Water Oxidation In Oxygen Evolving Complex Research Articles

Development of broken-symmetry (BS) methods in chemical reactions. A theoretical view of water oxidation in photosystem II and related systems

Fundamental concepts and basic theories for the broken-symmetry (BS) methods have been reviewed in relation to theoretical elucidation and understanding of the mechanism for water oxidation in the oxygen evolving complex (OEC) of photosystem II (PSII). The HOMO-LUMO mixings by the BS method have provided the BS orbitals which are mainly localized on the metal and oxygen sites of the high-valent transition-metal oxo (MO) bonds, respectively. The oxyl-radical character (M–O) is responsible for radical reactivity such as the radical coupling in accord with various experimental results. The Lewis acids play important roles for reduction of the oxyl-radical character, indicating the participation of the water-coordinated Ca(II) ion of the CaMn3O4 cubane to stabilization of the Mn(V)=O…Ca(II) bond for essentially non-radical reactions. Several chemical indices have been calculated to elucidate the radical character for quantitative purpose. Implications of the computational results are discussed in relation to possible mechanisms of water oxidation in OEC of PSII.

Theory of chemical bonds in metalloenzymes XXIII fundamental principles for the photo-induced water oxidation in oxygen evolving complex of photosystem II

Molecular quantum mechanics (MQM) investigations have been performed for elucidation of fundamental principles of the photo-induced water oxidation in oxygen evolving complex (OEC) of photosystem II (PSII). To this end, as a first theoretical step, broken symmetry (BS) quantum mechanics (QM) and QM(BS)/molecular mechanics (MM) calculations have been conducted for elucidation of geometrical, electronic and spin structures of the CaMn4Ox (X = 5, 6) cluster in the five steps Si (i = 0∼4) of the Kok cycle for water oxidation. The QM and QM/MM calculations have provided full optimised geometries of short-lived key intermediates and transition state structure for the O-O bond formation in the native solar-energy conversion. The interplay between theory and experiment have clearly indicated that the CaMn4O5 cluster in OEC of PSII exhibits typical physicochemical properties of strong correlation electron system (SCES) confined with effective protein field. Our QM and QM/MM computational results for key intermediates and transition structure for the O-O bond formation in the Kok cycle are now plentiful for derivation of fundamental principles (FP) for understanding of photo-induced water oxidation in OEC of PSII. We summarize twenty nine fundamental principles (FPs) in systematic manner by QM and QM/MM calculations for understanding of water oxidation in Oxygen Evolving Complex(OEC) of Photosystem II(PSII). One of our final aims is theoretical design of the next-generation artificial photosystem materials composed of abundant metal ions.

Theoretical and computational investigations of geometrical, electronic and spin structures of the CaMn4 OX (X = 5, 6) cluster in the Kok cycle Si (i = 0-3) of oxygen evolving complex of photosystem II.

The optimized geometries of the CaMn4 OX (X = 5, 6) cluster in the oxygen evolving complex (OEC) of photosystem II (PSII) by large-scale quantum mechanics (QM) and molecular mechanics (MM) calculations are compared with recent serial femtosecond crystallography (SFX) results for the Si (i = 0-3) states. The valence states of four Mn ions by the QM/MM calculations are also examined in relation to the experimental results by the X-ray emission spectroscopy (XES) for the Si intermediates. Geometrical and valence structures of right-opened Mn-hydroxide, Mn-oxo and Mn-peroxide intermediates in the S3 state are investigated in detail in relation to recent SFX and XES experiments for the S3 state. Interplay between theory and experiment indicates that the Mn-oxo intermediate is a new possible candidate for the S3 state. Implications of the computational results are discussed in relation to possible mechanisms of the oxygenoxygen bond formation for water oxidation in OEC of PSII.

