Water structural changes in lumirhodopsin, metarhodopsin I, and metarhodopsin II upon photolysis of bovine rhodopsin: Analysis by Fourier transform infrared spectroscopy
Difference Fourier transform infrared spectra of lumirhodopsin, metarhodopsin I, and metarhodopsin II versus rhodopsin were recorded with hydrated films of bovine rod outer segments at 200, 240, and 270 K. In the region between 3700 and 3450 cm-1, the O-H stretching vibrational bands of water were identified by H(2)18O and 2H2O shifts. Lumirhodopsin and metarhodopsin I exhibit almost identical spectral shape in this region. The O-H stretching vibration band of water was detected at 3533 cm-1 upon formation of lumirhodopsin and metarhodopsin I and at 3641 cm-1 upon formation of metarhodopsin II. The results suggest that hydrogen bonding of water molecules in the protein is stronger in lumirhodopsin and metarhodopsin I, intermediates with a protonated Schiff base, than in metarhodopsin II with an unprotonated Schiff base. This is similar to the case of photoreaction of bacteriorhodopsin, in which stronger hydrogen bonding of water is formed in the L intermediate than the M intermediate [Maeda, A., Sasaki, J., Shichida, Y., & Yoshizawa, T. (1992) Biochemistry 31, 462-467].