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 Mn
3
CaO
4
Mn metal cluster with three closely associated manganese ions linked to a single μ
4
-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
18
O-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 S
1
→S
2
transition should produce opposite effects on the two water-exchange rates.
The D1-V185N mutation alters substrate water exchange by stabilizing alternative structures of the Mn4Ca-cluster in photosystem II
