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Title: Computational insights on crystal structures of the oxygen-evolving complex of photosystem II with either Ca²⁺ or Ca²⁺ substituted by Sr²⁺

The oxygen-evolving complex of photosystem II can function with either Ca²⁺ or Sr²⁺ as the heterocation, but the reason for differing turnover rates remains unresolved despite reported X-ray crystal structures for both forms. Using quantum mechanics/molecular mechanics (QM/MM) calculations, we optimize structures with each cation in both the resting state (S₁) and in a series of reduced states (S₀, S₋₁, and S-₂). Through comparison with experimental data, we determine that X-ray crystal structures with either Ca²⁺ or Sr²⁺ are most consistent with the S-₂ state, Mn₄[III,III,III,II] with O4 and O5 protonated. As expected, the QM/MM models show that Ca²⁺/Sr²⁺ substitution results in elongation of the heterocation bonds and displaces terminal waters W3 and W4. The optimized structures also show that hydrogen-bonded W5 is displaced in all S states with Sr²⁺ as the heterocation, suggesting that this water may play a critical role during water oxidation.
Authors:
 [1] ;  [2] ;  [1] ;  [1] ;  [1]
  1. Yale Univ., New Haven, CT (United States)
  2. Yale Univ., New Haven, CT (United States); Brookhaven National Lab. (BNL), Upton, NY (United States)
Publication Date:
OSTI Identifier:
1183828
Report Number(s):
BNL--107849-2015-JA
Journal ID: ISSN 0006-2960; R&D Project: CO045; KC0301020
Grant/Contract Number:
SC00112704
Type:
Accepted Manuscript
Journal Name:
Biochemistry
Additional Journal Information:
Journal Volume: 54; Journal Issue: 3; Journal ID: ISSN 0006-2960
Publisher:
American Chemical Society (ACS)
Research Org:
Brookhaven National Laboratory (BNL), Upton, NY (United States)
Sponsoring Org:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY