A quantum chemical study of hydrogen abstraction from manganese-coordinated water by a tyrosyl radical: A model for water oxidation in photosystem II

Recently, water oxidation in photosystem II was proposed to involve direct abstraction of hydrogen atoms from water molecules terminally ligated to manganese ions in the oxygen-evolving complex by the oxidized tyrosine radical, Tyr(Z)(.). This model is tested here by performing quantum chemical calculations. An empirically parametrized hybrid density functional method is used, and both monomeric and dimeric manganese model systems are studied. It is found that, by coordination to a manganese center, the first O-H bond strength of water is lowered from 113.4 to 84.3 kcal/mol. This O-H bond strength is only 2.8 kcal/mol stronger than that in tyrosine. Using an extended basis set, we find that this difference decreases still further. The second hydrogen abstraction energy is quite similar. Since thermoneutrality in the reaction (or a weak exothermicity) is a requirement for the hydrogen abstraction model, the present calculations support this model. Possible functions of a coordinated chloride and a nearby calcium complex are suggested. Five-or six-coordination and ferro- or antiferromagnetic spin couplings of the manganese centers are discussed.