QM/MM computational studies of substrate water binding to the oxygen-evolving centre of photosystem II

Eduardo M. Sproviero, Katherine Shinopoulos, José A. Gascón, James P. McEvoy, Gary W. Brudvig, Victor S. Batista

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This paper reports computational studies of substrate water binding to the oxygen-evolving centre (OEC) of photosystem II (PSII), completely ligated by amino acid residues, water, hydroxide and chloride. The calculations are based on quantum mechanics/molecular mechanics hybrid models of the OEC of PSII, recently developed in conjunction with the X-ray crystal structure of PSII from the cyanobacterium Thermosynechococcus elongatus. The model OEC involves a cuboidal Mn3CaO4Mn metal cluster with three closely associated manganese ions linked to a single μ4-oxo-ligated Mn ion, often called the 'dangling manganese'. Two water molecules bound to calcium and the dangling manganese are postulated to be substrate molecules, responsible for dioxygen formation. It is found that the energy barriers for the Mn(4)-bound water agree nicely with those of model complexes. However, the barriers for Ca-bound waters are substantially larger. Water binding is not simply correlated to the formal oxidation states of the metal centres but rather to their corresponding electrostatic potential atomic charges as modulated by charge-transfer interactions. The calculations of structural rearrangements during water exchange provide support for the experimental finding that the exchange rates with bulk 18O-labelled water should be smaller for water molecules coordinated to calcium than for water molecules attached to the dangling manganese. The models also predict that the S1→S 2 transition should produce opposite effects on the two water-exchange rates.

Original languageEnglish
Pages (from-to)1149-1156
Number of pages8
JournalPhilosophical Transactions of the Royal Society B: Biological Sciences
Issue number1494
Publication statusPublished - Mar 27 2008



  • Density functional theory
  • Oxomanganese complexes
  • Oxygen evolution
  • Photosynthesis
  • Photosystem II
  • Quantum mechanics/molecular mechanics

ASJC Scopus subject areas

  • Biochemistry, Genetics and Molecular Biology(all)
  • Agricultural and Biological Sciences(all)

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