Substrate Water Exchange for the Oxygen Evolving Complex
in PSII in the S<sub>1</sub>, S<sub>2</sub>, and S<sub>3</sub> States
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Abstract
Detailed
mechanisms for substrate water exchange in the oxygen
evolving complex in photosystem II have been determined with DFT methods
for large models. Existing interpretations of the experimental water
exchange results have been quite different. By many groups, these
results have been the main argument against the water oxidation mechanism
suggested by DFT, in which the oxygen molecule is formed between a
bridging oxo and an oxyl radical ligand in the center of the OEC.
That mechanism is otherwise in line with most experiments. The problem
has been that the mechanism requires a rather fast exchange of a bridging
oxo ligand, which is not a common finding for smaller Mn-containing
model systems. However, other groups have actually favored a substrate
derived oxo ligand partly based on the same experiments. In the present
study, three S-states have been studied, and the rates have been well
reproduced by the calculations. The surprising experimental finding
that water exchange in S<sub>1</sub> is slower than the one in S<sub>2</sub> is reproduced and explained. The key to this rate difference
is the ease by which one of the manganese centers (<b>Mn3</b>) is reduced. This reduction has to occur to release the substrate
water from <b>Mn3</b>. The similar rate of the slow exchange
in S<sub>2</sub> and S<sub>3</sub> has been rationalized on the basis
of earlier experiments combined with the present calculations. The
results strongly support the previous DFT-suggested water oxidation
mechanism