Assignment of fingerprint vibrations in the resonance Raman spectra of rhodopsin, isorhodopsin, and bathorhodopsin: implications for chromophore structure and environment
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Abstract
13C- and 2H-labeled retinal derivatives have been used to assign normal modes in the 1100-1300-cm-1 fingerprint region of the resonance Raman spectra of rhodopsin, isorhodopsin, and bathorhodopsin. On the basis of the 13C shifts, C8-C9 stretching character is assigned at 1217 cm-1 in rhodopsin, at 1206 cm-1 in isorhodopsin, and at 1214 cm-1 in bathorhodopsin. C10-C11 stretching character is localized at 1098 cm-1 in rhodopsin, at 1154 cm-1 in isorhodopsin, and at 1166 cm-1 in bathorhodopsin. C14-C15 stretching character is found at 1190 cm-1 in rhodopsin, at 1206 cm-1 in isorhodopsin, and at 1210 cm-1 in bathorhodopsin. C12-C13 stretching character is much more delocalized, but the characteristic coupling with the C14H rock allows us to assign the "C12-C13 stretch" at approximately 1240 cm-1 in rhodopsin, isorhodopsin, and bathorhodopsin. The insensitivity of the C14-C15 stretching mode to N-deuteriation in all three pigments demonstrates that each contains a trans (anti) protonated Schiff base bond. The relatively high frequency of the C10-C11 mode of bathorhodopsin demonstrates that bathorhodopsin is s-trans about the C10-C11 single bond. This provides strong evidence against the model of bathorhodopsin proposed by Liu and Asato [Liu, R., & Asato, A. (1985) Proc. Natl. Acad. Sci. U.S.A. 82, 259], which suggests a C10-C11 s-cis structure. Comparison of the fingerprint modes of rhodopsin (1098, 1190, 1217, and 1239 cm-1) with those of the 11-cis-retinal protonated Schiff base in methanol (1093, 1190, 1217, and 1237 cm-1) shows that the frequencies of the C-C stretching modes are largely unperturbed by protein binding. In particular, the invariance of the C14-C15 stretching mode at 1190 cm-1 does not support the presence of a negative protein charge near C13 in rhodopsin. In contrast, the frequencies of the C8-C9 and C14-C15 stretches of bathorhodopsin and the C10-C11 and C14-C15 stretches of isorhodopsin are significantly altered by protein binding. The implications of these observations for the mechanism of wavelength regulation in visual pigments and energy storage in bathorhodopsin are discussed.