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RISS 인기검색어
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- 저자 : Nagata Toshi (검색결과 4 건)
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Yamazaki, Hirosato,Nagata, Toshi,Yagi, Masayuki Korean Society of Photoscience 2009 Photochemical & photobiological sciences Vol.8 No.2
It was earlier reported that $[(OH_2)(terpy)Mn({\mu}-O)_2Mn(terpy)(OH_2)]^{3+}$ (terpy = 2,2':6',2"-terpyridine) (1) adsorbed on layer compounds catalyzes water oxidation to $O_2$ (J. Am. Chem. Soc., 2004, 126, 8084). The derivative with 4'-chloro-2,2':6',2"-terpyridine (Clterpy), $[(OH_2)(Clterpy)Mn({\mu}-O)_2Mn(Clterpy)(OH_2)](NO_3)_3\;(2(NO_3)_3)$ was synthesized and characterized by UV-visible absorption spectroscopic and magnetic susceptibility measurements. 2 is instable in aqueous solution at room temperature, but the stability of 2 in solution significantly increased at $5^{\circ}C$. The reaction of a 2-mica adsorbate with $Ce^{IV}$ in water produced a significant amount of $O_2$, although the reaction of 2 with $Ce^{IV}$ in a homogenous solution did not. However, the maximum turnover number (TN = 0.52) of 2 on the mica adsorbate was less than unity, indicating the non-catalytic $O_2$ evolution by 2 on mica in contrast to the cooperative catalysis by 1 on mica with TN = 15. The kinetic analysis showed that $O_2$ evolution follows first order kinetics with respect to 2 adsorbed on mica, with the first-order rate constant given to be $6.8{\times}10^{-5}\;s{-1}$. The first order kinetics can be explained by $O_2$ evolution involved in the unimolecular decomposition of 2 adsorbed on mica, which might be ascribed to the destabilized higher oxidation state of 2 due to the electron-withdrawing chloro-substitution.
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Photooxidation of alcohols by a porphyrin/quinone/TEMPO system
Nagasawa, Takayuki,Allakhverdiev, Suleyman I.,Kimura, Yoshifumi,Nagata, Toshi Korean Society of Photoscience 2009 Photochemical & photobiological sciences Vol.8 No.2
Photooxidation of alcohols to the corresponding aldehydes with a porphyrin/quinone/TEMPO (TEMPO = 2,2,6,6-tetramethyl-1-piperidinyloxy free radical) system is described. This photoreaction is a combination of a photoinduced electron transfer from the porphyrin to the quinone and a TEMPO-catalyzed oxidation of alcohols triggered by one electron oxidation. The rates of oxidation were in the order of benzylic $\approx$ allylic > primary $\gg$ secondary, which is consistent with the intermediacy of the oxoammonium cation derived from TEMPO. Examination of the initial rates suggested that the reaction proceeded via the triplet excited state of the zinc porphyrin. The dependence of initial rates on the oxidation potentials of the porphyrin showed a characteristic bell shape, which is caused by two competitive factors, the efficiency of photoinduced electron transfer and the equilibrium of electron exchange between the porphyrin cation radical and TEMPO. The potential significance of this reaction in photosynthetic model chemistry is briefly discussed.
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Kurashov, Vasily N.,Allakhverdiev, Suleyman I.,Zharmukhamedov, Sergey K.,Nagata, Toshi,Klimov, Vyacheslav V.,Semenov, Alexey Yu.,Mamedov, Mahir D. Korean Society of Photoscience 2009 Photochemical & photobiological sciences Vol.8 No.2
An electrometric technique was used to investigate the generation of a photovoltage ($\Delta\psi$) by Mn-depleted spinach photosystem II (PS II) core particles incorporated into liposomes. In the presence of $MnCl_2$, the fast kinetically unresolvable phase of $\Delta\psi$ generation, related to electron transfer between the redox-active tyrosine $Y_Z$ and the primary plastoquinone acceptor $Q_A$ was followed by an additional electrogenic phase (${\tau}\;{\sim}\;20\;{\mu}s$, ~5% of the phase attributed to ${Y_Z}^{OX}{Q_A}^-$). The latter phase was ascribed to the transfer of an electron from the Mn, bound to the Mn-binding site of the PS II reaction center to the ${Y_Z}^{OX}$. An additional electrogenicity observed upon addition of synthetic trinuclear Mn complex-1 has a ${\tau}\;{\sim}\;50\;{\mu}s$ (~4% of the ${Y_Z}^{OX}Q_A$) and ${\tau}\;{\sim}\;160\;ms$ (~25%). The fast electrogenic component could be ascribed to reduction of ${Y_Z}^{OX}$ ox by Mn, delivered to the Mn-binding site in Mn-depleted samples after the release of the tripod ligands from the complex-1 while the slow electrogenic phase to the electron transfer from theMn-containing complex-1 attached to the protein-water boundary to the oxidized Mn at the protein-embedded Mn-binding site.
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Hydrogen photoproduction by use of photosynthetic organisms and biomimetic systems
Allakhverdiev, Suleyman I.,Kreslavski, Vladimir D.,Thavasi, Velmurugan,Zharmukhamedov, Sergei K.,Klimov, Vyacheslav V.,Nagata, Toshi,Nishiharad, Hiroshi,Ramakrishna, Seeram Korean Society of Photoscience 2009 Photochemical & photobiological sciences Vol.8 No.2
Hydrogen can be important clean fuel for future. Among different technologies for hydrogen production, oxygenic natural and artificial photosyntheses using direct photochemistry in synthetic complexes have a great potential to produce hydrogen, since both use clean and cheap sources: water and solar energy. Artificial photosynthesis is one way to produce hydrogen from water using sunlight by employing biomimetic complexes. However, splitting of water into protons and oxygen is energetically demanding and chemically difficult. In oxygenic photosynthetic microorganisms such as algae and cyanobacteria, water is split into electrons and protons, which during primary photosynthetic process are redirected by photosynthetic electron transport chain, and ferredoxin, to the hydrogen-producing enzymes hydrogenase or nitrogenase. By these enzymes, $e^-$ and $H^+$ recombine and form gaseous hydrogen. Biohydrogen activity of hydrogenase can be very high but it is extremely sensitive to photosynthetic $O_2$. In contrast, nitrogenase is insensitive to $O_2$, but has lower activity. At the moment, the efficiency of biohydrogen production is low. However, theoretical expectations suggest that the rates of photon conversion efficiency for $H_2$ bioproduction can be high enough (>10%). Our review examines the main pathways of $H_2$ photoproduction by using of photosynthetic organisms and biomimetic photosynthetic systems.
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