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 di rect abstraction of hydrogen atoms from water molecules terminally lig ated to manganese ions in the oxygen-evolving complex by the oxidized tyrosine radical, Tyr(Z)(.). This model is tested here by performing q uantum chemical calculations. An empirically parametrized hybrid densi ty functional method is used, and both monomeric and dimeric manganese model systems are studied. It is found that, by coordination to a man ganese center, the first O-H bond strength of water is lowered from 11 3.4 to 84.3 kcal/mol. This O-H bond strength is only 2.8 kcal/mol stro nger than that in tyrosine. Using an extended basis set, we find that this difference decreases still further. The second hydrogen abstracti on energy is quite similar. Since thermoneutrality in the reaction (or a weak exothermicity) is a requirement for the hydrogen abstraction m odel, 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 coupling s of the manganese centers are discussed.