicity of alpha-tocopherol which has two methyl-groups neighboring the OH group.
  • Keishi Ohara, Ayumi Shimizu, Yuki Wada, Shin-ichi Nagaoka
    JOURNAL OF PHOTOCHEMISTRY AND PHOTOBIOLOGY A-CHEMISTRY 210 2-3 173 - 180 2010年02月 [査読有り]
     研究論文(学術雑誌) 
    In order to investigate effect of the inhomogeneous dispersion system on photogeneration and decay dynamics of the tocopheroxyl radical (Toc(center dot)), transient absorption and steady-state and time-resolved EPR measurements on photolysis of vitamin E (VE) were carried out. Tocopherol (TocH) is photo-decomposed through electron-detachment in the excited state and the subsequent proton releasing, which produce Toc(center dot) and the solvated electron (e(solv)(-)). The decay of Toc(center dot) is explained by (i) a rapid decay within 2 mu s by recombination between Toc(center dot) and e(solv)(-), (ii) a second-order decay observed in the range of 5-150 mu s by reaction between Toc(center dot) and the superoxide anion radical (O(2)(center dot-)), and (iii) a slow decay in >10 ms by bimolecular self-quenching of Toc(center dot). The Toc(center dot) decay in the VE emulsion was very different from that in the homogeneous solution. In the emulsion, the Toc(center dot) yield was small, the contribution of the reaction with O(2)(center dot-) was rather small, and the decay due to the bimolecular reaction of Toc(center dot) was much accelerated. In the emulsion, Toc(center dot) could not be detected by steady-state EPR, although it could in ethanol. These differences should be caused by the oil-droplet formation in the emulsion. From these results, we can expect that the photostability of the VE emulsion is superior to that of the VE homogeneous solutions. (C) 2010 Elsevier By. All rights reserved.
  • Keishi Ohara, Kaori Kikuchi, Takashi Origuchi, Shin-ichi Nagaoka
    JOURNAL OF PHOTOCHEMISTRY AND PHOTOBIOLOGY B-BIOLOGY 97 3 132 - 137 2009年12月 [査読有り]
     研究論文(学術雑誌) 
    Singlet oxygen ((1)O(2)) quenching by trolox C (TC, 6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid), which is a water-soluble vitamin E analogue, in ethanol-water solutions and in aqueous SDS and CTAC micelles was studied by measuring the time-profiles of (1)O(2) phosphorescence at 1274 nm. The second-order rate-constant for (1)O(2) quenching by TC was determined in the ethanol-water solution to be 1.03 x 10(8), 6.22 x 10(7) and 6.23 x 10(7) M(-1) s(-1) at pH 2.0, 7.0, and 8.4, respectively. These values mean that the non-dissociated form of TC under acidic conditions has superior activity to the mono-anion form. In aqueous micelle systems, the decay rate of (1)O(2) at first decreased and then increased with increase of the concentration of TC. This behavior is explained in terms of the (1)O(2) quenching by TC in the bulk phase and in terms of shifting the environment surrounding (1)O(2) to lipophilic by dissolving TC in the hydrophobic region inside the micelle. The present investigation on (1)O(2) emission dynamics in inhomogeneous solutions made it possible to detect a little change in the solutions, which affects the environment around (1)O(2) , such as the micelle formation and dissolving solute in the micelle. (C) 2009 Elsevier B.V. All rights reserved.
  • 長岡 伸一, 大内 綾, 垣内 拓大, 小原 敬士, 向井 和男
    ビタミン 83 9 521 - 527 日本ビタミン学会 2009年09月 
    Recent studies on the kinetics of the regeneration reactions of vitamin E in solutions by means of stopped-flow spectroscopy confirmed that the quantum effect often plays an important role. Vitamin E may inhibit the autooxidation of lipids in cellular membranes by taking advantage of proton tunneling. It is interesting that the microscopic quantum-mechanical tunneling effect manifests itself in a macroscopic vital function. Although this finding is in conflict with our intuition at first glance, living cells might, in reality, know quantum mechanics very well and use them effectively.
  • Bilkish Rahman, Hiroki Akutsu, Jun-ichi Yamada, Keishi Ohara, Shin'ichi Nakatsuji
    SYNTHETIC METALS 159 12 1123 - 1130 2009年06月 [査読有り]
     研究論文(学術雑誌) 
    Series of radical salts derived from a 2,4,6-triphenylpyridinium (TPP) carrying TEMPO-substituent and transition metal complexes were prepared by the anion exchange of the TPP-TEMPO tetrafluoroborate with [n-Bu(4)N][Ni(dmit)(2)], [n-Bu(4)N](2)[Pd(dmit)(2)], [n-Bu(4)N(2)][Pt(dmit)(2)], [n-Bu(4)N][Ni(mnt)(2)], or [n-Bu(4)N](2)[Ni(mnt)(2)] to give crystals with different structural features. While weak antiferromagnetic interactions of the Curie-Weiss behavior of S = 1/2 spin are predominantly observed in the salts of Pd(dmit)(2), Pt(dmit)(2) and Ni(mnt)(2), the magnetic behavior of 1:1 Ni(dmit)(2) salt can be explained by the additional contribution from the spins on Ni(dmit)(2) anions (chi - chi(TEMPO)), obeying the 2D Heisenberg model. A couple of the oxidized salts, [TPP][Ni(dmit)(2)] as well as [TPP](2)[Pd(dmit)(2)], exhibit semi-conducting properties with relatively high room temperature conductivities. (C) 2009 Elsevier B.V. All rights reserved.
  • Keishi Ohara, Yoko Ichimura, Shin-ichi Nagaoka
    BULLETIN OF THE CHEMICAL SOCIETY OF JAPAN 82 6 689 - 691 2009年06月 [査読有り]
     研究論文(学術雑誌) 
    The second-order rate constants (k(Q)) of singlet-oxygen quenching by caffeic acid and related phenols were measured in ethanol and toluene. Caffeic acid derivatives deactivate singlet-oxygen largely by physical quenching at k(Q) = 10(6) - 10(7) M-1 s(-1) through the charge-transfer transition state.
  • Kazuo Mukai, Aya Ouchi, Akiko Mitarai, Keishi Ohara, Chihiro Matsuoka
    BULLETIN OF THE CHEMICAL SOCIETY OF JAPAN 82 4 494 - 503 2009年04月 [査読有り]
     研究論文(学術雑誌) 
    In order to understand the dynamics of antioxidant actions of vitamin E (alpha-, beta-, gamma-, and delta-tocopherols, TocH) in biological systems, kinetic study of the formation and decay reactions of vitamin E radicals (alpha-, beta-, gamma-, and delta-tocopheroxyls, Toc center dot) has been performed in organic solvents, using stopped-flow spectrophotometry. By mixing alpha-, beta-, gamma-, and delta-TocH with aryloxyl radical (ArO center dot) in ethanol, the peaks of the UV-vis absorption due to alpha-, beta-, gamma-, and delta-Toc center dot radical appeared rapidly at ca. 430-340 nm, showed maxima, and then decayed gradually. The second-order rate constants (k(f) and 2k(d)) for the formation and decay (that is, bimolecular disproportionation) reactions of alpha-Toc center dot were determined by comparing the observed curves with the simulation ones obtained by the numerical calculation of differential equations related to the above reactions. From the results, the wavelengths of absorption maxima (lambda(i)(max)) and molar extinction coefficients (epsilon(i)) (i = 1-4) of the optical spectra were determined for alpha-Toc center dot radical. Notable solvent effects have been observed for the reaction rates (k(f) and 2k(d)) and absorption spectra (lambda(i)(max) and epsilon(i)) of alpha-Toc center dot radical. The scheme of the formation and decay reactions of alpha-, beta-, gamma-, and delta-Toc center dot radicals has been discussed based on the results obtained.
  • Takuhiro Kakiuchi, Kazuo Mukai, Keishi Ohara, Shin-ichi Nagaoka
    BULLETIN OF THE CHEMICAL SOCIETY OF JAPAN 82 2 216 - 218 2009年02月 [査読有り]
     研究論文(学術雑誌) 
    A study of the kinetics of the proton-transfer reaction in flavonoid in ethanol solution by means of stopped-flow spectroscopy indicated that proton tunneling plays an important role in the antioxidant reaction.
  • Keishi Ohara, Yoshimi Hashimoto, Chiaki Hamada, Shin-ichi Nagaoka
    JOURNAL OF PHOTOCHEMISTRY AND PHOTOBIOLOGY A-CHEMISTRY 200 2-3 239 - 245 2008年12月 [査読有り]
     研究論文(学術雑誌) 
    The photochemical reactions of vitamin K (VK) with antioxidant vitamins (vitamin E (VE) and vitamin C (VC)) in aqueous hexadecyltrimethylammonium chloride (CTAC), sodium dodecyl sulfate (SDS), and Triton X-100 micelle systems, and in an aerosol OT (AOT) reversed micelle system were investigated by a time-resolved EPR (TR-EPR). The photolysis of VK with VE in the aqueous micelle solutions gave the TR-EPR spectra having strong intensity and net emissive polarization, suggesting that the excited triplet state of VK ((3)VK(center dot)) was rapidly quenched by VE coexisting inside the micelle. On the other hand. the photolysis of VK with VC in the aqueous SDS and CTAC micelle systems gave the spectra having weak intensity, showing that the reaction between (3)VK(center dot) and VC was inefficient in these micelle systems, probably because (3)VK(center dot) scarcely diffused Out from the micelle. The photolysis of VK with VC in the AOT reversed micelle solution gave the spin-correlated radical pair CIDEP spectrum. The result suggests that the long-lived radical pair was generated from the reaction between (3)VK(center dot) and VC in the water/oil interface region of the AOT micelle, although one of the reactants dissolved in the oil phase and another did in the separated water phase. (C) 2008 Elsevier B.V. All rights reserved.
  • Keishi Ohara, Takashi Origuchi, Kanako Kawanishi, Shin-ichi Nagaoka
    Bulletin of the Chemical Society of Japan 81 3 345 - 347 日本化学会 2008年03月 [査読有り]
     研究論文(学術雑誌)
  • MUKAI K, TOKUNAGA A, ITOH S, KANESAKI Y, OUCHI A, OHARA K, NAGAOKA S, ABE K
    IOS Press, BioFactors 32 1-4 49 - 58 2008年 [査読有り]
     研究論文(国際会議プロシーディングス)
  • Kazuo Mukai, Aiko Tokunaga, Shingo Itoh, Yu Kanesaki, Aya Ouchi, Keishi Ohara, Shin-ichi Nagaoka, Kouichi Abe
    BIOFACTORS 32 1-4 49 - 58 2008年 [査読有り]
     研究論文(学術雑誌) 
