Faculty Profiles - SHEN Jian-Ren

Papers - SHEN Jian-Ren

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Structural basis for energy transfer pathways in the plant PSI-LHCI supercomplex Reviewed International journal

Xiaochun Qin, Michihiro Suga, Tingyun Kuang, Jian-Ren Shen

SCIENCE 348 ( 6238 ) 989 - 995 2015.5

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER ASSOC ADVANCEMENT SCIENCE

Photosynthesis converts solar energy to chemical energy by means of two large pigment-protein complexes: photosystem I (PSI) and photosystem II (PSII). In higher plants, the PSI core is surrounded by a large light-harvesting complex I (LHCI) that captures sunlight and transfers the excitation energy to the core with extremely high efficiency. We report the structure of PSI-LHCI, a 600-kilodalton membrane protein supercomplex, from Pisum sativum (pea) at a resolution of 2.8 angstroms. The structure reveals the detailed arrangement of pigments and other cofactors-especially within LHCI-as well as numerous specific interactions between the PSI core and LHCI. These results provide a firm structural basis for our understanding on the energy transfer and photoprotection mechanisms within the PSI-LHCI supercomplex.

DOI： 10.1126/science.aab0214

A synthetic Mn4Ca-cluster mimicking the oxygen-evolving center of photosynthesis Reviewed International journal

Chunxi Zhang, Changhui Chen, Hongxing Dong, Jian-Ren Shen, Holger Dau, Jingquan Zhao

SCIENCE 348 ( 6235 ) 690 - 693 2015.5

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER ASSOC ADVANCEMENT SCIENCE

Photosynthetic splitting of water into oxygen by plants, algae, and cyanobacteria is catalyzed by the oxygen-evolving center (OEC). Synthetic mimics of the OEC, which is composed of an asymmetric manganese-calcium-oxygen cluster bound to protein groups, may promote insight into the structural and chemical determinants of biological water oxidation and lead to development of superior catalysts for artificial photosynthesis. We synthesized a Mn4Ca-cluster similar to the native OEC in both the metal-oxygen core and the binding protein groups. Like the native OEC, the synthetic cluster can undergo four redox transitions and shows two magnetic resonance signals assignable to redox and structural isomerism. Comparison with previously synthesized Mn3CaO4-cubane clusters suggests that the fourth Mn ion determines redox potentials and magnetic properties of the native OEC.

DOI： 10.1126/science.aaa6550

Damage Management in Water-Oxidizing Catalysts: From Photosystem II to Nanosized Metal Oxides Reviewed

Mohammad Mahdi Najafpour, Monika Fekete, Davood Jafarian Sedigh, Eva-Mari Aro, Robert Carpentier, Julian J. Eaton-Rye, Hiroshi Nishihara, Jian-Ren Shen, Suleyman I. Allakhverdiev, Leone Spiccia

ACS CATALYSIS 5 ( 3 ) 1499 - 1512 2015.3

Language：English Publisher：AMER CHEMICAL SOC

Current energy resources largely rely on fossil fuels that are expected to be depleted in 50-200 years. On a global scale, the intensive use of this energy source has resulted in highly detrimental effects to the environment. Hydrogen production by water splitting, with sunlight as the main energy source, is a promising way to augment the production of renewable energy; however, the development of an efficient and stable water-oxidizing catalyst remains a key task before a technological breakthrough based on water splitting can be realized. A main issue hampering the development of commercially viable, non-precious-metal-based catalysts is their susceptibility to degradation. To efficiently address this major drawback, self-healing catalysts that can repair their structure without human intervention will be necessary. In this review, we focus on water oxidation by natural and artificial Mn-, Co-, and Ni-based catalysts and then discuss the self-healing properties that contribute to sustaining their catalytic activity.

DOI： 10.1021/cs5015157

Theoretical studies of the damage- free S-1 structure of the CaMn4O5 cluster in oxygen-evolving complex of photosystem II Reviewed

Mitsuo Shoji, Hiroshi Isobe, Shusuke Yamanaka, Michihiro Suga, Fusamichi Akita, Jian-Ren Shen, Kizashi Yamaguchi

