Low-frequency fourier transform infrared spectroscopy of the oxygen-evolving complex in Photosystem II.

In this communication, we report our progress on the development of low-frequency Fourier transform infrared (FTIR) spectroscopic techniques to study metal-substrate and metal-ligand vibrational modes in the Photosystem II/oxygen-evolving complex (PS II/OEC). This information will provide important structural and mechanistic insight into the OEC. Strong water absorption in the low-frequency region (below 1000 cm(-1)), a lack of suitable materials, and temperature control problems have limited previous FTIR spectroscopic studies of the OEC to higher frequencies (>1000 cm(-1)). We have overcome these technical difficulties that have blocked access to the low-frequency region and have developed successive instruments that allow us to move deeper into the low-frequency region (down to 350 cm(-1)), while increasing both data accumulation efficiency and S/N ratio. We have detected several low-frequency modes in the S(2)/S(1)spectrum that are specifically associated with these two states. Our results demonstrate the utility of FTIR techniques in accessing low-frequency modes in Photosystem II and in proteins generally.