    Detailed kinetic studies have been performed for the reaction of aroxyl (ArO center dot) radical with vitamin E (alpha-, beta-, gamma-, delta-tocopherol, TocH), ubiquinol-10, and related antioxidants in micellar solution, using a stopped-flow spectrophotometer. The second-order reaction rates (k(s)) obtained increased in the order of hydroquinone < tocol <delta-T6cH < ubiquinol-0 <gamma-TocH similar to beta-TocH < ubiquinol-10 <alpha-TocH at pH 4 similar to 9. The antioxidants which have lower oxidation potentials (E(p)) showed higher reactivities. The reaction rates obtained in micellar solution were pH dependent because of the dissociation of OH groups in the antioxidants. For instance, by comparing the k(s) values with the mole fraction of each molecular form of ubiquinol-10, the reaction rate k(s1) (1.21 x 10(5) M(-1)s(-1)) for undissociated form, k(s2) (1.04 x 10(6) M(-1) s(-1)) for monoanion and k(s3) (0 M(-1)s(-1)) for dianion, and the pK(a1) and PK(a2) values (11.4 and 12.7) were determined. It was found that the relative ratio of K(s) values (100:21:20:2.9) of alpha-, beta-, gamma-, delta-tocopherols in micellar dispersion has good correlation with the relative biological activities for rat fetal resorption, rat haemolysis, and chicken muscle dystrophy. The relative antioxidant activities of alpha-tocopherol and ubiquinol-10 have been discussed on the basis of the products of the k(s) values and their concentrations in serum and several tissues (heart, muscle, liver, kidney, and brain).
  • Shin-ichi Nagaoka, Akiko Fujii, Megumi Hino, Mai Takemoto, Misaki Yasuda, Mariko Mishima, Keishi Ohara, Akane Masumoto, Hidemitsu Uno, Umpei Nagashima
    JOURNAL OF PHYSICAL CHEMISTRY B 111 45 13116 - 13123 2007年11月 [査読有り]
     研究論文(学術雑誌) 
    The UV protection and singlet oxygen quenching of aloesaponarin I have been studied by means of laser spectroscopy. The excited-state intramolecular proton transfer that provides the UV protection takes place along only one of the molecule's two intramolecular hydrogen bonds, and this can be understood by considering the nodal pattern of the wave function. The functional groups participating in the excited-state intramolecular proton transfer also play important roles in the singlet oxygen quenching. Aloesaponarin I has a quenching rate constant larger than that of vitamin E and has a long duration of action due to its resistance to UV degradation and chemical attacks by singlet oxygen and free radicals.
  • Shin-ichi Nagaoka, Takuhiro Kakiuchi, Keishi Ohara, Kazuo Mukai
    CHEMISTRY AND PHYSICS OF LIPIDS 146 1 26 - 32 2007年03月 [査読有り]
     研究論文(学術雑誌) 
    The rate constant and activation energy of the regeneration reaction of natural vitamin E by vitamin C were determined with a double-mixing stopped-flow spectrophotometer. The formation of vitamin C radical was observed in the absorption spectrum. The kinetic effect of methyl substitution on the aromatic ring of vitamin E radical indicates that partial charge-transfer plays a role in the reaction. Since a substantial deuterium kinetic isotope effect was not found, the tunneling effect may not play an important role under the present experimental conditions. (c) 2007 Elsevier Ireland Ltd. All rights reserved.
  • Kazuo Mukai, Aiko Tokunaga, Shingo Itoh, Yu Kanesaki, Keishi Ohara, Shin-ichi Nagaoka, Kouichi Abe
    JOURNAL OF PHYSICAL CHEMISTRY B 111 3 652 - 662 2007年01月 [査読有り]
     研究論文(学術雑誌) 
    The reaction rates (k(s)) of vitamin E (alpha-, beta-, gamma-, delta-tocopherols, TocH), ubiquinol-10, and related antioxidants (tocol, ubiquinol-0, and hydroquinone) with aroxyl (ArO center dot) radical have been measured in micellar solution by stopped-flow spectrophotometer. The k(s) values increased in the order of hydroquinone < tocol < delta-TocH < ubiquinol-0 < gamma-TocH approximate to beta-TocH < ubiquinol-10 < alpha-TocH at pH 4 similar to 8. The antioxidants which have lower oxidation potentials showed higher reactivities. The k(s) values of alpha-, beta-, gamma-, delta-tocopherol, and tocol remained constant between pH 4 and 10, and decreased rapidly at pH 11 similar to 12 by increasing pH value. From the pH dependence of k(s) values, the pK(a) values (= 13.1 similar to 12.6) have been determined for these tocopherols. The k(s) values of ubiquinol-10 also remained constant between pH 4 and 9, and increased rapidly at pH 9.5. Ubiquinol-10 is dibasic acid and can exist in three different molecular forms, depending on the pH value. By comparing the k(s) values with the mole fraction of each molecular form of ubiquinol-10, the reaction rate k(s1) (= 1.21 x 10(5) M(-1)s(-1)) for the undissociated form, k(s2) (= 1.04 x 10(6) M(-1)s(-1)) for monoanion and k(s3) (= 0 M (-1)s (-1)) for dianion, and the pK(a1) and pK(a2) values (= 11.4 and 12.7) were determined. The k(s2) value is 8.6 times as large as the k(s1) value. Similar analyses were performed for ubiquinol-0 and hydroquinone. It was found that the relative ratio of k(s) values (100:21:20:2.9) of alpha-, beta-, gamma-, delta-tocopherols in micellar dispersion has good correlation with the relative biopotency ratios for rat fetal resorption, rat hemeolysis, and chicken muscle dystrophy. The relative antioxidant activities of alpha-tocopherol and ubiquinol-10 have been discussed based on the k(s) values obtained and their concentrations in serum and several tissues (heart, muscle, liver, kidney, and brain).
  • Keishi Ohara, Yoko Ichimura, Kumi Tsukamoto, Mayumi Ogata, Shin-ichi Nagaoka, Kazuo Mukai
    BULLETIN OF THE CHEMICAL SOCIETY OF JAPAN 79 10 1501 - 1508 2006年10月 [査読有り]
     研究論文(学術雑誌) 
    A kinetic study involving 4-hydroxycinnamic acid derivatives (HCAs) was performed in order to clarify the mechanism for free radical-scavenging and vitamin E-regenerating. The second-order rate constants of the radical-scavenging reaction (k(s)) observed for caffeic acid derivatives are larger than those for ferulic acid derivatives. The result may be explained by i) the lower oxidation potential coming from the existence of the more electron-donating hydroxy group comparing with methoxy group, and ii) the stabilization effect of the corresponding radicals produced in the radical-scavenging reaction through intramolecular hydrogen bonding. The radical-scavenging activity of HCAs in ethanol mainly occurs via the hydrogen atom transfer from the phenolic OH-protons. The pH dependence of k(s) for ferulic acid in the aqueous Triton X-100 micelle solutions suggests the importance of the phenolic OH proton on the radical-scavenging reaction. On the other hand, the pH dependence of k(s) for caffeic acid and chlorogenic acid suggests the occurrence of intramolecular hydrogen bonding and an electron-transfer process in the radical-scavenging reaction.
  • Keishi Ohara, Debora M. Martino, Hans van Willigen
    JOURNAL OF PHOTOCHEMISTRY AND PHOTOBIOLOGY A-CHEMISTRY 181 2-3 325 - 332 2006年07月 [査読有り]
     研究論文(学術雑誌) 
    The anomalous net absorptive CIDEP spectra observed in the photoinduced hydrogen abstraction reaction of chromone (CR) and chromone2-carboxylic acid (CRCA) from 2-propanol with addition of hydrochloric acid (HCl) were investigated with FT-EPR. In neat 2-propanol, the spectra of the ketyl and 2-hydroxypropan-2-yl (2-HP) radicals display E-*/A (low-field side emission and high-field side absorption, the asterisk denoting excess net polarization) or E/A type polarization, which is explained by the STO mixing radical pair mechanism and minor contributions of the triplet mechanism (TM). The addition of HCl to these systems gives the net absorptive CIDEP spectra, while the triplet states of CR and CRCA should have emissive polarizations. The analysis of the time developments of the EPR signals indicates that the radical generation rate increases with the increase of the HCl concentration, while the signal intensity of radicals does not increase. The net absorptive polarization might be explained by non-reactive quenching of the triplet by the spin-sublevel dependent back charge-transfer process in the intermediate exciplex. (c) 2005 Elsevier B.V. All rights reserved.
  • K Ohara, A Fujii, Y Ichimura, K Sato, K Mukai
    BULLETIN OF THE CHEMICAL SOCIETY OF JAPAN 79 3 421 - 426 2006年03月 [査読有り]
     研究論文(学術雑誌) 
    A kinetic study was performed for edaravone (3-methyl-1-phenyl-2-pyrazolin-5-one) in order to clarify the mechanism of its free-radical-scavenging and vitamin E-regenerating actions. The second-order rate constants for the radical-scavenging reaction of edaravone were measured in several organic solvents and in a water/methanol mixed solvent at various pH. The keto-enol tautomerism and the acid-base dissociation equilibrium of edaravone produce keto, enol, and anion forms in solutions, and their contributions and activities varied depending on the properties of the solutions. From the results of NMR and kinetic studies, it has been clarified that the keto-enol tautomerism of edaravone actually exists, and the keto form has larger radical-scavenging activity than the enol form. Furthermore, the pH dependence of the rate constants suggests that the anion form produced by the acid-base dissociation equilibrium of edaravone has the highest radical-scavenging activity in the keto, enol, and anion forms.
  • K Mukai, D Shiba, K Yoshida, K Mukai, H Hisatou, K Ohara, Y Hosokoshi, N Azuma
    BULLETIN OF THE CHEMICAL SOCIETY OF JAPAN 78 12 2114 - 2123 2005年12月 [査読有り]
     研究論文(学術雑誌) 
    Four kinds of 1:1 salts of 3-(4- and 3-alkyl-pyridinium)-1,5-diphenylverdazyl (alkyl = ethyl and methyl) radical cations with the Ni(dmit)(2) anion ([p-EtPyDV](+)[Ni(dmit)(2)](-) (1), [m-EtPyDV](+)[Ni(dmit)(2)](-) (2), [p-MePyDV](+)[Ni(dmit)(2)](-) (3), and [m-MePyDV](+)[Ni(dmit)(2)](-) (4)) have been prepared. The magnetic properties of salts 1-4 were discussed based on the results obtained by magnetic susceptibility and ESR measurements of 1-4 and the crystal structure analysis of 1. The results of the crystal structure analysis of salt 1 indicate the dimer formation in Ni(dmit)(2) anion molecules, and the dimers are sandwiched between two verdazyl cations [p-EtPyDV](+), indicating the formation of a magnetic linear tetramer in 1. The magnetic susceptibility data for salt 1 have been fitted to a four-spin linear tetramer model using an end exchange interaction of 2J(1)/k(B) = -780 K and a central interaction of 2J(2)/k(B) = -200 K. The conductivity (sigma) of salts 1, 2, 3, and 4 at room temperature was sigma = 2.3 x 10(-5), 1.8 X 10(-4), 1.4 x 10(-5), and 5.4 x 10(-6) S cm(-1) with an activation energy of E-A = 0.28, 0.52, 0.21, and 0.33 eV, respectively. The 1: 1 salts 1-4 are new molecular paramagnetic semiconductors.
  • K Mukai, D Shiba, K Mukai, K Yoshida, H Hisatou, K Ohara, Y Hosokoshi, N Azuma
    POLYHEDRON 24 16-17 2513 - 2521 2005年11月 [査読有り]
     研究論文(学術雑誌) 
