Resonance Raman spectra of the hydrogen out-of-plane (HOOP) vibrational modes in the retinal chromophore of octopus bathorhodopsin with deuterium label(s) along the polyene chain have been obtained. In clear contrast with bovine bathorhodopsin's HOOP modes, there are only two major HOOP bands at 887 and 940 cm−1 for octopus bathorhodopsin. On the basis of their isotopic shifts upon deuterium labeling, we have assigned the band at 887 cm−1 to C10H and C14H HOOP modes, and the band at 940 cm−1 to C11H=Cı2H Au-like HOOP mode. Except for a 26 cm−1 downward shift, the C11H=C12H Au-like wag appears to be little disturbed in octopus bathorhodopsin from the chromophore in solution since its changes upon deuterium labeling are close to those found in solution model-compound studies. We found also that the C10H and C14H HOOP wags are also similar to those in the model-compound studies. However, we have found that the interaction between the C7H and CgH HOOP internal coordinates of the chromophore in octopus bathorhodopsin is different from that of the chromophore in solution. The intensity of the C11H=C12H and the other HOOP modes suggests that the chromophore of octopus bathorhodopsin is somewhat torsionally distorted from a planar trans geometry. Importantly, a twist about C11=C12 double bond is inferred. Such a twist breaks the local symmetry, resulting in the observation of the normally Raman-forbidden C11H=C12H Au-like HOOP mode. The twisted nature of the chromophore, semi-quantitatively discussed here, likely affects the λmax of the chromophore and its enthalpy. The nature of the HOOP modes of octopus bathorhodopsin differs substantially from those found in bovine bathorhodopsin. Thus, while the λmax values for the two batho products are essentially the same as is their enthalpies (relative to their respective rhodopsins),. it seems likely that the various molecular factors that determine these quantities are present to different degrees. For example, the twist about C11=C12 double bond is likely larger in bovine bathorhodopsin than in octopus bathorhodopsin to account for the decoupling of C12H wag from C11H wag in bovine bathorhodopsin. This difference can be caused by the perturbation of a negatively charged protein residue in bovine bathorhodopsin (Eyring et al., 1982), which is much weaker in octopus bathorhodopsin. These issues are discussed in some detail.
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