CHEMICAL PHYSICS LETTERS 623 1 - 7 2015.3

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

The structure of the oxygen evolving complex (OEC) of photosystem II (PSII) was recently analyzed by crystallography at 1.95 angstrom resolution using X-ray free electron laser (XFEL), but the positions of hydrogen atoms were not determined. We have examined the XFEL structure theoretically under the assumption off our protonation cases. The spin densities obtained by the high-spin UB3LYP revealed that a partial internal reduction of the high-valent Mn ions in the CaMn4O4X(H2O)(3)Y cluster occurs for the O-(5)(=X) = O2- case, entailing its protonation (X = OH-) in the XFEL structure. (C) 2015 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.cplett.2015.01.030

Theoretical modelling of biomolecular systems I. Large-scale QM/MM calculations of hydrogen-bonding networks of the oxygen evolving complex of photosystem II Reviewed

Mitsuo Shoji, Hiroshi Isobe, Shusuke Yamanaka, Yasufumi Umena, Keisuke Kawakami, Nobuo Kamiya, Jian-Ren Shen, Takahito Nakajima, Kizashi Yamaguchi

MOLECULAR PHYSICS 113 ( 3-4 ) 359 - 384 2015.2

Language：English Publishing type：Research paper (scientific journal) Publisher：TAYLOR & FRANCIS LTD

Quantum mechanical (QM)/molecular mechanics (MM) calculations by the use of a large-scale QM model (QM Model V) have been performed to elucidate hydrogen-bonding networks and proton wires for proton release pathways (PRP) of water oxidation reaction in the oxygen evolving complex (OEC) of photosystem II (PSII). Full geometry optimisations of PRP by the QM/MM model have been carried out starting from the geometry of heavy atoms determined by the recent high-resolution X-ray diffraction (XRD) experiment of PSII refined to 1.9 angstrom resolution. Computational results by the QM/MM calculations have elucidated the hydrogen-bonding O center dot center dot center dot O(N) and O center dot center dot center dot H distances and O(N)-H center dot center dot center dot O angles in PRP, together with the Cl-O(N) and Cl center dot center dot center dot H distances and O(N)-H center dot center dot center dot Cl angles for chloride anions. The optimised hydrogen-bonding networks are well consistent with the XRD results and available experiments such as extended X-ray absorption fine structure, showing the reliability of channel structures of OEC of PSII revealed by the XRD experiment. The QM/MM computations have elucidated possible roles of chloride anions in the OEC of PSII. The QM/MM computational results have provided useful information for understanding and explanation of accumulated mutation experiments of key amino acid residues in the OEC of PSII. Implications of the present results are discussed in relation to three steps for theoretical modelling of water oxidation in the OEC of PSII and bio-inspired working hypotheses for developments of artificial water oxidation systems by use of 3d transition-metal complexes.

DOI： 10.1080/00268976.2014.960021

Biochemical and structural study of Arabidopsis hexokinase 1 Reviewed International journal

Juan Feng, Shun Zhao, Xuemin Chen, Wenda Wang, Wei Dong, Jinghua Chen, Jian-Ren Shen, Lin Liu, Tingyun Kuang

ACTA CRYSTALLOGRAPHICA SECTION D-BIOLOGICAL CRYSTALLOGRAPHY 71 ( Pt 2 ) 367 - 375 2015.2

Language：English Publishing type：Research paper (scientific journal) Publisher：WILEY-BLACKWELL

Hexokinase 1 from Arabidopsis thaliana (AtHXK1) plays a dual role in glycolysis and sugar sensing for vital metabolic and physiological processes. The uncoupling of glucose signalling from glucose metabolism was demonstrated by the analysis of two mutants (AtHXK1(G104D) and AtHXK1(S177A)) that are catalytically inactive but still functional in signalling. In this study, substrate-binding experiments indicate that the two catalytically inactive mutants have a high affinity for glucose, and an ordered substrate-binding mechanism has been observed for wild-type AtHXK1. The structure of AtHXK1 was determined both in its inactive unliganded form and in its active glucose-bound form at resolutions of 1.8 and 2.0 angstrom, respectively. These structures reveal a domain rearrangement of AtHXK1 upon glucose binding. The 2.1 angstrom resolution structure of AtHXK1(S177A) in the glucose-bound form shows similar glucose-binding interactions as the wild type. A glucose-sensing network has been proposed based on these structures. Taken together, the results provide a structural explanation for the dual functions of AtHXK1.

