Hydrogen Bonds of Water and CO Groups Coordinate Long-Range Structural Changes in the L Photointermediate of Bacteriorhodopsin

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Abstract
Fourier transform infrared spectra of light-adapted bacteriorhodopsin exhibit a band at 1618 cm-1 that shifts to 1625 cm-1 upon formation of the L intermediate. It is assigned to the peptide CO of Val49 from the fact that it shifts in [1-13C]valine-labeled bacteriorhodopsin and appears perturbed in the Val49→Met mutant. The intensity of the BR→L difference band is reduced in the Thr46→Val mutant but restored by the additional mutation of Asp96→Asn. These intensity changes are closely correlated with the H-bonding change of water molecules, suggesting that the peptide CO of Val49 is hydrated. This could arise in the Thr46→Val mutant because of perturbation of the CO of Val46, which points toward Val49. The Val49→Ala mutation influences a peptide N−H, presumably of Val49, and the carboxylic CO of Asp96, as well as water molecules proximal to Asp85. Conversely, the water molecule assumed to be in the cavity that arises from the missing two methyl groups in V49A could be affected in the mutant of Asp96→Asn. We propose that the perturbation exerted on Asp85 by the Schiff base in the l intermediate is transmitted to Asp96 through H-bonding of water molecules in the Asp85−Val49 region, the CO of Val49, H-bonding between Val49 and Thr46, and H-bonding between Thr46 and Asp96.

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