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Rationalizing the 2.25 Å Resolution Crystal Structure of the Water Oxidising Complex of Photosystem II in the S 3 State
Author(s) -
Petrie Simon,
Stranger Rob,
Pace Ron J.
Publication year - 2017
Publication title -
chemphyschem
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.016
H-Index - 140
eISSN - 1439-7641
pISSN - 1439-4235
DOI - 10.1002/cphc.201700640
Subject(s) - extended x ray absorption fine structure , photosystem ii , oxidation state , oxygen evolving complex , chemistry , crystallography , crystal structure , metal , absorption spectroscopy , physics , photosynthesis , biochemistry , organic chemistry , quantum mechanics
Quantum chemical calculations are described which rationalize the recent X‐ray diffraction (XRD) structure at 2.25 Å of the Mn 4 Ca water oxidising complex (WOC) of photosystem II (PSII) in the S 3 intermediate state. The new S 3 XRD structure shows remarkable similarity to earlier atomic resolution (1.9, 1.95 Å) WOC structures in the dark stable S 1 state and is inconsistent with most current proposals, from computational chemistry and other sources, regarding the Mn oxidation state levels in the WOC cluster and the nature of water substrate binding, particularly in S 3 . This mirrors earlier failures to rationalise the WOC geometry in the 1.9 and 1.95 Å S 1 XRD structures, assuming “high” paradigm Mn oxidation models. However, we recently showed that a lower Mn oxidation assumption closely reproduces the S 1 XRD structures, computationally. This same “low” Mn oxidation model, now computationally applied in S 3 , not only reproduces the latest 2.25 Å XRD structure but also rationalises a number of other important, experimental features of the WOC, including the metal–metal distances inferred from EXAFS studies as well as earlier S 3 state XRD structures of lower resolution (4–5 Å). As found previously for S 1 , the WOC in the S 3 state is computationally revealed to be structurally variable, consistent with some EXAFS and lower‐resolution XRD data. This is a direct consequence of at least two Mn III ions being present in all metastable S states.