Summary
Vibrational spectroscopy, which includes infrared and Raman spectroscopies, provides structural information of molecules by detecting molecular vibrations based on chemical bonds and interactions. These methods have been applied to the study of various cofactors in Photosystem II. In particular, light-induced Fourier transform infrared (FTIR) difference spectroscopy has proven to be a powerful method to reveal detailed structures of the binding sites of cofactors including protein moieties and water molecules. Information available by FTIR difference spectroscopy includes hydrogen bonding and protonation state of chemical groups, which play an essential role in proton transfer and also in controlling redox reactions, but are often not available by X-ray crystallography. The FTIR investigations cover all the redox cofactors of Photosystem II in both the main and peripheral electron-transfer pathways, i.e., the manganese-cluster, the redox-active tyrosines Yz and YD, the primary donor P680, the primary acceptor pheophytin, the quinone acceptors QA and QB, the non-heme iron, cytochrome b559, chlorophyll Z, and β-carotene. This article reviews how the structures and reactions of these cofactors have been studied using mainly FTIR spectroscopy with the assistance of Raman spectroscopy.