A resonance Raman study of the C=N configurations of octopus rhodopsin, bathorhodopsin, and isorhodopsin

The resonance Raman spectra of octopus rhodopsin, bathorhodopsin, and isorhodopsin at 120 K have been obtained as well as those of pigments regenerated with isotopically labeled retinals near the C14-C15 bond. Deuteration of the Schiff base nitrogen induces relatively large changes in the C-C stretch region between 1100 and 1300 cm^-1, including a large frequency shift of the C14-C15 stretch mode located at 1206-1227 cm^-1 in the three octopus species, as revealed by the Raman spectra of their 14,15- ¹³C₂ derivatives. Such results are different compared to those of the bovine pigments, in which no significant frequency shift of the C14-C15 stretch mode was observed upon Schiff base N deuteration. In an earlier Raman study of a Schiff base model compound which contained only one single bond adjacent to two double bonds, we have found that the stretch mode of this C- C single bond at 1232 cm^-1 shifts up by 15 cm^-1 and its intensity is also greatly reduced upon Schiff base N deuteration when the C=N configuration is anti [Deng et al., (1994) J. Phys. Chem. 98, 4776-4779]. The same study has also shown that when the C=N configuration is syn, the C-C stretch mode should be at about 1150 cm^-1. Since the C14-C15 stretch mode frequency is relatively high in the spectra of octopus rhodopsin and bathorhodopsin (> 1200 cm^-1) and since the normal mode pattern near the Schiff base is similar to the model, we suggest that the C=N configuration in these two species is anti. The different responses of the C14-C15 stretch mode to the Schiff base nitrogen deuteration in bovine and octopus pigments are due to the fact that the coupled C14-C15 stretch and the C12-C13 stretch motions in the model compound or in bovine rhodopsin are altered in octopus rhodopsin so that the stretch motion of the C14-C15 bond is more localized, similar to the C-C stretch motion in the small Schiff base model compound. In clear contrast with the bovine rhodopsin Raman spectrum, which is very similar to that for the 11-cis-retinal Schiff base, the drastically different octopus rhodopsin spectrum indicates large protein perturbations on the C11=C12-C13 moiety, either by steric or by electrostatic interactions. Further studies are required to determine if such spectral differences indicate a difference of the energy conversion mechanisms in the primary photochemical event of these two pigments.