    Five kinds of (1:1) and (1:3) salts of 3-(4- and 3-alkyl-pyridinium)-1,5-diphenylverdazyl (alkyl = ethyl and methyl) radical cations with Ni(dmit)(2) anion ([p-EtPyDV](+)[Ni(dmit)(2)](3)(-) (1), [m-EtPyDV](+)[Ni(dmit)(2)](-) (2), [p-MePyDV](+)[Ni(dmit)(2)](-) (3), [m-MePyDV](+)[Ni(dmit)(2)](-) (4), and [p-MePyDV](+)[Ni(dmit)(2)](3)(-) (5)) (dmit = 1,3-dithiol-2-thione-4,5-dithiolate) have been prepared, and the magnetic susceptibilities (chi(M)'s) have been measured between 1.8 and 300 K. The chi(M) of salt 1 can be well explained by a four-spin linear tetramer model ([p-EtPyDV](+)-[Ni(dmit)(2)](-)-[Ni(dmit)(2)](-)-[p-EtPyDV](+)) with an end exchange interaction 2J(1)/k(B) = -780 K and a central interaction 2J(2)/k(B) = -200 K. The formation or magnetic tetramer in salt 1 has been ascertained by the crystal structure analysis of 1. The chi(M) of salt 2 was explained by the SUM of the contributions from: (i) a dimer system (2J/k(B) = -320 K) due to Ni(dmit)(2) anions and (ii) the one-dimensional (1D) Heisenberg ferromagnetic linear-chain system (2J k(B) = +2.5 K) due to verdazyl cations. The chi(M) of salt 3 is similar to that of salt 1, suggesting a formation of magnetic tetramer in salt 3. The chi(M) of salt 4 shows a broad maximum at T-max = 100 +/- 2 K, and can be explained by a 1D Heisenberg antiferromagnetic alternating-chain model with 2J(1)/k(B) = -165 K (alternation parameter alpha = J(2)/J(1) = 0.6). The conductivity (sigma) of salts 1, 2, 3 and 4 at 293 K was sigma = 2.3 x 10(-5), 1.1 x 10(-3), 1.4 x 10(-5) and 5.4 x 10(-6) S cm(-1) with an activation energy of E-A = 0.28, 0.52, 0.21 and 0.33 eV, respectively. The (1:3) salt 5 (sigma = 1.0 S cm(-1), E-A = 0.048 eV) showed three to five orders of magnitude higher conductivity than those of the (1:1) salts 1-4 at room temperature. All the (1:1) and (1:3) salts 1-5 are new molecular paramagnetic semiconductors. (c) 2005 Elsevier Ltd. All rights reserved.
  • H Yamada, Y Yamashita, M Kikuchi, H Watanabe, T Okujima, H Uno, T Ogawa, K Ohara, N Ono
    CHEMISTRY-A EUROPEAN JOURNAL 11 21 6212 - 6220 2005年10月 [査読有り]
     研究論文(学術雑誌) 
    A novel alpha-diketone precursor of pentacene, 6,13-dihydro-6,13-ethanopentacene-15,16-dione, was prepared and converted successfully to pentacene in 74% yield by photolysis of the precursor in toluene: Irradiation of the diketone solution in toluene with light of 460 nm under an Ar atmosphere caused the solution to change from yellow to fluorescent orange-pink within a few minutes, after which, purple precipitates appeared. After 35 min, the solution changed to colorless and the purple precipitates were filtered to give pentacene in 74% yield. By contrast, in the presence of oxygen, the color of the solution changed from yellow to pale yellow., and only 6,13-endoperoxide of pentacene was quantitatively obtained. The rate of the reaction upon photolysis was measured by observing the decay of n-pi* absorption of the precursor at 460 nm, and was found to be similar in both the presence and absence of oxygen. Therefore, the photoreaction of the alpha-diketone precursor seemed to occur via the singlet excited state. Because the T-T absorption of pentacene was observed upon photolysis of the precursor in the nanosecond transient absorption measurement under an Ar atmosphere, the excited triplet state of the pentacene generated singlet oxygen by sensitization, and it reacted with the ground-state pentacene to give the 6,13-endoperoxide. The alpha-diketone deposited on glass was also converted successfully to pentacene film by photoirradiation. In addition, diketone precursors of a mixture of 2,8- and 2,9-di-bromopentacene and 2,6-trianthrylene were also prepared and their photoconversion was performed.
  • K Mukai, S Nagai, K Ohara
    FREE RADICAL BIOLOGY AND MEDICINE 39 6 752 - 761 2005年09月 [査読有り]
     研究論文(学術雑誌) 
    A kinetic study of the quenching reaction of singlet oxygen (O-1(2)) with catechins (catechin (CA), epicatechin (EC), epigallocatechin (EGC), epicatechin gallate (ECG), epigallocatechin gallate (EGCG)) and related compounds (5-methoxyresorcinol (MR), 4-methylcatechol (MC), and n-propyl gallate (PG)) was performed in ethanol at 35 degrees C. MR, MC, and PG are considered to be a model of resorcinol (A)-, catechol (B)-, and gallate (G)-rings in catechins, respectively. The overall rate constants, k(Q) ( k(q) + k(r) physical quenching + chemical reaction), for the reaction of catechins with 10, increased in the order of PG < MR < MC < CA < EC < EGC < ECG < EGCG. In a comparison of the rate constants, the relationship between quenching rates and chemical structures is discussed. The catechins which have lower peak oxidation potentials, E-P, show higher reactivities. It was observed that the chemical reaction (k(r)) is almost negligible in the quenching reaction Of O-1(2) by catechins. The kQ values of EGCG (1.47 x 10(8) M-1 s(-1)) and ECG (7.81 X 10(7)) were found to be larger than those of lipids (1.3 x 10(5-)1.9 X 10(5) M-1 s(-1)), amino acids (< 3.7 x 10(7)), and DNA (5.1 x 10). Further, these values are similar to those (1.15 x 10(8)-2.06 x 10(8) M-1 s(-1)) of alpha- and gamma-tocopherol, ubiquinol-10, and gamma-tocopherol hydroquitione (plastoquinol model). The result suggests that catechins may contribute to the protection of oxidative damage in biological systems, by quenching O-1(2). (c) 2005 Elsevier Inc. All rights reserved.
  • K Mukai, S Mitani, K Ohara, SI Nagaoka
    FREE RADICAL BIOLOGY AND MEDICINE 38 9 1243 - 1256 2005年05月 [査読有り]
     研究論文(学術雑誌) 
    The reaction rates (k(r)) of 5,7-diisopropyl-tocopheroxyl radical (Toc(center dot)) with catechins (epicatechin (EC), epicatechin gallate (ECG), epigallocatechin (EGC), epigoallocatechin gallate (EGCG)) and related compounds (methyl gallate (MG), 4-methyleatechol (MC), and 5-Foethoxyresorcinol (MR)) have been measured by stopped-flow spectrophotorneter. The k(r), values increased in the order of MR << MG < EC < MC similar to ECG < FGC < EGCG in ethanol and 2-propanol/H2O (5/1, v/v) solutions, indicating that the reactivity of the OH groups in catechins increased in the order of resorcinol A-ring << gallate G-ring < catechol B-ring < pyrogallol B-ring. The catechins which have lower oxidation potentials show higher reactivities. The rate constants for catechins in micellar solution showed notable pH dependence with one or two peaks around pH 9-11, because of the dissociation of various phenolic hydroxyl protons in catechins. The Structure-activity relationship in the free-radical-scaveril-ing reaction by catechins has been clarified by the detailed analyses of the pH dependence of kr values. The reaction rates increased remarkably with increasing the anionic character of catechins, that is, the electron-donating capacity of catechins. The mono anion form at catechol B-and resorcinol A-rings and dianion form at pyrogallol B-and gallate G-rings show the highest activity for free-radical-scavenging. It was found that catechins (EC, ECG, EGC, and EGCG) have activity similar to or higher than that of vitamin C in vitamin E regeneration at pH 7-12 in micellar solution. (c) 2005 Elsevier Inc. All rights reserved.
  • K Ohara, W Mizukami, A Tokunaga, S Nagaoka, H Uno, K Mukai
    BULLETIN OF THE CHEMICAL SOCIETY OF JAPAN 78 4 615 - 621 2005年04月 [査読有り]
     研究論文(学術雑誌) 
    A kinetic Study was performed for the free-radical scavenging actions of curcumin and half-curcumin, which has a half-structure of curcumin, in order to clarify the mechanism of free-radical-scavenging in curcumin. The second-order rate constants for radical-scavenging reactions of curcumin and half-curcumin were measured by a stopped-flow spectrophotometer in several organic solvents (methanol, ethanol, acetonitrile, chloroform, and benzene) and in aqueous Triton X-100 (5.0 wt %) micelle solutions at various pH. The difference in the rate constant and solvent dependence between curcumin and half-curcumin suggests that the enol structure with the intramolecular hydrogen-bond Of Curcumin strongly enhances the radical-scavenging activity. Furthermore, notable pH dependences were observed for the rate constants of curcumin and half-curcumin in micelle solutions, suggesting that the acid-base dissociation equilibrium of phenol-protons in curcumin and half-curcumin affects their radical-scavenging activities.
  • S Nagai, K Ohara, K Mukai
    JOURNAL OF PHYSICAL CHEMISTRY B 109 9 4234 - 4240 2005年03月 [査読有り]
     研究論文(学術雑誌) 
    The quenching rate of singlet oxygen (O-1(2)) by seven kinds of flavonoids (flavone, flavonol, chrysin, apigenin, rutin, quercetin, and myricetin) with 2,3-double bonds has been measured spectrophotometrically in ethanol at 35 degreesC. The overall rate constants k(Q) (=k(q) + k(r), physical quenching + chemical reaction) increased as the number of OH groups substituted to the flavone skeleton (that is, the total electron-donating capacity of flavonoids) increases. The existence of catechol or pyrogallol structure in the B-ring is essential for the O-1(2) quenching of flavonoids. Log k(Q) was found to correlate with their peak oxidation potentials, E-P; the flavonoids that have smaller E-P values show higher reactivities. Similarly, log k(Q) values of flavonoids correlate with the energy level of the highest occupied molecular orbital (E-HOMO), calculated by the PM3 MO method, and the longest wavelength pipi* excitation energy (E-ex). The contribution of the chemical reaction (k(r)) was found to be negligible in these flavonoids. The k(Q) values of rutin, quercetin, and myricetin [(1.21similar to5.12) x 10(8) M-1 s(-1)] were found to be larger than those of lipids [(0.9similar to6.4) x 10(4) M-1 s(-1)], amino acids (< 3.7 x 10(7) M-1 s(-1)),and DNA (5.1 x 10(5) M-1 s(-1)). The result suggests that these flavonoids may contribute to the protection of oxidative damage in foods and plants, by quenching O-1(2).
  • Yoshio Teki, Mituhiro Kimura, Shinsuke Narimatsu, Keishi Ohara, Kazuo Mukai
    Bulletin of the Chemical Society of Japan 77 1 95 - 99 2004年01月 [査読有り]
     研究論文(学術雑誌) 
    The excited high-spin quartet (S = 3/2) state of the intramolecular triplet-radical pair between the excited triplet state of pyrene and a dangling verdazyl radical was reported. A novel verdazyl radical with the pyrene moiety was designed and synthesized. The time-resolved ESR spectrum and a DFT calculation show the photo-induced ferromagnetic spin alignment between the excited triplet state of the pyrene moiety and the dangling verdazyl radical. This is the first observation of the high-spin (S > 1) excited state of the π-conjugated pyrene-radical pair.
