A Vibrational Analysis of Rhodopsin and Bacteriorhodopsin Chromophore Analogues: Resonance Raman and Infrared Spectroscopy of Chemically Modified Retinals and Schiff Bases

Resonance Raman spectroscopy has been used to study chemically modified retinal analogues involving chain substitutions, ring substitutions, or Schiff-base linkages. In addition, retinal fragments and fully deuterated retinals were investigated, and infrared spectra of the four isomers of retinal were obtained. Low-frequency resonance Raman spectra are also reported for all of the isomers of retinal, for the protonated and unprotonated Schiff bases of frans-retinal, for β-ionone, and for trans-3-dehydroretinal. Band assignments were made to specific vibrational motions, and these assignments have led to a detailed understanding of the spectral features observed in the resonance Raman spectra of the retinylidene chromophore in rhodopsin and bacteriorhodopsin.