Theory of chemical bonds in metalloenzymes XXII: a concerted bond-switching mechanism for the oxygen–oxygen bond formation coupled with one electron transfer for water oxidation in the oxygen-evolving complex of photosystem II

ABSTRACTQM(UB3LYP)/MM(AMBER) calculations were performed for the locations of the transition structure (TS) of the oxygen–oxygen (O–O) bond formation in the S4 state of the oxygen-evolving complex (OEC) of photosystem II (PSII). The natural orbital (NO) analysis of the broken-symmetry (BS) solutions was also performed to elucidate the nature of the chemical bonds at TS on the basis of several chemical indices defined by the occupation numbers of NO. The computational results revealed a concerted bond switching (CBS) mechanism for the oxygen–oxygen bond formation coupled with the one-electron transfer (OET) for water oxidation in OEC of PSII. The orbital interaction between the σ-HOMO of the Mn(IV)4–O(5) bond and the π*-LUMO of the Mn(V)1=O(6) bond plays an important role for the concerted O–O bond formation for water oxidation in the CaMn4O6 cluster of OEC of PSII. One electron transfer (OET) from the π-HOMO of the Mn(V)1=O(6) bond to the σ*-LUMO of the Mn(IV)4–O(5) bond occurs for the formation of electron transfer diradical, where the generated anion radical [Mn(IV)4–O(5)]-• part is relaxed to the •Mn(III)4 … O(5)- structure and the cation radical [O(6)=Mn(V)1]+ • part is relaxed to the +O(6)–Mn(IV)1• structure because of the charge-spin separation for the electron-and hole-doped Mn–oxo bonds. Therefore, the local spins are responsible for the one-electron reductions of Mn(IV)4->Mn(III)4 and Mn(V)1->Mn(IV)1. On the other hand, the O(5)- and O(6)+ sites generated undergo the O–O bond formation in the CaMn4O6 cluster. The Ca(II) ion in the cubane- skeleton of the CaMn4O6 cluster assists the above orbital interactions by the lowering of the orbital energy levels of π*-LUMO of Mn(V)1=O(6) and σ*-LUMO of Mn(IV)4–O(5), indicating an important role of its Lewis acidity. Present CBS mechanism for the O–O bond formation coupled with one electron reductions of the high-valent Mn ions is different from the conventional radical coupling (RC) and acid-base (AB) mechanisms for water oxidation in artificial and native photosynthesis systems. The proton-coupled electron transfer (PC-OET) mechanism for the O–O bond formation is also touched in relation to the CBS-OET mechanism.

Theory of chemical bonds in metalloenzymes XXI. Possible mechanisms of water oxidation in oxygen evolving complex of photosystem II

ABSTRACTPossible mechanisms for water cleavage in oxygen evolving complex (OEC) of photosystem II (PSII) have been investigated based on broken-symmetry (BS) hybrid DFT (HDFT)/def2 TZVP calculations in combination with available XRD, XFEL, EXAFS, XES and EPR results. The BS HDFT and the experimental results have provided basic concepts for understanding of chemical bonds of the CaMn4O5 cluster in the catalytic site of OEC of PSII for elucidation of the mechanism of photosynthetic water cleavage. Scope and applicability of the hybrid DFT (HDFT) methods have been examined in relation to relative stabilities of possible nine intermediates such as Mn-hydroxide, Mn-oxo, Mn-peroxo, Mn-superoxo, etc., in order to understand the O–O (O–OH) bond formation in the S3 and/or S4 states of OEC of PSII. The relative stabilities among these intermediates are variable, depending on the weight of the Hartree–Fock exchange term of HDFT. The Mn-hydroxide, Mn-oxo and Mn-superoxo intermediates are found to be preferable in the weak, intermediate and strong electron correlation regimes, respectively. Recent different serial femtosecond X-ray (SFX) results in the S3 state are investigated based on the proposed basic concepts under the assumption of different water-insertion steps for water cleavage in the Kok cycle. The observation of water insertion in the S3 state is compatible with previous large-scale QM/MM results and previous theoretical proposal for the chemical equilibrium mechanism in the S3 state . On the other hand, the no detection of water insertion in the S3 state based on other SFX results is consistent with previous proposal of the O–OH (or O–O) bond formation in the S4 state . Radical coupling and non-adiabatic one-electron transfer (NA-OET) mechanisms for the OO-bond formation are examined using the energy diagrams by QM calculations and by QM(UB3LYP)/MM calculations . Possible reaction pathways for the O–O and O–OH bond formations are also investigated based on two water-inlet pathways for oxygen evolution in OEC of PSII. Future perspectives are discussed in relation to post HDFT calculations of the energy diagrams for elucidation of the mechanism of water oxidation in OEC of PSII.