A RESONANCE RAMAN-STUDY OF THE C=N CONFIGURATIONS OF OCTOPUS RHODOPSIN, BATHORHODOPSIN, AND ISORHODOPSIN

The resonance Raman spect-ra 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 chan ges 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 1206-12 27 cm(-1) in the three octopus species, as revealed by the Raman spect ra of their 14,15-C-13(2) derivatives. Such results are different comp ared to those of the bovine pigments, in which no significant frequenc y shift of the C14-C15 stretch mode was observed upon Schiff base N de uteration. In an earlier Raman study of a Schiff base model compound w hich contained only one single bond adjacent to two double bonds, we h ave 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 upo n Schiff base N deuteration when the C=N configuration is anti [Deng e t al., (1994) J. Phys. Chem. 98, 4776-4779]. The same study has also s hown that when the C=N configuration is syn, the C-C stretch mode shou ld be at about 1150 cm(-1) Since the C14-C15 stretch mode frequency is relatively high in the spectra of octopus rhodopsin and bathorhodopsi n (>1200 cm(-1)) and since the normal mode pattern near the Schiff bas e is similar to the model, we suggest, that the C=N configuration in t hese two species is anti. The different responses of the C14-C15 stret ch 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 a re 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 s mall Schiff base model compound. In clear contrast with the bovine rho dopsin Raman spectrum, which is very similar to that for the Il-cis-re tinal Schiff base, the drastically different octopus rhodopsin spectru m indicates large protein perturbations on the C11=C12-C13 moiety, eit her by steric or by electrostatic interactions. Further studies are re quired to determine if such spectral differences indicate a difference of the energy conversion mechanisms in the primary photochemical even t of these two pigments.