DOI： 10.1107/S1399004714026091

Comparison of nano-sized Mn oxides with the Mn cluster of photosystem II as catalysts for water oxidation Reviewed International journal

Mohammad Mandi Najafpour, Mohadeseh Zarei Ghobadi, Behzad Haghighi, Tatsuya Tomo, Jian-Ren Shen, Suleyman I. Allakhverdiey

BIOCHIMICA ET BIOPHYSICA ACTA-BIOENERGETICS 1847 ( 2 ) 294 - 306 2015.2

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

"Back to Nature" is a promising way to solve the problems that we face today, such as air pollution and shortage of energy supply based on conventional fossil fuels. A Mn cluster inside photosystem II catalyzes light-induced water-splitting leading to the generation of protons, electrons and oxygen in photosynthetic organisms, and has been considered as a good model for the synthesis of new artificial water-oxidizing catalysts. Herein, we surveyed the structural and functional details of this cluster and its surrounding environment. Then, we review the mechanistic findings concerning the cluster and compare this biological catalyst with nano-sized Mn oxides, which are among the best artificial Mn-based water-oxidizing catalysts. (C) 2014 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.bbabio.2014.11.006

Optical identification of the long-wavelength (700-1700 nm) electronic excitations of the native reaction centre, Mn4CaO5 cluster and cytochromes of photosystem II in plants and cyanobacteria Reviewed International journal

Jennifer Morton, Fusamichi Akita, Yoshild Nakajima, Jian-Ren Shen, Elmars Krausz

BIOCHIMICA ET BIOPHYSICA ACTA-BIOENERGETICS 1847 ( 2 ) 153 - 161 2015.2

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

Visible/UV absorption in PS II core complexes is dominated by the chl-a absorptions, which extend to similar to 700 nm. A broad 700-730 nm PS II core complex absorption in spinach has been assigned [1] to a charge transfer excitation between Chl(D1) and Chl(D2). Emission from this state, which peaks at 780 nm, has been seen [2] for both plant and cyanobacterial samples. We show that Thermosynechococcus vulcanus PS II core complexes have parallel absorbance in the 700-730 nm region and similar photochemical behaviour to that seen in spinach. This establishes the low energy charge transfer state as intrinsic to the native PS II reaction centre. High-sensitivity MCD measurements made in the 700-1700 nm region reveal additional electronic excitations at similar to 770 nm and similar to 1550 nm. The temperature and field dependence of MCD spectra establish that the system peaking near 1550 nm is a heme-to-Fe(III) charge transfer excitation. These transitions have not previously been observed for cyt b(559) or cyt C-550. The distinctive characteristics of the MCD signals seen at 770 nm allow us to assign absorption in this region to a d(z)(2) -> d(x2) (-) (y2) transition of Mn(III) in the Ca-Mn4O5 cluster of the oxygen evolving centre. Current measurements were performed in the S-1 state. Detailed analyses of this spectral region, especially in higher S states, promise to provide a new window on models of water oxidation. (C) 2014 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.bbabio.2014.11.003

Native structure of photosystem II at 1.95 angstrom resolution viewed by femtosecond X-ray pulses Reviewed International journal

Michihiro Suga, Fusamichi Akita, Kunio Hirata, Go Ueno, Hironori Murakami, Yoshiki Nakajima, Tetsuya Shimizu, Keitaro Yamashita, Masaki Yamamoto, Hideo Ago, Jian-Ren Shen

NATURE 517 ( 7532 ) 99 - U265 2015.1

Language：English Publishing type：Research paper (scientific journal) Publisher：NATURE PUBLISHING GROUP

Photosynthesis converts light energy into biologically useful chemical energy vital to life on Earth. The initial reaction of photosynthesis takes place in photosystem II (PSII), a 700-kilodalton homodimeric membrane protein complex that catalyses photo-oxidation of water into dioxygen through an S-state cycle of the oxygen evolving complex (OEC). The structure of PSII has been solved by X-ray diffraction (XRD) at 1.9 angstrom resolution, which revealed that the OEC is a Mn4CaO5-cluster coordinated by a well defined protein environment(1). However, extended X-ray absorption fine structure (EXAFS) studies showed that the manganese cations in the OEC are easily reduced by X-ray irradiation(2), and slight differences were found in the Mn-Mn distances determined by XRD1, EXAFS(3-7) and theoretical studies(8-14). Here we report a 'radiation-damage-free' structure of PSII from Thermosynechococcus vulcanus in the S-1 state at a resolution of 1.95 angstroms using femtosecond X-ray pulses of the SPring-8 angstrom compact free-electron laser (SACLA) and hundreds of large, highly isomorphous PSII crystals. Compared with the structure from XRD, the OEC in the X-ray free electron laser structure has Mn-Mn distances that are shorter by 0.1-0.2 angstroms. The valences of each manganese atom were tentatively assigned as Mn1D(III), Mn2C(IV), Mn3B(IV) and Mn4A(III), based on the average Mn-ligand distances and analysis of the Jahn-Teller axis on Mn(III). One of the oxo-bridged oxygens, O5, has significantly longer distances to Mn than do the other oxo-oxygen atoms, suggesting that O5 is a hydroxide ion instead of a normal oxygen dianion and therefore may serve as one of the substrate oxygen atoms. These findings provide a structural basis for the mechanism of oxygen evolution, and we expect that this structure will provide a blueprint for the design of artificial catalysts for water oxidation.