  • K Mukai, N Senba, T Hatanaka, H Minakuchi, K Ohara, M Taniguchi, Y Misaki, Y Hosokoshi, K Inoue, N Azuma
    INORGANIC CHEMISTRY 43 2 566 - 576 2004年01月 [査読有り]
     研究論文(学術雑誌) 
    Four kinds of 1:1 and 1:3 salts of 3-[4-(trimethylammonio)phenyl]-1,5-diphenyl-6-oxoverdazyI radical cation ([1](+)) and its mono- and dimethyl derivatives ([2](+) and [3](+)) with Ni(dmit)(2) anions (d(mit) = 1,3-dithiol-2-thione-4,5-dithiolate) ([1](+)[Ni(dmit)(2)](-) (4), [2](+)[Ni(dmit)(2)](-) (5), [3](+)[Ni(dmit)(2)](-) (6), and [1](+)[Ni(dmit)(2)](3)(-) (7)) have been prepared, and the magnetic susceptibilities (chi(M)) have been measured between 1.8 and 300 K. The chi(M) values of salts 5 and 7 can be well reproduced by the sum of the contributions from (i) a Curie-Weiss system with a Curie constant of 0.376 (K emu)/mol and negative Weiss constants (Theta) of -0.4 and -1.7 K and (ii) a dimer system with strong negative exchange interactions of 2J/k(B) = -354 and -258 K, respectively. The dimer formations in Ni(dmit)(2) anions have been ascertained by the crystal structure analyses of salts 4-6. In salts 4 and 6, Ni(dmit)(2) dimer molecules are sandwiched between two verdazyl cations, indicating the formation of a linear tetramer in 4 and 6. The magnetic susceptibility data for salts 4 and 6 have been fitted to a linear tetramer model using an end exchange interaction of 2J(1)/k(B) = -600 K and a central interaction of 2J(2)/k(B) = -280 K for 4 and 2J(1)/k(B) = -30 K and 2J(2)/k(B) = -580 K for 6, respectively. The results of the temperature dependence of the 9(T) value in salts 4-6 obtained by ESR measurement also support the above analyses. The 1:1 salts 4-6 are insulators. On the other hand, the conductivity of the 1:3 salt 7 at 20 degreesC was sigma = 0.10 S cm(-1) with an activation energy E-A = 0.099 eV, showing the semiconductor property. Salt 7 is a new molecular paramagnetic semiconductor.
  • K Ohara, R Watanabe, Y Mizuta, S Nagaoka, K Mukai
    JOURNAL OF PHYSICAL CHEMISTRY B 107 41 11527 - 11533 2003年10月 [査読有り]
     研究論文(学術雑誌) 
    The photoinitiated reaction between vitamin C and xanthone in sodium lauryl sulfate (SDS), hexadecyltrimethylammonium chloride (CTAC), and Triton X-100 micelle solutions at various pH was investigated by time-resolved electron paramagnetic resonance (TR-EPR). The TR-EPR spectra were explained by superimpositions of the xanthone ketyl and the vitamin C radicals, showing that a fast hydrogen abstraction reaction of the excited xanthone from vitamin C progresses around the water-oil interface region of the micelles. The EPR signal intensity of the vitamin C radical showed the notable pH dependence, which seems to be attributable to the acid-base dissociation equilibrium of vitamin C. The results suggested that the present reaction is controlled by the transportation of the excited xanthone and vitamin C to the reaction-progressing region, which is the surface or inside of the micelle, and by the difference of the reactivity between the dissociation forms of vitamin C.
  • Yoshinori Nishioku, Keishi Ohara, Kazuo Mukai, Shin-ichi Nagaoka
    Journal of Physical Chemistry B 105 21 5032 - 5038 2002年05月 [査読有り]
     研究論文(学術雑誌) 
    The models of vitamin K and vitamin E linked molecule, (1-(1,4-naphthoquinone-2-oxy)-6-(hydroxy-2,5,7,8-tetramethylchroman-2-carbonylox y)-hexane), was synthesized as a model of vitamin K and vitamin E which coexist in biological membranes. The time-resolved EPR (TR-EPR) spectra of this molecule could be assigned to vitamin K and E radicals and showed that the excited triplet state of thh vitamin K moiety was rapidly quenched by abstraction of hydrogen from the vitamin E moiety. The broad and emissive CIDEP spectra observed in ethanol suggested that the intramolecular interaction between vitamin K and vitamin E radicals was strong there. The relatively resolved spectra observed in a TX-100 micelle system, on the other hand, indicated that the interaction between these radicals was very weak. These results suggest that the vitamin K and E radicals existed around the water-oil interface of the micelle independently. The absence of a deuterium effect in our experiments showed that tunneling does not contribute significantly to the quenching reaction.
  • N Azuma, N Senba, K Okuda, K Ohara, Y Hosokoshi, K Inoue, K Mukai
    MOLECULAR CRYSTALS AND LIQUID CRYSTALS 376 341 - 346 2002年 [査読有り]
     研究論文(学術雑誌) 
    Three kinds of (1:1) salts of TCNQF(4) anion with 3-(4trimethylaminophenyl)-1,5-diphenyl-6-oxo-verdazyl radical cation [1(+)] and its mono-methyl [2(+)] and di-methyl [3(+)] derivatives have been prepared, and magnetic property and crystal structure of the salts have been studied. The salts were found to be new genuine organic magnetic compounds consisting of open-shell verdazyl radical cation and electron acceptor TCNQF(4) anion.
  • K Ohara, K Mukai
    CHEMICAL PHYSICS LETTERS 317 6 619 - 623 2000年02月 [査読有り]
     研究論文(学術雑誌) 
    The addition effect of hydrochloric acid (HCl) on the photoreduction of chromone-2-carboxylic acid (CRCA) is studied by time-resolved EPR, The EPR lines of CRCA ketyl radical show an enhanced absorption in the presence of HCl, while without HCl these show an emissive character. On the other hand, the lines of the CRCA alkyl type radical show an emissive character whether HCl is included or not. The simultaneous reactions of the closely-lying two excited tripler states (T-1 and T-2) of CRCA may induce the above anomalous CIDEP behavior. (C) 2000 Elsevier Science B.V. All rights reserved.
  • S Nagaoka, M Inoue, C Nishioka, Y Nishioku, S Tsunoda, C Ohguchi, K Ohara, K Mukai, U Nagashima
    JOURNAL OF PHYSICAL CHEMISTRY B 104 4 856 - 862 2000年02月 [査読有り]
     研究論文(学術雑誌) 
    Studies of the kinetics of the proton- or hydrogen-transfer reactions concerning vitamin E in solutions and in micellar dispersions by means of stopped-flow and absorption spectroscopy indicated that proton tunneling plays an important role in the antioxidant and regeneration reactions that are advantageous in vivo but not as a part of the harmful prooxidant action.
  • K Ohara, S Nagaoka, K Mukai
    BULLETIN OF THE CHEMICAL SOCIETY OF JAPAN 73 1 37 - 42 2000年01月 [査読有り]
     研究論文(学術雑誌) 
    The photosensitized reaction of maleimide (MI) with xanthone (Xn) in 2-propanol was investigated by the time-resolved EPR method. The emissive CIDEP spectrum observed in neat 2-propanol is predominantly assigned to two kinds of MI alkyl-type radicals. On the other hand, the absorptive spectrum of the MI anion radical has been newly observed in 2-propanol including hydrochloric acid. The addition effect of hydrochloric acid indicates the existence of two mechanisms for the photosensitization of MI by Xn. One is a typical triplet-triplet energy transfer followed by hydrogen abstraction by the triplet state of MI from the solvent. The other is an electron-transfer process to MI from the Xn ketyl radical produced primarily by a fast reaction of the triplet Xn with HCl. The spin-polarization transfer mechanism based on these photosensitization processes can successfully explain the present CIDEP results. The effect of water and the sensitizer dependence on the above reaction were also investigated.
  • K Mukai, K Suzuki, K Ohara, JB Jamali, N Achiwa
    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 68 9 3078 - 3085 1999年09月 [査読有り]
     研究論文(学術雑誌) 
    The magnetic susceptiblities, ESR and powder X-ray patterns of verdazyl radical alloy, (p-CDpOV)(1-x)(p-BDpOV)(x) (x = 0-1), were measured to study the doping effects of magnetic impurities for the quasi-1D Heisenberg ferromagnet (p-CDpOV) and antiferromagnet (p-BDpOV) with S = 1/2, which have similar van der Waals radius to each other. The susceptibility curves systematically change with impurity concentration x. The most noticeable point is that the impurity effect is rather mild on the 1D ferromagnetic Heisenberg system. The characteristic feature of 1D ferromagnet still survives even in the heavily doped sample with x = 0.50. The positive Weiss constant (theta) and the ferromagnetic intrachain exchange interaction (2J(intrachain)) remain constant between x = 0 and 0.50. The results indicate that the p-BDpOV radical functions as an ideal dopant for the 1D ferromagnet; p-CDpOV. This is the first report of the effects of magnetic impurities on the quasi-1D ferromagnetic chain system.
  • K Ohara, N Hirota, DM Martino, H van Willigen
    JOURNAL OF PHYSICAL CHEMISTRY A 102 28 5433 - 5439 1998年07月 [査読有り]
     研究論文(学術雑誌) 
    CIDEP spectra from free radicals produced by the photolysis of xanthone (Xn) in 2-propanol were investigated with FT-EPR. The spectra were assigned to the 2-hydroxypropan-2-yl (2HP) and xanthone ketyl (XnH) radicals. In pure 2-propanol and 2-propanol containing 10% H2O, the spectra display low field emission/ high field absorption with net emission (E*/A) type polarization. The observed CIDEP pattern is mainly due to the S-To radical pair mechanism (RPM) with minor contributions from the triplet mechanism (TM) and radical triplet pair mechanism (RPM). Upon addition of HCl, the polarization changes to net absorption. The rise time of the absorptive signals is determined by the response time of the spectrometer (similar to 3 x 10(-8) s). Transient optical absorption measurements show that the triplet state of xanthone ((3)Xn*) is quenched by HCl, and the change in spin polarization produced by HCl addition is attributed to this quenching process. The dependence of the CIDEP pattern and triplet xanthone lifetime on HCl concentration shows that both involve a diffusion-controlled process. The main 3Xn(*) quenching process was found to be nonreactive, but it is proposed that its spin selectivity produces spin polarization in the products of the hydrogen abstraction reaction that runs in parallel with this quenching process.
  • 長岡 伸一, 西奥 義憲, 小原 敬士, 向井 和男
    日本結晶学会日本結晶学会誌 40 1 119-123,133-133 - 123 The Crystallographic Society of Japan 1998年02月 
