Water structural changes in the bacteriorhodopsin photocycle: analysis by Fourier transform infrared spectroscopy

The Fourier transform infrared difference spectra between light-adapted bacteriorhodopsin (BR) and its photointermediates, L and M, were analyzed for the 3750-3450-cm-1 region. The 0-H stretching vibrational bands were identified from spectra upon substitution with (H2O)-H-2. Among them, the 3642-cm-1 band of BR was assigned to water by substitution with (H2O)-O-18. By a comparison with the published infrared spectra of the water in model systems [Mohr, S. C., Wilk, W. D., & Barrow, G. M. (1965) J. Am. Chem. Soc. 87, 3048-3052], it is shown that the O-H bonds of the water in BR interact very weakly. Upon formation of L, the interaction becomes stronger. The O-H bonds of the protein side chain undergo similar changes. On the other hand, M formation further weakens the interaction of the same water molecules in BR. The appearance of a sharp band at 3486 cm-1, which was assigned tentatively to the N-H stretching vibration of the peptide bond, is unique to L. The results suggest that the water molecules are involved in the perturbation of Asp-96 in the L intermediate and that they are exerted from the protonated Schiff base which changes position upon the light-induced reaction.