DOI： 10.1038/nature13991

Isolation and characterization of a PSI-LHCI super-complex and its sub-complexes from a siphonaceous marine green alga, Bryopsis Corticulans Reviewed International journal

Xiaochun Qin, Wenda Wang, Lijing Chang, Jinghua Chen, Peng Wang, Jianping Zhang, Yikun He, Tingyun Kuang, Jian-Ren Shen

PHOTOSYNTHESIS RESEARCH 123 ( 1 ) 61 - 76 2015.1

Language：English Publishing type：Research paper (scientific journal) Publisher：SPRINGER

A novel super-complex of photosystem I (PSI)-light-harvesting complex I (LHCI) was isolated from a siphonaceous marine green alga, Bryopsis corticulans. The super-complex contained 9-10 Lhca antennas as external LHCI bound to the core complex. The super-complex was further disintegrated into PSI core and LHCI sub-complexes, and analysis of the pigment compositions by high-performance liquid chromatography revealed unique characteristics of the B. corticulans PSI in that one PSI core contained around 14 a-carotenes and 1-2 e-carotenes. This is in sharp contrast to the PSI core from higher plants and most cyanobacteria where only beta-carotenes were present, and is the first report for an alpha-carotenetype PSI core complex among photosynthetic eukaryotes, suggesting a structural flexibility of the PSI core. Lhca antennas from B. corticulans contained seven kinds of carotenoids (siphonaxanthin, all-trans neoxanthin, 9'-cis neoxanthin, violaxanthin, siphonein, e-carotene, and acarotene) and showed a high carotenoid: chlorophyll ratio of around 7.5: 13. PSI-LHCI super-complex and PSI core showed fluorescence emission peaks at 716 and 718 nm at 77 K, respectively; whereas two Lhca oligomers had fluorescence peaks at 681 and 684 nm, respectively. By comparison with spinach PSI preparations, it was found that B. corticulans PSI had less red chlorophylls, most of them are present in the core complex but not in the outer light-harvesting systems. These characteristics may contribute to the fine tuning of the energy transfer network, and to acclimate to the ever-changing light conditions under which the unique green alga inhabits.

DOI： 10.1007/s11120-014-0039-z

Direct electron transfer from photosystem II to hematite in a hybrid photoelectrochemical cell Reviewed International journal

Wangyin Wang, Zhiliang Wang, Qingjun Zhu, Guangye Han, Chunmei Ding, Jun Chen, Jian-Ren Shen, Can Li

CHEMICAL COMMUNICATIONS 51 ( 95 ) 16952 - 16955 2015

Language：English Publishing type：Research paper (scientific journal) Publisher：ROYAL SOC CHEMISTRY

A hybrid photoanode integrating the cyanobacterial photosystem II (PSII) with a hematite film for water oxidation is constructed. Direct electron transfer from PSII to the excited Ti/Fe2O3 electrode occurs under light irradiation, resulting in a significant improvement of the photocurrent.

DOI： 10.1039/c5cc06900a

The Structure of Photosystem II and the Mechanism of Water Oxidation in Photosynthesis Reviewed International journal

Jian-Ren Shen

ANNUAL REVIEW OF PLANT BIOLOGY, VOL 66 66 23 - 48 2015

Language：English Publishing type：Part of collection (book) Publisher：ANNUAL REVIEWS

Oxygenic photosynthesis forms the basis of aerobic life on earth by converting light energy into biologically useful chemical energy and by splitting water to generate molecular oxygen. The water-splitting and oxygen-evolving reaction is catalyzed by photosystem II (PSII), a huge, multisubunit membrane-protein complex located in the thylakoid membranes of organisms ranging from cyanobacteria to higher plants. The structure of PSII has been analyzed at 1.9-angstrom resolution by X-ray crystallography, revealing a clear picture of the Mn4CaO5 cluster, the catalytic center for water oxidation. This article provides an overview of the overall structure of PSII followed by detailed descriptions of the specific structure of the Mn4CaO5 cluster and its surrounding protein environment. Based on the geometric organization of the Mn4CaO5 cluster revealed by the crystallographic analysis, in combination with the results of a vast number of experimental studies involving spectroscopic and other techniques as well as various theoretical studies, the article also discusses possible mechanisms for water splitting that are currently under consideration.

