Mechanism of Photosynthetic Oxygen Evolution

University Of California-Riverside, Riverside CA

Investigators

Abstract

The objective of this project is to provide fundamental new insight into the mechanism of water oxidation in Photosystem II. The experiments are designed to complement the high-resolution structure of Photosystem II that is expected to appear within the next 5 years. The overall goal is to understand how the reactivity of the (Mn)4-Ca cluster, located at the heart of the catalytic site, is controlled by its local protein environment. One specific goal is to characterize the changes in bonding and bond strengths that accompany the oxidation of the (Mn)4-Ca cluster through its catalytic cycle, particularly from its dark-stable S1 state to its S2 and S3 states. To relate vibrational modes to individual amino acid residues or to the (Mn)4-Ca cluster itself, FTIR difference spectra will be obtained of PSII particles that have been labeled isotopically or that contain site-directed mutations. Both mid-frequency (2000 - 1000 cm-1) and low-frequency (1000 - 350 cm-1) FTIR difference spectroscopy will be employed. A second goal of the study is to characterize the structural changes that facilitate the production of the Mn2+-Mn3+ intermediate that is formed during the light-driven assembly of the (Mn)4-Ca cluster. These structural changes will also be characterized by FTIR difference spectroscopy. A third goal is to trap and characterize the transient S4 state by freeze quenching. The formation of this state, the most oxidized state of the (Mn)4-Ca cluster, immediately precedes O2 release. The characterization of this state will provide crucial experimental constraints for proposed mechanisms of O-O bond formation.