    Since a vitamin E radical produced by the antioxidant reaction of vitamin E causes damage to vital functions, ubiquinol and vitamin C regenerate a vitamin E molecule from a vitamin E radical <I>in vivo</I>. The kinetic studies of the regeneration reactions have been carried out in solutions by means of stopped-flow spectroscopy. Substantial deuterium kinetic-isotope-effects on the second-order rate constants have been observed in both the reactions of ubiquinol and vitamin C. Furthermore, a deviation from a linear relationship in the Arrhenius plot has been observed in the reaction of ubiquinol. These results suggest that the tunneling effect plays an important role in the regeneration reactions in various tissues and mitochondria.
  • Kazuo Mukai, Kentaro Suzuki, Keishi Ohara, Javad, B. Jamali, Norio Achiwa
    J. Phys. Chem. B 102 5 782 - 787 1998年 [査読有り]
     研究論文(学術雑誌) 
    Magnetic susceptibilities of 8 kinds of 3-(4-R-phenyl)-1,5-diphenyl-6-oxo- and -thioxoverdazyl radical crystals (p-MDpOV (R = OCH3), p-MeDpOV (R = CH3), p-CyDpOV (R = CN), p-NDpOV (R = NO2), p-MDpTV (R = OCH3), p-MeDpTV (R = CH3), p-CyDpTV (R = CN), and p-NDpTV (R = NO2)) were measured between 4.2 and 300 K. The susceptibilities of p-MDpOV, p-CyDpOV, and p-MDpTV can be well explained by a one-dimensional (1D) antiferromagnetic (AFM) Heisenberg linear chain model with exchange interaction of 2 J/k = −27.3, −27.3, and −10.9 K, respectively. The susceptibility of p-MeDpTV can be interpreted in terms of a 1D AFM Heisenberg alternating chain model with 2 J1/k = −92.0 K (alternation parameter α = J2/J1 = 0.7). On the other hand, the susceptibilities of p-MeDpOV, p-NDpOV, and p-CyDpTV follow the Curie−Weiss law with positive Weiss constants of +2.5, +1.5, and +2.9 K, respectively, and the exchange interactions 2 J/k were estimated to be +7.0, +5.0, and +7.0 K, assuming the one-dimensionality for these radical crystals. The susceptibility of p-NDpTV follows the Curie−Weiss law with a negative Weiss constant of −0.9 K. Ab initio molecular orbital (MO) calculation was performed for five kinds of 6-oxoverdazyl radicals to clear the origin of the intermolecular ferromagnetic exchange interaction that is observed with high probability for these verdazyl radicals. A very strong spin polarization effect has been found, which is advantageous to the intermolecular ferromagnetic interaction.
  • T Koga, K Ohara, K Kuwata, H Murai
    JOURNAL OF PHYSICAL CHEMISTRY A 101 43 8021 - 8025 1997年10月 [査読有り]
     研究論文(学術雑誌) 
    The hydrochloric-acid-addition effect on the photochemical reaction of xanthone in alcohol is studied by a time-resolved ESR technique. The CIDEP spectra of the transient radicals peculiarly reveal an enhanced absorption by the addition of hydrochloric acid, while the ordinary triplet mechanism of xanthone is known to be emissive. The spin-polarization inversion phenomenon is explained by the triplet mechanism induced by the excitation of a ground-state weak charge-transfer complex of xanthone and HCl.
  • K Ohara, Y Miura, M Terazima, N Hirota
    JOURNAL OF PHYSICAL CHEMISTRY A 101 4 605 - 611 1997年01月 [査読有り]
     研究論文(学術雑誌) 
    The magnetic field dependence of the chemically induced dynamic electron polarization (CIDEP) spectra of spin-correlated radical pairs (SCRPs) was studied by time-resolved EPR at three different external magnetic fields, i.e., X band (9.2 GHz, 330 mT), S band (3.0 GHz, 100 mT), and L band (1.5 GHz, 50 mT). The CIDEP spectra were obtained by the photolysis of three systems, xanthone and 2,6-di-tert-butylphenol (2,6-DBP) in a sodium dodecyl sulfate (SDS) micelle solution, zinc tetrakis(4-sulfonatophenyl)porphyrin (ZnTPPS) and p-benzoquinone (p-BQ) in a cetyltrimethylammonium chloride (CTAC) micelle solution, and acetone in 2-propanol, at low temperatures. In the two micelle systems, the SCRP spectra scarcely depend on the external magnetic field, and the decay times do not change much on going from the X band to the L band. These observations are discussed in terms of the cage escape rate and the spin-lattice relaxation rate. The strong magnetic field dependence found for the net emissive polarization in the CIDEP spectra of the xanthone and 2,6-DBP system is ascribed to the magnetic field dependence of the triplet mechanism (TM). On the other hand, the intensity of the SCRP spectrum in the acetone system drastically decreases with decreasing the magnetic field. This observation is rationalized on the basis of the restricted motions of the SCRPs in the 2-propanol solution at low temperatures.
  • K Ohara, N Hirota
    BULLETIN OF THE CHEMICAL SOCIETY OF JAPAN 69 6 1517 - 1526 1996年06月 [査読有り]
     研究論文(学術雑誌) 
    The Chemically Induced Dynamic Electron Polarization (CIDEP) mechanisms in the hydrogen abstraction reactions of excited azaaromatic compounds; quinoxaline, phenazine, and quinoline are investigated in detail. It is shown that several polarization mechanisms are involved and the observed CIDEP spectra vary strongly depending on various factors such as the concentration, the temperature, the delay time, and the excitation light intensity. The net emissive spin polarization is ascribed mainly to the radical-triplet pair mechanism (RTPM), which is strongly dependent on the concentrations of the radical and triplet molecules. It is shown that the triplet molecules are long-lived and the condition to produce the RTPM polarization is satisfied. The observed time evolutions of the transient EPR signals are qualitatively explained on the basis of the rate equations, which include the RTPM polarization and the relaxation.
  • K Ohara, N Hirota, CA Steren, H vanWilligen
    JOURNAL OF PHYSICAL CHEMISTRY 100 8 3070 - 3074 1996年02月 [査読有り]
     研究論文(学術雑誌) 
    The secondary quenching reactions of the 2-hydroxypropan-2-yl radical produced by photolysis of pyrazine, quinoxaline, and p-benzoquinone in 2-propanol were investigated with time-resolved EPR. The quenching rate constants are 6.6 x 10(6), 1.2 x 10(8), and similar to 10(8) M(-1) s(-1) for pyrazine, quinoxaline, and p-benzoquinone, respectively. The time profiles of the transient EPR signals of pyrazinyl and 2-hydroxypropan-2-yl radicals were measured by FT-EPR and analyzed in detail in terms of a reaction scheme involving the primary hydrogen abstraction reaction and the subsequent quenching reaction. It is found that half of the signal intensity of the pyrazinyl radical arises from polarization transfer accompanying 2-hydroxypropan-2-yl quenching by pyrazine.
  • K Ohara, M Terazima, N Hirota
    JOURNAL OF PHYSICAL CHEMISTRY 99 51 17814 - 17821 1995年12月 [査読有り]
     研究論文(学術雑誌) 
    A CIDEP study on the triplet mechanism (TM) was carried out on three systems, zinc tetraphenylporphyrin (TPP) and p-benzoquinone (p-BQ) in alcohols, pyrazine/2-propanol, and maleic anhydride/2-propanol, in three different microwave frequency regions, i.e. X-(9.5 GHz), S-(3.0 GHz), and L-(1.5 GHz) bands. The CIDEP spectra involving TM contributions drastically change depending on the microwave (MW) frequency or the magnetic field. Tn the ZnTPP and p-BQ systems the TM contribution increases with a decrease of the MW frequency, whereas in the other two systems the TM contribution increases with an increase of the MW frequency. The results can be explained by the frequency dependence of the initial spin polarization produced by the TM which is strongly influenced by the rotational correlation time, the zero field splitting, and the reaction rate of the triplet molecule. The experimental results are compared with the predictions of the Atkins-Evans theory. Reasonable agreements between the experimental results and the theoretical predictions are obtained.
  • K OHARA, JN HIROTA, CA STEREN, H VANWILLIGEN
    CHEMICAL PHYSICS LETTERS 232 1-2 169 - 175 1995年01月 [査読有り]
     研究論文(学術雑誌) 
    The results of an FT-EPR study of the CIDEP of 2-propanolyl radical produced by the photochemical reaction of acetone with triethylamine are reported. The time profiles of the signals are explained satisfactorily on the basis of a hydrogen abstraction reaction involving the acetone singlet and/or triplet excited states depending upon the amine concentration and CIDEP mechanisms involving singlet and/or triplet geminate pairs and triplet F pairs together with spin-lattice relaxation.
  • M TERAZIMA, Y MIURA, K OHARA, N HIROTA
    CHEMICAL PHYSICS LETTERS 224 1-2 95 - 100 1994年07月 [査読有り]
     研究論文(学術雑誌) 
    Chemically induced dynamic electron polarization (CIDEP) in an acetone/2-propanol system is investigated in the L band microwave frequency region at various temperatures. The polarization due to the ST- mechanism is analyzed in terms of the mixing of hyperfine-independent (ST-M-i) and hyperfine-dependent (ST-M-d) mechanisms. At higher temperature, ST-M-d dominates over ST-M-i in the ST-M polarization, while the relative contribution of ST-M-i increases with decreasing temperature. A mechanism of ST-M-d is discussed based on the radical diffusion in solution and a possible contribution of the ST+M polarization in the CIDEP spectrum is discussed.
  • K OHARA, H MURAI, K KUWATA
    BULLETIN OF THE CHEMICAL SOCIETY OF JAPAN 65 6 1672 - 1678 1992年06月 [査読有り]
     研究論文(学術雑誌) 
    A triplet photosensitization reaction of maleic anhydride with alcohols and the subsequent secondary reaction were studied by a time-resolved ESR method. The hydrogen abstraction reaction of the C=C bond site of maleic anhydride in the excited triplet state was examined and discussed as the primary step. The adduct radical formed in the secondary reaction between maleic anhydride and alcohol radicals showed a characteristic E/A-shaped hyperfine line(s). This phenomenon can be explained by the CIDEP ''memory transfer'' of the multiplet (E/A) spin polarization of the alcohol radicals to the adduct radical. The spin-lattice relaxation times of the excited triplet state of xanthone as the sensitizer in 2-propanol and cyclohexanol were also estimated.
  • T TAKEMURA, K OHARA, H MURAI, K KUWATA
    CHEMISTRY LETTERS 19 9 1635 - 1638 1990年09月 [査読有り]
     研究論文(学術雑誌)
  • K OHARA, H MURAI, K KUWATA
    BULLETIN OF THE CHEMICAL SOCIETY OF JAPAN 62 8 2435 - 2440 1989年08月 [査読有り]
     研究論文(学術雑誌)

