Effects of arginine-82 on the interactions of internal water molecules in bacteriorhodopsin.

Arg82, one of the residues near the protonated Schiff base of bacteriorhodopsin, facilitates proton release to the medium during the L-to-M reaction of the photocycle, but retards the rate of proton transfer from the Schiff base to Asp85. In order to understand the role of Arg82 in these processes, the structural changes upon formation of the M intermediate were studied by Fourier transform infrared spectroscopy of the hydrated films of Arg82 mutants at pH 9.5. The negative band at 1700 cm-1 in the BR --> M spectrum due to the deprotonation of Glu204 was absent when Arg82 was replaced with alanine (R82A), but present with small amplitude when residue 82 was a glutamine (R82Q), or a lysine (R82K), with a shift to 1696 cm-1. The O-H stretch of water at 3643 cm-1 is shifted toward a lower frequency in R82Q, R82K, and R82A in the unphotolyzed state. However, R82Q retains a fraction of the unshifted band. Another O-H stretch is prominent in R82Q around 3625 cm-1 but absent in R82A and probably in R82K. In parallel, R82Q retains a fraction of the slow component of the formation of the M intermediate, which is almost completely absent in R82K and R82A. These results, along with previous data for the mutants of Glu204, suggest that the guanidium group of Arg82 influences the H-bonding of water molecules located close to Asp85 and Arg82-Glu204 regions, and the rate of proton transfer from the Schiff base to Asp85. The amide group of Gln82 can substitute for it but weakly.