Nucleophilic water attack is not a possible mechanism for O-O bond formation in photosystem II.

Two different types of mechanisms are at present suggested for the O-O bond-formation step in photosystem II. The first one is a coupling between an oxyl radical and a bridging oxo. The second one is a nucleophilic water attack on a terminal oxo (or oxyl) group. In the present short paper, the six most reasonable versions of the latter mechanism have been studied and compared with the oxo-oxyl mechanism. The barriers are found to be much too high for the water attack, and that mechanism can therefore safely be ruled out. The reason is that the protonated peroxide product is always very high in energy.