書籍

  • ビタミンE研究の進歩XIII
    ビタミンE研究会 2009年
  • Kinetic Study of Vitamin E Regeneration Reaction with Catechins (EC, EGC, ECG, and EGCG) in Homogeneous and Micellar Solution
    Kazuo Mukai, Shuji Mitani, Souichi Nagai, Keishi Ohara (担当:分担執筆, 範囲:Proceedings of the 3rd China-Japan International Conference on Vitamins', Guilin, China, 2004)
    2004年
  • ビタミンE研究の進歩Ⅹ
    ビタミンE研究会 2002年
  • ビタミンE研究の進歩Ⅸ
    (担当:分担執筆, 範囲:時間分解ESR法によるビタミンE類の一重項酸素消光速度の測定(pp1-6), 時間分解ESR法による光酸化開始に対するビタミンE類の抗酸化機構の研究(pp7-12))
    ビタミンE研究会 2000年
  • CIDEP studies of photochemical reactions at different microwave frequencies, ST0M and ST-M.
    Keishi Ohara, Noboru Hirota (担当:分担執筆, 範囲:Modern Applications of EPR/ESR: From Biophysics to Materials Science, Proceedings of the Asia-Pacific EPR/ESR Symposium, 1st, Kowloon, Hong Kong, Jan. 20-24, 1997, p114-122)
    Springer-Verlag, Singapore 1998年

作品等

  • 生物・分子機能性化学における近赤外発光の新展開
    2004年

MISC

共同研究・競争的資金等の研究課題

  • 日本学術振興会:科学研究費補助金(基盤研究(C))
    研究期間 : 2014年04月 -2017年 
    代表者 : 小原 敬士
  • 文部科学省:科学研究費補助金(基盤研究(C))
    研究期間 : 2010年 -2013年 
    代表者 : 小原 敬士
     
    希少な天然抽出物質等の一重項酸素消去速度定数を微量の試料で計測するための近赤外発光寿命スポット計測法を開発し、微量溶液やゲル・固形試料に対しファイバープローブの0.4 mmのスポットで抗酸化評価のための計測を可能にした。0.05 mgのカロテノイド異性体やヒドロゲル試料の一重項酸素消去活性の測定に成功した。また、脂溶性香辛料成分のシクロデキストリンによる水溶液分散化による一重項酸素消去活性評価について検討し、水環境化の構造活性相関を明らかにした。一方、1測定10分を要する現状では評価対象や増感剤の計測中のダメージがスポット計測の障害であり、測定時間を大幅に短縮する必要がある。
  • 飲料・食品のための簡便・短時間で計測可能な抗酸化力評価法の開発
    科学技術振興機構(JST):研究成果最適展開支援プログラム(A-STEP)FSステージ探索タイプ
    研究期間 : 2011年08月 -2012年03月 
    代表者 : 小原 敬士
     
    飲料サンプル用の標準抗酸化力評価キットの作成と高感度・短時間で試料を測定する計測法の最適化を実施し、含有成分の抗酸化力の簡便迅速な標準評価手法を新たに開発し提案することを目標とする。
  • 文部科学省:科学研究費補助金(基盤研究(C))
    研究期間 : 2007年 -2008年 
    代表者 : 小原 敬士
     
    飲料・食品・化粧品・医薬品などで注目されている分散・懸濁系での天然抗酸化剤による活性酸素種消去ダイナミクスの詳細を明らかにするため、油水分散エマルジョン, 水油分散の逆ミセル系の油水界面で起こる抗酸化反応を過渡吸収法及び一重項酸素(1O_2)近赤外発光寿命法などの時間分解計測を用いて研究した。スーパーオキシドラジカルや1O_2 が天然抗酸化剤により油水界面近傍で消去されるダイナミクスを直接観測することに成功し、反応速度定数が求められた。
  • 文部科学省:科学研究費補助金(基盤研究(C))
    研究期間 : 2007年 -2008年 
    代表者 : 長岡 伸一, 小原 敬士
     
