Vibrational Analysis of Carboxylate Ligands in the Water Oxidizing Center

Abstract

Photosynthetic water oxidation performed by photosystem II (PSII) provides electrons that are required to fix CO2, which in turn supports life on Earth by liberating molecular oxygen. The high-resolution X-ray structure of PSII recently showed that the water-oxidizing center (WOC) consists of an Mn cluster, six carboxylate, four water, and one imidazole ligands. In addition, previous FTIR studies have revealed significant structural changes of the WOC during the water oxidation process, particularly within the carboxylate groups. However, not only the assignment of the carboxylate vibrations to the spectra but also the roles of the carboxylate ligands in water oxidation still remain unclear. In this chapter, to simulate S2-minus-S1 FTIR difference spectra in the carboxylate stretching region upon the S1 → S2 transition, we carried out a normal mode analysis of the WOC using quantum mechanics/molecular mechanics (QM/MM) calculations. We determined that high-oxidation models with Mn(III)2Mn(IV)2 satisfactorily reproduced experimental data from intact and Ca-depleted PSII compared with low-oxidation models by the evaluation of WOC models with different protonation and oxidation states. Furthermore, it is suggested that the carboxylate ligands bridging the Mn and Ca2+ ions tune the reactivity of the water ligands coordinated with Ca2+ by shifting the charge through their π conjugation.