Abstract
Light-driven proton transport in bacteriorhodopsin (BR) is achieved by dynamic rearrangement of the hydrogen-bonding network inside the membrane protein. Arg82 is located between the Schiff base region and proton release group, and has a major influence on the pK(a) values of these groups. It is believed that Arg82 changes its hydrogen-bonding acceptors during the pump cycle of BR, stages of which are correlated with proton movement along the transport pathway. In this study, we compare low-temperature polarized FTIR spectra of [eta(1,2)-(15)N]arginine-labeled BR in the 2750-2000 cm(-1) region with those of unlabeled BR for the K, L, M, and N intermediates. In the K-minus-BR difference spectra, (15)N-shifted modes were found at 2292 (-)/2266 (+) cm(-1) and at 2579 (-)/2567 (+) cm(-1). The former corresponds to strong hydrogen bonding, while the latter corresponds to very weak hydrogen bonding. Both N-D stretches probably originate from Arg82, the former oriented toward water 406 and the latter toward the extracellular side, and both hydrogen bonds are somewhat strengthened upon retinal photoisomerization. This perturbation of arginine hydrogen bonding is entirely relaxed in the L intermediate where no (15)N-isotope shifts are observed in the difference spectrum. In the M intermediate, the frequency is not significantly altered from that in BR. However, the polarized FTIR spectra strongly suggest that the dipolar orientation of the strongly hydrogen bonded N-D group of Arg82 is changed from perpendicular to parallel to the membrane plane. Such a change is presumably related to the motion of the Arg82 side chain from the Schiff base region to the extracellular proton release group. Additional bands corresponding to weak hydrogen bonding were observed in both the M-minus-BR and N-minus-BR spectra. Changes in hydrogen-bonding structures involving Arg82 are discussed on the basis of these FTIR observations.