Preresonance Raman Spectra of Crystals of Retinal Isomers

Preresonance Raman spectra of crystals of all-trans-, 9-cis-, 11-cis-, and 13-cis-retinal are reported. The solid state spectra of these retinal isomers show a detailed vibrational fine structure observed in several instances in the resonance Raman spectra of various rhodopsins but not seen in solution spectra. A systematic variation in the C=C stretching vibration among the isomers is noted, and a splitting in the stretching vibration of the terminal carbonyl group has also been detected which is apparently sensitive to the conformation of this group. Furthermore, a doublet band is reported in the 1005-1031-cm^-1 region which appears to arise from 5-CH₃,9-CH₃, and 13-CH₃ vibrations with the conjugated portion of the molecule and is characteristic of each individual isomer. The vibrational frequencies of the doublet are interpreted in terms of resonance structures. In addition, various other vibrational modes between 700 and 1700 cm^-1 are catalogued which also reflect the conformation of retinal.