Fourier transform infrared analysis of the photosynthetic oxygen-evolving center

Fourier transform infrared (FTIR) difference spectroscopy is a powerful method to study the detailed structures of active sites in enzymes. It can detect basically all molecules involved in trigger reactions and the structural information obtained is complementary to that by X-ray crystallography. Light-induced FTIR difference spectra of the photosynthetic oxygen-evolving center (OEC), which consists of the Mn4Ca cluster embedded in photosystem II protein complexes, were obtained as spectral changes upon the first S1-to-S2 transition or during the S-state cycle. Band assignments were performed by isotopic substitution, quantum chemical calculations, and site-directed mutagenesis, while the structures and reactions of OEC were analysed by applying certain perturbations on the OEC and detecting concomitant spectral changes. This review summarizes the FTIR studies on OEC thus far performed for understanding the molecular mechanism of photosynthetic oxygen evolution.