    励起状態における水素結合に沿ったプロトン移動は、正確な測定と定量的理論解析に適している。今回我々は、プロトン移動が重要な役割を示す生理活性物質に我々が提唱している理論モデルを適用できることを見いだした。また、さらに長波長領域の励起状態である一重項酸素消去における分子内水素結合を含む生体関連分子が示す役割を研究すると共に、逆に短波長領域である軟X 線領域にも研究のウイングを広げた。
  • 文部科学省:科学研究費補助金(特定領域研究)
    研究期間 : 2003年 -2006年 
    代表者 : 向井 和男, 田嶋 邦彦, 小原 敬士, 長岡 伸一, 美藤 正樹
     
    人の老化は活性酸素・フリーラジカルと脂質、蛋白、核酸などの分子との反応によって引き起こされる。生体内には、ビタミンE(α-,β-,γ-,δ-Tocopherol, TocH)、ユビキノール-10、ビタミンC、ポリフェノール類(フラボン類、カテキン類)などの、種々の抗酸化剤が存在し、活性酸素・フリーラジカルを速やかに消去し、老化を防いでいる。本研究においては、上記の抗酸化剤について、i)フリーラジカル消去作用、ii)ビタミンEラジカル還元・再生作用、iii)一重項酸素(^1O_2)消去作用の速度論的研究を行ない、種々の興味ある結果を得た。以下にその主な例を示す。1)Stopped-Flow分光光度計を用い、ミセル溶液中でビタミンE(α-,β-,γ-,δ-tocopherol)とユビキノールのAroxylラジカル消去速度(k_s)をpHを変えて測定した。k_sの値と種々の組織中の濃度の積の値を比較し、心臓、筋肉、臓、腎臓、脳のミトコンドリア中ではユビキノールが、血清中ではビタミンEがラジカル消去の高い活性を示すことを明らかにした。2)Endoperoxideから発生させた^1O_2と、7種のフラボン誘導体、4種のカテキン類との反応を行い、これらのポリフェノール類がビタミンEに匹敵する速い^1O_2の消去速度(k_Q)を有することを明らかにした。k_Qの値が、脂質、蛋白、核酸に比べて速いことから、ポリフェノールが植物や食物の光劣化を防御していることを明らかにした。3)ビタミンEの抗酸化反応によって生じるEラジカル(α-Toc・,β-Toc・,γ-Toc・,δ-Toc・)の生成・消滅反応の追跡を行なった。これらの反応に関与する反応式、すなわち、3元連立微分方程式の解を、4次のRunge-Kutta法を用いて求め、実測値との比較から、生成・消滅反応の速度(k_f and 2k_d)を求めることに成功した。Eラジカルの消滅は何れも2分子反応によって起こることが明らかになった。得られた結果から、ビタミンEラジカル類の生成・消滅ダイナミクス機構について詳細な検討を行った。4)Double-Mixing Stopped-Flow分光光度計を用い、生体中における老化防御のKey反応として知られる、ビタミンCによるビタミンEラジカル(α-Toc・,β-Toc・,γ-Toc・)の再生反応速度(尾)を求めることに初めて成功した。k_rに及ぼす重水素置換効果を測定し、このような速い反応にはトンネル効果が寄与しないことを明らかにした。
  • 文部科学省:科学研究費補助金(基盤研究(C))
    研究期間 : 2004年04月 -2005年 
    代表者 : 小原 敬士
     
    本研究は、生体系等の光学的透明性が低い懸濁液で、複数の天然抗酸化剤が含まれる多成分系が示す活性酸素種に対する抗酸化過程の挙動を、蛍光またはESRの時間分解測定により速度論的に研究するための手法確立を目的とする。平成16-17年度の研究実績は以下の通りである。1 蛍光検出ストップトフロー測定装置により、懸濁溶液系での抗酸化反応速度の時間分解法による定量的な測定に初めて成功した。条件検討の結果、モデルフリーラジカル還元体の蛍光増加、或いはビタミンE(VE)等の抗酸化剤の蛍光減少により抗酸化反応過程が追跡可能であることがわかり、均一溶液系で従来法と遜色ない測定結果を得た。この方法で、エタノール・水混合溶媒にVEを分散したエマルジョン溶液で抗酸化反応速度を測定し、その速度定数が均一領域と大きく異なることを見いだした。さらにDPPHやGalvinoxyl等のラジカルに対するエマルジョン系の抗酸化反応速度測定に成功し、この方法が懸濁系での抗酸化反応において汎用的であることを示した。2 1275mmの^1O_2近赤外発光の直接測定により、VEエマルジョン溶液における^1O_2消去ダイナミクスを検討した。均一溶液でVEは10^7-10^8M^<-1>s^<-1>程度の^1O_2消去速度を示すが、エマルジョン系ではVE濃度の増大とともに^1O_2寿命が急激に増大した。この結果、VEエマルジョン系では^1O_2が油滴中にとけ込み、^1O_2ダイナミクスが主にVE油滴近傍で起こっていることが示された。3 天然VEやH_2O_2水溶液の266nm光による直接光分解で生成する中間体ラジカルの生成・減衰ダイナミクスを時間分解ESRにより検討中である。本研究の成果により、エマルジョン系でのフリーラジカル・^1O_2に対する天然抗酸化剤の抗酸化反応過程が時間分解法で定量的に研究可能となり、その均一系と異なる挙動を解明する糸口が得られた。
  • 文部科学省:科学研究費補助金(基盤研究(C))
    研究期間 : 2003年 -2004年 
    代表者 : 向井 和男, 小原 敬士
     
    近年、有機強磁性体や有機超伝導体に関する研究が多くの注目を集め、前者では20種、後者では80種に及ぶ例がこれまで見いだされている。しかしながら、強磁性と超伝導性の両機能を併せ持つ分子性強磁性超伝導体はこれまで見出されていない。本研究においては研究代表者の向井がこれまで詳細な磁性研究を行なって来た、安定フェルダジルラジカル(1,3,5-Triphenyl-verdazyl,1,5-diphenyl-3-phenyl-6-oxoverdazylなど)をスピン源とし、導電性分子(TCNQ、Me_2TCNQ、Ni(dmit)_2^-、Pd(dmit)_2^<2->など)との組み合わせによる分子性錯体を合成し、伝導性・磁性の複合機能を有する新しい分子性超伝導・磁性体、特に、初めての純有機強磁性・超伝導体の開発を目指した研究を行なった。本研究では、1.まず、これらのフェルダジルラジカルに正電荷を持った置換基を導入した分子(3-(4- and 3-Methylpyridinium)-1,5-diphenylverdazyl(p- and m-MePyDV))など、多くの分子の合成を試みた。2.次に、Verdazylカチオンラジカルと電子受容体アニオン([TCNQ]^-、[Me_2TCNQ]^-、Ni(dmit)_2^-、Pd(dmit)_2^<2->など)との反応により、先ず、(1:1)錯体を、更に、その酸化などにより(1:2)、(1:3)錯体結晶([p-MePyDV]^+[TCNQ]_2^-、[p-MePyDV]^+[Me_2TCNQ]_2^-、[1]^+[Ni(dmit)_2]_3^-、[1]^+[Pd(dmit)_2]_3^-、など)の合成・単離に成功した。3.合成した分子性錯体の、磁化率、ESR、電気伝導度の測定、結晶構造解析を行ない、10種に近い新しい純有機常磁性・半導体と分子性常磁性・半導体を得る事に成功した。強磁性・超伝導体を得るには至らなかったが、このような磁性と伝導性の両機能を同時に有する分子性化合物の例は大変限られて居り、本研究で得られた結果の意義は大きい。
  • 文部科学省:科学研究費補助金(特定領域研究(B))
    研究期間 : 1999年 -2001年 
    代表者 : 向井 和男, 小原 敬士, 東 長雄
     
    純有機反強磁性超伝導体或いは純有機強磁性伝導体を得るために、次のような種々の合成と、測定を試みた。1)分子内に正の電荷を有する種々の新規フェルダジルラジカルを合成し、TCNQ、Me_2TCNQ、Cl_2TCNQ、DDQ等の電子受容体との(1:1)及び(1:2)塩を作成した。その磁化率、ESR、伝導度の測定と、結晶構造解析結果から、4種の(1:2)TCNQ塩が純有機磁気半導体であることを見出した。低温での比熱測定を行ない、反強磁性への転移を見出すことを試みた。しかしながら、TCNQ分子間のスピン間反強磁性相互作用が大きいため、低温でその磁性が消えてしまい、反強磁性転移を見出すに至らなかった。現在、分子間の相互作用を小さくするため、Me_2TCNQやCl_2TCNQとの塩について詳細な研究を進めている。2)1)と同様、分子内に正の電荷を有するフェルダジルラジカルとNi(dmit)_2等の塩を合成した。(1:3)塩は何れも磁気半導体としての性質を示した。今後、加圧下での伝導度の測定を行い、超伝導への転移を見出すことを試みる。3)分子内に負の電荷を有する種々の新規フェルダジルラジカルを合成し、TTF、BEDT-TTF、BETS、等の電子供与体との塩を作成し、磁気伝導体の開発を試みている。現在までに、カルボキシル基を有する数種のラジカルの合成に成功している。
  • 文部科学省:科学研究費補助金(基盤研究(B))
    研究期間 : 1999年 -2000年 
    代表者 : 向井 和男, 小原 敬士, 長岡 伸一
     
