アイテム詳細

要旨

Light-induced water oxidation in photosynthesis occurs in a single biological supercomplex called photosystem II at a protein-bound Mn₄O₅Ca cluster, which passes through 5 intermediate states (Si-state cycle). (1) A detailed electronic model of the catalytic center has been developed based on structural data from X-ray crystallography (2) and magnetic resonance (EPR and ENDOR) techniques. (3-6) Based on ⁵⁵Mn ENDOR spin and oxidation states of the Mn ions and the coupling between them could be determined; together with chemical modifications of the cluster a model of the catalytic center has been obtained. High field ELDOR-detected NMR (EDNMR) at 94GHz (see figure) is applied to detect the interaction of the cluster (S2 state) with magnetic nuclei of amino acids (¹⁴N) and attached water molecules. In samples exchanged with H₂¹⁷O, ¹⁷O hyperfine couplings of three different types of water molecules were detected, (6) see figure. They could be assigned based on the structural model from X-ray crystallography (2) which has been refined using densitiy functional theory (5) and comparison with model complexes. Additional experiments like Ca/Sr exchange (4) and the use of inhibitors (NH₃) (7) helped in this endeavor. These data further refine the reaction pathway fpr O-O bond formation supporting an oxo/oxyl coupling mechanism in the catalytically active state (S4) of the cycle. (8,1) A mechanistic model for water oxidation in photosynthesis is proposed.