Interpretation of the resonance Raman spectrum of bathorhodopsin based on visual pigment analogs

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Abstract
Resonance Raman spectra of visual pigment analogs were used to evaluate various models for the structure of the retinal chromophore in bathorhodopsin. Deuteration or removal of the 18-methyl on the .beta.-ionyl ring or of the 19-methyl on the polyene chain demonstrates that the 3 intense low wavenumber bands of bathorhodopsin at 853, 875 and 920 cm-1 are not due to exomethylene or ring modes. Assignment of these lines to out-of-plane vinyl hydrogen motions on the chain best accounts for the experimental data. Calculations show that the intensity of these vibrations can be explained by twists of 10-30.degree. about chain single bonds. The 1100-1400 cm-1 Raman fingerprint indicates that the configuration of the double bonds is trans. This suggests that the structure of the bathorhodopsin chromophore is twisted all-trans.