Identification of a Mn−O−Mn Cluster Vibrational Mode of the Oxygen-Evolving Complex in Photosystem II by Low-Frequency FTIR Spectroscopy†

We have developed conditions for recording the low-frequency S2/S1 Fourier transform infrared difference spectrum of hydrated PSII samples. By exchanging PSII samples with buffered 18O water, we found that a positive band at 606 cm-1 in the S2/S1 spectrum in 16O water is clearly downshifted to 596 cm-1 in 18O water. By taking double-difference (S2/S1 and 16O minus 18O) spectra, we assign the 606 cm-1 mode to an S2 mode and also identify a corresponding S1 mode at about 625 cm-1. In addition, by Sr and 44Ca substitution experiments, we found that the 606 cm-1 mode is upshifted to about 618 cm-1 by Sr2+ substitution but that this mode is not affected by substitution with the 44Ca isotope. On the basis of these results and also on the basis of studies of Mn model compounds, we assign the 625 cm-1 mode in the S1 state and the 606 cm-1 mode in the S2 state to a Mn−O−Mn cluster vibration of the oxygen-evolving complex (OEC) in PSII. This structure may include additional bridge(s), which could be another oxo, ca...