Manganese oxyl radical intermediates and O-O bond formation in photosynthetic oxygen evolution and a proposed role for the calcium cofactor in photosystem II

Spin state considerations are proposed to sharply limit the possible O-O bo nd-forming steps in water oxidation by the oxygen evolving center of Photos ystem II. A series of intermediates are proposed for the Kok S states on th e basis of quantum chemical studies on simple model complexes; these are al so consistent with the main biophysical data. Only one Mn atom in the activ e site cluster is thought to be redox-active and mediate O-O bond formation . A key concept is the formation of an unreactive Mn=O oxo at the S-2 state , followed by its conversion to a reactive Mn-O . oxyl form at the S-3 leve l, with radical character on the oxyl oxygen, at which point O-O bond forma tion can occur by a coupling between the oxyl and an outer-sphere water mol ecule. An MnOOH intermediate at S-3 is proposed to lose a hydrogen atom to give O-2. The role of the Ca cofactor is to bring about a 5- to 6-coordinat ion change at S-2, necessary for formation of a reactive oxo in S-3. The ch loride cofactor is assigned the role of charge neutralization.