    天然の抗酸化剤として良く知られている、カテキン類、コーヒー酸類について、その活性酸素・フリーラジカル消去反応速度を、1)酸素吸収法、2)ストップトフロー分光光度計法、3)時間分解ESR法を用いて測定し、抗酸化活性の評価を行った。また、その結果に基づき、抗酸化の機構について検討した。1)Stopped-Flow分光光度計を用いて、有機溶媒中及びミセル溶液中でカテキン類とコーヒー酸類のビタミンE再生反応速度とフェノキシルラジカル消去反応速度を測定した。また、この結果を過酸化ラジカル消去反応速度と比較検討した。更に、pHを変えて反応速度の測定を行い、その解析結果から反応の構造活性相関を明らかにした。また、サイクリックボルタンメトリー法を用いて酸化電位を測定し、ラジカル消去速度との相関性を見出した。尚、本研究は主に愛媛大学で行った。2)圧力Transducer法(酸素吸収法)により、有機溶媒中及びミセル溶液中でカテキン類とコーヒー酸類の脂質過酸化ラジカル消去速度を測定し、これらの生体関連天然抗酸化剤の抗酸化活性について定量的な評価を行うと共に、その結果をビタミンEやユビキノールと比較検討した。尚、本研究は、研究代表者の向井の研究室の大学院生3名がカナダのマウントアリソン大学のBarclay教授の研究室にそれぞれ4ヶ月間滞在し、共同研究により行った。3)ビタミンEの抗酸化反応、再生反応、酸化促進反応に対する重水素置換効果を測定した。その結果から、これらの反応は、何れも水素のトンネル効果が重要な役割を果たしていることを明らかにした。
  • 文部科学省:科学研究費補助金(奨励研究(A))
    研究期間 : 1999年 -2000年 
    代表者 : 小原 敬士
     
    本研究は、ミセル・脂質二分子膜に導入されたCIDEPスピンプローブを用いて局所場での一重項酸素の寿命を時間分解ESRにより測定することにより、その動態を検討することを目的とする。本年度は本課題を実施するにあたり、以下のことを行った。(1)CIDEPスピンプローブとなるアルキル鎖及びペプチド鎖を持ったニトロキシドラジカル4種を設計・合成し、これがミセル中に取り込まれることを蛍光法及びESR法により確認した。3種の表面電荷の異なるミセル系において、油溶性のキサントン及び水溶性のTPPS(ポルフィリン誘導体)を用い、ミセル内外に存在するこれらの分子の励起状態とスピンプローブのミセルの水・油界面での相互作用について、時間分解ESR法により検討した。その結果、アルキル鎖によりミセルの疎水性部分に保持されたプローブは水相の励起分子との相互作用が小さく、アルキル鎖のないラジカル単体では比較的強い相互作用が観測されたことから、アルキル鎖のないラジカルがミセル内から水相に拡散している様子が示された。これらの結果から、合成した分子は不均一系のプローブとして有効に機能することがわかった。現在、脂質二分子膜モデルとなるフォスファチジルコリンを用いた系で検討を行っている。(2)水相で発生させた一重項酸素とプローブの相互作用について検討したが、一重項酸素の低収率と感度不足のため充分な信号強度が得られなかった。また、局所的に一重項酸素を発生させる試みとして、光により励起状態となる分子を持つミセル・ペプチド分子の合成を試みたが、中間体の副反応や低収率のため現在まで合成に至っていない。現在も引き続きこれらの検討を行っている。(3)ミセル系でのビタミンE及びビタミンCの挙動を励起分子としてキサントンを用いた系で検討し、分子の可溶化位置と反応速度のミセル,pHの依存性を時間分解ESR法で検討した。得られた顕著なミセル、pH依存性から、これらの抗酸化剤の界面での挙動が明かとなった。スピンプローブとこれらの抗酸化剤をミセル系に導入した系で時間分解ESRを測定することにより、界面での抗酸化反応機構について検討中である。
  • 文部科学省:科学研究費補助金(基盤研究(C))
    研究期間 : 1997年 -1998年 
    代表者 : 長岡 伸一, 小原 敬士, 向井 和男
     
    プロトン移動は最も単純な化学反応であり、正確な測定と定量的理論解析に適している。研究代表者は既に、波動関数の節を考慮すれば励起状態でプロトン移動が起こるかどうかを予測できることを示唆した。本研究においては、励起状態において分子のどこに節が生じるか、我々の節の考えがプロトン移動における置換基効果、溶媒効果、励起状態依存性を説明できるかどうか、及び生体系におけるプロトン移動反応に適用できるかどうかを検討した。具体的には、サリチルアルデヒド誘導体やアミノアントラキノン誘導体では、プロトン移動反応の電子状態依存性及び分子構造や置換基がプロトン移動に与える影響について研究した。また、ヒドロキシアンスリルベンゾチアゾール誘導体では、同じ官能基から構成される分子でも置換基の相対的な位置によってプロトン移動反応の性質が異なることを明らかにした。こうした研究結果は全て、我々の節の考えを支持している。さらに、ビタミンEとキノン類が結合した分子を合成し、生体系において光照射によって生じる有害な活性分子をビタミンEがどのようにして消去していくかを検討した。以上のような研究は、微視的な電子の動きが結果として現れる反応にどのように影響するかを理解する上で興味深い。プロトン移動は、多くの研究者によって研究されているが、節の考えに基づいて波動関数の性質とプロトン移動反応を直接結びつけようとしているのは我々のグループのみである。
  • 文部科学省:科学研究費補助金(基盤研究(B))
    研究期間 : 1996年 -1998年 
    代表者 : 向井 和男, 小原 敬士
     
    30種を超える新規Verdazylラジカルやその電荷移動錯体を合成し、SQUID、ESR、X線、比熱などの測定を行い、以下に示すような興味ある結果を得た。1) Verdazylラジカル結晶p-DaTPOVがT_c=0.28KのCurie温度を持つ準一次元有機強磁性体であることを見出した。2) 弱強磁性体TOVラジカル結晶を非磁性還元体(TOV-H)で希釈した混晶(TOV)_<1-x>(TOV-H)_xを作り、その磁性の研究を行った。TOVにおける反強磁性的交換相互作用から、混晶x=0.03,0.05,0,09においては強磁性的交換相互作用が観測された。X線解析結果から、上記の磁性変化の原因を明らかにした。3) スピンパイエルス転移化合物p-CyDOVについて、強磁場下における磁化測定を行い、p-CyDOVの磁気相図の完成に成功した。純有機中性ラジカル磁性体においては初めての例である。4) 上記p-CyDOVラジカル結晶について、スピンパイエルス転移に対する非磁性および磁性不純物効果の研究を行い、磁気相図の作成や転移点(T_c)以下の磁気的性質について検討を行った。5) Verdazylラジカルと電子受容体(TCNQ、TCNQF_4など)の分子性錯体を合成し、その磁性の研究を行った。6) 一次元強磁性体と一次元反強磁性体のラジカル合金に関する研究を行い、ラジカル置換による磁性の変化に対する詳細な検討を行った。
  • 文部科学省:科学研究費補助金(国際学術研究)
    研究期間 : 1996年 -1996年 
    代表者 : 広田 襄, 小原 敬士, 中川 公一, 寺嶋 正秀, WILLIGEN Han, WILLIGEN, Hans Van
     
    1996年8月にノボシビルスクにおいて開催された、スピンおよび磁場効果に関する国際シンポジウムにおいて、広田、van Willigenおよび小原はその後の共同研究の計画に関して詳細な打ち合わせを行った。その結果、これまでのCW-ESRの研究において、未解決の問題の多いXanthoneの光反応の初期過程を、FT-ESRを用いて研究することに決め、まず小原がボストンで実験を行い、以下の成果を得た。1)Xanthone/2-propanol系でEの分極の生成は水素引き抜き反応の速度により決まり、主な分極の機構はラジカル対機構(RPM)である。2)Xanthone/2-propanol+H_2O+HC1で観測される吸収(A)の分極の立ち上がは非常に速く、装置の分解能により決まっており、三重項機構(TM)によると考えられるが、分極の強度は弱い。さらに、11月に広田がボストンにおいて共同でFT-ESRの実験を行い、上記2)の系についてHC1の濃度を変化させて詳しい研究を行った。その結果HC1の低濃度ですでに2-propanol由来のラジカルに関しては、EからAへの分極の反転が起こることが明らかになり、さらにXanthoneラジカルにおいても、AからEへの分極の反転が起こるなど新しい現象が見出された。これらの新しく得られた結果を含めて、Xanthoneの反応初期過程を明らかにするには、過渡吸収による補足的な実験の必要性が痛感され、1月にvan Willigen教授来日の際、京都で過渡吸収の実験を試みたが、装置上の問題で満足な結果が得られず、その後京都において実験を完成した。これらの結果をまとめて論文を作成してJ.Phys.Chem.誌に投稿した。以上の研究に加えて、中川は以前より共同研究を行っているphenothiazine(PTH)とアルキル直鎖を持つPTHの誘導体のSDSミセル溶液中の光イオン化で生じる水和電子の研究を行った。その結果、レーザー励起後50ns以内での水和電子のシグナルの位相の詳細、水和電子の消失速度とPTHのミセル内の位置との関係、水和電子のクエンチの動力学などについて知見を得た。

委員歴

  • 2004年04月 - 現在   日本分光学会   中国四国支部幹事, 代議員   日本分光学会
  • 2014年05月 - 2016年02月   日本化学会中国四国支部   愛媛地区幹事

担当経験のある科目

  • 構造化学演習愛媛大学
  • コンピューター化学愛媛大学
  • 化学入門愛媛大学
  • 化学愛媛大学
  • 化学実験愛媛大学
  • 物理化学愛媛大学
  • 分子分光学愛媛大学
  • 化学反応論愛媛大学

愛媛大学教員活動実績

教育活動(B)

担当授業科目(B01)

  • 2019, 前期, 学部, 分子分光学
  • 2019, 前期, 学部, 卒業研究Ⅰ
  • 2019, 前期, 学部, 化学Ⅰ
  • 2019, 前期, 学部, 化学実験Ⅱ
  • 2019, 前期, 学部, 化学実験Ⅲ
  • 2019, 前期, 修士, 化学物質管理の基礎知識
  • 2019, 前期, 修士, 化学物質管理の基礎知識
  • 2019, 前期, 博士, 物質機能科学特論Ⅲ
  • 2019, 前期, 学部, 化学実験Ⅱ
  • 2019, 前期, 学部, 化学実験Ⅲ


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