A Resonance Raman Study Of the C=C Stretch Modes in Bovine and Octopus Visual Pigments with Isotopically Labeled Retinal Chromophores

Abstract— 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‐D2‐labeled retinals, with additional 13C labeling at C8, C10, C11 and C14, respectively, are used to determine the difference of specific C=C stretch modes between bovine and octopus pigments. Our results show that the C9=C10 and C13=C14 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 C9=C10 stretch mode in octopus rhodopsin is about 10 cm−1 lower than in bovine rhodopsin, while other C=C stretches are similar.