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-fre quency Fourier transform infrared (FTIR) spectroscopic techniques to study metal-substrate and metal-ligand vibrational modes in the Photosystem II/ox ygen-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 dee per into the low-frequency region (down to 350 cm(-1)), while increasing bo th 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 w ith these two states. Our results demonstrate the utility of FTIR technique s in accessing low-frequency modes in Photosystem II and in proteins genera lly.