An investigation of the roles of calcium, manganese, and alternate electron donors in photosystem II

Photosynthetic oxygen evolution occurs in Photosystem II (PSII) at an active site that consists of a tetranuclear manganese (Mn4) cluster, Ca and Cl cofactors, and a redox-active tyrosine. The oxidation of H 2O to O2 and H+ requires oxidation of the Mn4 cluster and the presence of Ca. When water oxidation is impaired, alternate electron donation pathways exist that prevent oxidative damage to PSII by the primary oxidant, P680.;In the first part of this thesis, the structures of cofactors that are involved in or affect electron transfer to P680 are investigated. The first of these is a redox-active carotenoid (Car). Resonance Raman was used to confirm that this Car is oxidized under conditions that disable water oxidation. This result suggests that the Car participates in the alternate electron donor pathway. Another cofactor studied is the membrane-associated low-potential cytochrome (Cyt) c550. To understand the origin of the extremely low potential of Cyt c550, its structure was studied spectroscopically. It was found that Cyt c 550 is structurally similar to other c-type hemes, which, coupled with recent electrochemical measurements, suggests that its reduction potential when membranebound may be greater than in solution.;In the second part of this thesis, the structure and function of the Ca-binding site of PSII and several bioinorganic models of the Mn4 cluster are investigated. Using a series of metal ion inhibitors, it was found that the Ca-binding site is comprised of a rigid coordination environment that is size-selective for Ca. Of all metal ions sharing the same size and charge as Ca, only Sr, with a pKa similar to that of Ca, restores activity. This suggests that Ca plays a chemical role in water oxidation. Complementary to the studies on the structure of the Ca site, the structures of several bioinorganic models of the Mn4 cluster were investigated. The Raman spectra of a series of models were obtained to aid in the assignment of Raman spectra of the Mn4 cluster. Further studies of a model complex capable of catalytic water oxidation suggests that its mechanism of O-O bond formation proceeds via a Mn(V) = O species. These results and those obtained for Ca are tied together in a proposed mechanism for photosynthetic water oxidation that involves nucleophilic attack of a Ca-bound H2O molecule on a Mn(V) = O species.