Hydroxide instead of bicarbonate in the structure of the oxygen evolving complex

The energy diagram for the catalytic cycle of dioxygen evolution in photosystem II has been recomputed using a slightly different model than the one used previously, since the results showed an erroneous trend in the critical region from S(2) to S(3). By replacing the bicarbonate ligand, which was bridging between calcium and the outer manganese, by a hydroxide a significant improvement of the energy diagram is obtained. Most notably, the S(2) to S(3) transition is now exergonic as it should be. However, for the S(2)(0) state an artificial constraint is needed on the hydrogen bonding, indicating that there is still some problem with the model used. This hydroxide model was used to study the effects of replacing calcium with strontium, magnesium and cadmium. The computed results reproduce the results observed experimentally that magnesium and cadmium suppress water oxidation and that strontium slows it down. For both magnesium and cadmium the process stops already at S(2).