Theory of chemical bonds in metalloenzymes - Manganese oxides clusters in the oxygen evolution center - Manganese o

K. Yamaguchi, M. Shoji, T. Saito, H. Isobe, S. Yamada, S. Nishihara, T. Kawakami, Y. Kitagawa, S. Yamanaka, M. Okumura

Research output: Chapter in Book/Report/Conference proceedingConference contribution

8 Citations (Scopus)

Abstract

In early 1980 we have initiated broken-symmetry (BS) MO theoretical calculations of transition-metal oxo species M=O (M=Ti,V,Cr,Mn,Fe,Ni,Cu) to elucidate the nature of dσ-pσ and dπ-pπ bonds. It has been concluded that high-valent M=O species such as [Mn(IV)=O]2+ and [Fe(IV)=O]2+ exhibit electrophilic property in a sharp contrast with nucleophilic character of low-valent M=O bonds: [M(II)O2-] 0, and closed-shell dπ-pπ bonds of high-valent M=O species often suffer the triplet-instability, giving rise to open-shell (BS) configurations with significant metal-diradical (MDR) character: •M-O•: note that these bonds are therefore regarded as typical examples of strongly correlated electron systems. Because of the MDR character, 1,4-metal diradical mechanism was indeed preferable to four-centered mechanism in the case of addition reaction of naked Mn(IV)=O to ethylene. Recently the manganese-oxo species have been receiving renewed interest in relation to catalytic cycle of oxygen evolution from water molecules in the photosynthesis II (PSII) system. Accumulated experimental results indicate that this process is catalyzed with four manganese oxide clusters coordinated with calcium ion (CaMn4O4). Past decade we have performed BS MO theoretical investigations of manganese oxide clusters related to CaMn4O 4. These calculations have elucidated that high-valent Mn(X)=O (X=IV,V) bonds exhibit intermediate MDR character (y=40-60%) in the case of total low-spin (LS) configuration but the MDR character decreases with coordination of Ca2+ and water molecules. While the MDR character of the Mn-oxo bonds becomes very high at the high-spin (HS) configuration. Our computational results enabled us to propose two possible mechanisms on the theoretical ground: (A) electrophilic (EP) mechanism and (B) radical coupling (RC) mechanism. The theoretical results indicate that the EP mechanism is preferable for the low-spin (LS) state in polar media like in the protein environments (native OEC), whereas the RC mechanism is feasible at the state without such environmental stabilization: local singlet and local triplet diradical mechanisms are proposed for the OO coupling process. Possibilities of EP and RC mechanisms are examined in comparison with a lot of experimental results accumulated and theoretical results with several groups.

Original languageEnglish
Title of host publicationInternational Conference of Computational Methods in Sciences and Engineering 2009, ICCMSE 2009
Pages63-79
Number of pages17
DOIs
Publication statusPublished - 2012
EventInternational Conference of Computational Methods in Sciences and Engineering 2009, ICCMSE 2009 - Rhodes, Greece
Duration: Sep 29 2009Oct 4 2009

Publication series

NameAIP Conference Proceedings
Volume1504
ISSN (Print)0094-243X
ISSN (Electronic)1551-7616

Other

OtherInternational Conference of Computational Methods in Sciences and Engineering 2009, ICCMSE 2009
CountryGreece
CityRhodes
Period9/29/0910/4/09

Keywords

  • CaMnO
  • Electrophilic mechanism
  • Local singlet diradical
  • Local triplet diradical
  • Mn(V)=O
  • OEC
  • Radical mechanism
  • metal-oxo

ASJC Scopus subject areas

  • Physics and Astronomy(all)

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    Yamaguchi, K., Shoji, M., Saito, T., Isobe, H., Yamada, S., Nishihara, S., Kawakami, T., Kitagawa, Y., Yamanaka, S., & Okumura, M. (2012). Theory of chemical bonds in metalloenzymes - Manganese oxides clusters in the oxygen evolution center - Manganese o. In International Conference of Computational Methods in Sciences and Engineering 2009, ICCMSE 2009 (pp. 63-79). (AIP Conference Proceedings; Vol. 1504). https://doi.org/10.1063/1.4771704