A resonance Raman study of the C=C stretch modes in bovine and octopus visual pigments with isotopically labeled retinal chromophores

Previous resonance Raman spectroscopic studies of bovine and octopus rhodopsin and bathorhodopsin in the C-C stretch fingerprint region have shown drastically different spectral patterns, which suggest different chromophore-protein interactions. We have extended our resonance Raman studies of bovine and octopus pigments to the C=C stretch region in order to reveal a more detailed picture about the difference in retinal-protein interactions between these two pigments. The C=C stretch motions of the protonated retinal Schiff base are strongly coupled to form highly delocalized ethylenic modes located in the 1500 to 1650 cm(-1) spectral region. In order to decouple these vibrations, a series of 11,12-D-2-labeled retinals, with additional 13C labeling at C-8, C-10, C-11 and C-14, respectively, are used to determine the difference of specific C=C stretch modes between bovine and octopus pigments. Our results show that the C-9=C-10 and C-13=C-14 stretch mode are about 20 cm(-1) lower in the Raman spectrum of octopus bathorhodopsin than in bovine bathorhodopsin, while the other C=C stretch modes in these two bathorhodopsins are similar. In contrast, only the C-9=C-10 stretch mode in octopus rhodopsin is about 10 cm(-1) lower than in bovine rhodopsin, while other C=C stretches are similar.