A vibrational analysis of rhodopsin and bacteriorhodopsin chromophore analogs: resonance Raman and infrared spectroscopy of chemically modified retinals and Schiff bases

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Abstract
Resonance Raman spectroscopy was used to study chemically modified retinal analogs involving chain substitutions, ring substitutions or Schiff-base linkages. Retinal fragments and fully deuterated retinals were investigated, and IR spectra of the 4 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 trans-retinal, .beta.-ionone and trans-3-dehydroretinal. Band assignments were made to specific vibrational motions, and these assignments led to a detailed understanding of the spectral features observed in the resonance Raman spectra of the retinylidene chromophore in rhodopsin and bacteriorhodopsin.