DOI： 10.1146/annurev-arplant-050312-120129

Durability of oxygen evolution of photosystem II incorporated into lipid bilayers Reviewed

Tomoyasu Noji, Masaharu Kondo, Keisuke Kawakami, Jian-Ren Shen, Mamoru Nango, Takehisa Dewa

RESEARCH ON CHEMICAL INTERMEDIATES 40 ( 9 ) 3231 - 3241 2014.11

Language：English Publishing type：Research paper (scientific journal) Publisher：SPRINGER

Photosystem II (PSII) has attracted a lot of attention for use in the construction of artificial photosynthetic materials due to its high activity of oxidation of water molecules. However, the robustness of PSII needs to be improved for in vitro application. In this study, we incorporated PSII (Thermosynechococcus vulcanus) into various phospholipid membranes to examine the activity and durability of oxygen evolution. PSII was incorporated into anionic 1,2-dioleoyl-sn-glycero-3-phospho-(1'-rac-glycerol) (PSII-DOPG), zwitterionic 1,2-dioleoyl-sn-glycero-3-phosphocholine (PSII-DOPC), and cationic 1,2-dioleoyl-sn-glycero-3-ethylphosphocholine (PSII-EDOPC). Structural integrity of PSII was examined by absorption and fluorescence spectroscopy. Compared with PSII dissolved in a micellar solution of n-dodecyl-beta-d-maltoside (PSII-micelle), durability of PSII-DOPC and PSII-DOPG were enhanced by 1.3- and 1.5-fold, respectively. The activity and durability of PSII-EDOPC was significantly low. Lipid-dependent activity and durability were discussed in terms of kinetic parameters of V (max) and K (m), and inhibition of the electron acceptor, phenyl-p-benzoquinone.

DOI： 10.1007/s11164-014-1829-9

Water exchange in manganese-based water-oxidizing catalysts in photosynthetic systems: From the water-oxidizing complex in photosystem II to nano-sized manganese oxides Reviewed International journal

Mohammad Mahdi Najafpour, Mohsen Abbasi Isaloo, Julian J. Eaton-Rye, Tatsuya Tomo, Hiroshi Nishihara, Kimiyuki Satoh, Robert Carpentier, Jian-Ren Shen, Suleyman I. Allakhverdiev

BIOCHIMICA ET BIOPHYSICA ACTA-BIOENERGETICS 1837 ( 9 ) 1395 - 1410 2014.9

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER

The water-oxidizing complex (WOC), also known as the oxygen-evolving complex (OEC), of photosystem II in oxygenic photosynthetic organisms efficiently catalyzes water oxidation. It is, therefore, responsible for the presence of oxygen in the Earth's atmosphere. The WOC is a manganese-calcium (Mn4CaO5(H2O)(4)) cluster housed in a protein complex. In this review, we focus on water exchange chemistry of metal hydrates and discuss the mechanisms and factors affecting this chemical process. Further, water exchange rates for both the biological cofactor and synthetic manganese water splitting are discussed. The importance of fully unveiling the water exchange mechanism to understand the chemistry of water oxidation is also emphasized here. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: Keys to Produce Clean Energy. (C) 2014 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.bbabio.2014.03.008

Photosynthesis research for sustainability: Keys to produce clean energy Reviewed International journal

Suleyman I. Allakhverdiev, Jian-Ren Shen

BIOCHIMICA ET BIOPHYSICA ACTA-BIOENERGETICS 1837 ( 9 ) 1377 - 1383 2014.9

Two new C-type cytochromes found in cyanobacteria: Tll0287 and PsbV2 Reviewed

Thanh-Lan Lai, Miwa Sugiura, Michihiro Suga, Jian-Ren Shen, Alain Boussac

JOURNAL OF BIOLOGICAL INORGANIC CHEMISTRY 19 S838 - S838 2014.8

A nano-sized manganese oxide in a protein matrix as a natural water-oxidizing site Reviewed International journal

Mohammad Mandi Najafpour, Mohadeseh Zarei Ghobadi, Behzad Haghighi, Tatsuya Tomo, Robert Carpentier, Jian-Ren Shen, Suleyman I. Allakhverdiev

PLANT PHYSIOLOGY AND BIOCHEMISTRY 81 3 - 15 2014.8

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER FRANCE-EDITIONS SCIENTIFIQUES MEDICALES ELSEVIER

The purpose of this review is to present recent advances in the structural and functional studies of water-oxidizing center of Photosystem II and its surrounding protein matrix in order to synthesize artificial catalysts for production of clean and efficient hydrogen fuel. (C) 2014 Elsevier Masson SAS. All rights reserved.

