Protons bound to the Mn cluster in photosystem II oxygen evolving complex detected by proton matrix ENDOR

Protons in the vicinity of the oxygen-evolving manganese cluster in photosystem II were studied by proton matrix ENDOR. Six pairs of proton ENDOR signals were detected in both the S0 and S2 states of the Mn-cluster. Two pairs of signals that show hyperfine constants of 2.3/2.2 and 4.0 MHz, respectively, disappeared after D2O incubation in both states. The signals with 2.3/2.2 MHz hyperfine constants in S0 and S2 state multiline disappeared after 3 h of D2O incubation in the S0 and S1 states, respectively. The signal with 4.0 MHz hyperfine constants in S0 state multiline disappeared after 3 h of D2O incubation in the S0 state, while the similar signal in S2 state multiline disappeared only after 24 h of D2O incubation in the S1 state. The different proton exchange rates seem to be ascribable to the change in affinities of water molecules to the variation in oxidation state of the Mn cluster during the water oxidation cycle. Based on the point dipole approximation, the distances between the center of electronic spin of the Mn cluster and the exchangeable protons were estimated to be 3.3/3.2 and 2.7 Å, respectively. These short distances suggest the protons belong to the water molecules ligated to the manganese cluster. We propose a model for the binding of water to the manganese cluster based on these results.