DOI： 10.1016/j.plaphy.2014.01.020

Unraveling the Molecular Dynamics of Thylakoids Under Light Stress Reviewed

Yasusi Yamamoto, Jian-Ren Shen, Yuichiro Takahashi

PLANT AND CELL PHYSIOLOGY 55 ( 7 ) 1203 - 1205 2014.7

Determination of damage-free crystal structure of an X-ray-sensitive protein using an XFEL Reviewed International journal

Kunio Hirata, Kyoko Shinzawa-Itoh, Naomine Yano, Shuhei Takemura, Koji Kato, Miki Hatanaka, Kazumasa Muramoto, Takako Kawahara, Tomitake Tsukihara, Eiki Yamashita, Kensuke Tono, Go Ueno, Takaaki Hikima, Hironori Murakami, Yuichi Inubushi, Makina Yabashi, Tetsuya Ishikawa, Masaki Yamamoto, Takashi Ogura, Hiroshi Sugimoto, Jian-Ren Shen, Shinya Yoshikawa, Hideo Ago

NATURE METHODS 11 ( 7 ) 734 - U174 2014.7

Language：English Publishing type：Research paper (scientific journal) Publisher：NATURE PUBLISHING GROUP

We report a method of femtosecond crystallography for solving radiation damage-free crystal structures of large proteins at sub-angstrom spatial resolution, using a large single crystal and the femtosecond pulses of an X-ray free-electron laser (XFEL). We demonstrated the performance of the method by determining a 1.9-angstrom radiation damage-free structure of bovine cytochrome c oxidase, a large (420-kDa), highly radiation-sensitive membrane protein.

DOI： 10.1038/NMETH.2962

Water Oxidation Chemistry of a Synthetic Dinuclear Ruthenium Complex Containing Redox-Active Quinone Ligands Reviewed International journal

Hiroshi Isobe, Koji Tanaka, Jian-Ren Shen, Kizashi Yamaguchi

INORGANIC CHEMISTRY 53 ( 8 ) 3973 - 3984 2014.4

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER CHEMICAL SOC

We investigated theoretically the catalytic mechanism of electrochemical water oxidation in aqueous solution by a dinuclear ruthenium complex containing redox-active quinone ligands, [Ru-2(X)(Y)(3,6-tBu(2)Q)(2)(btpyan)](m+) [X, Y = H2O, OH, O, O-2; 3,6-tBu(2)Q = 3,6-di-tert-butyl-1,2-benzoquinone; btpyan = 1,8-bis(2,2':6',2"-terpyrid-4'-yl)anthracene] (m = 2, 3, 4) (1). The reaction involves a series of electron and proton transfers to achieve redox leveling, with intervening chemical transformations in a mesh scheme, and the entire molecular structure and motion of the catalyst 1 work together to drive the catalytic cycle for water oxidation. Two substrate water molecules can bind to 1 with simultaneous loss of one or two proton(s), which allows pH-dependent variability in the proportion of substrate-bound structures and following pathways for oxidative activation of the aqua/hydroxo ligands at low thermodynamic and kinetic costs. The resulting bis-oxo intermediates then undergo endothermic O-O radical coupling between two Ru(III)-O-center dot units in an anti-coplanar conformation leading to bridged mu-peroxo or mu-superoxo intermediates. The mu-superoxo species can liberate oxygen with the necessity for the preceding binding of a water molecule, which is possible only after four-electron oxidation is completed. The magnitude of catalytic current would be limited by the inherent sluggishness of the hinge-like bending motion of the bridged mu-superoxo complex that opens up the compact, hydrophobic active site of the catalyst and thereby allows water entry under dynamic conditions. On the basis of a newly proposed mechanism, we rationalize the experimentally observed behavior of electrode kinetics with respect to potential and discuss what causes a high overpotential for water oxidation by 1.

DOI： 10.1021/ic402340d

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