Molecular Flow Resonance Raman Effect from Retinal and Rhodopsin

We have performed resonance enhanced Raman measurements of retinal isomers in solution (alltrans, 11-cis, 9-cis, and 13-cis) and cetyltrimethylammonium bromide (CTAB) detergent extracts of bovine rhodopsin near physiological temperatures (17 °C). In order to measure these photolabile systems, we have developed a general technique which allows Raman measurements of any photosensitive material. This technique involves imposing a molecular velocity transverse to the Raman exciting laser beam sufficient to ensure that any given molecule moves through the beam so that it has little probability of absorbing a photon. We have also measured the resonance Raman spectra of crystals of the same retinal isomers. The data show that each isomer has a distinct and characteristic Raman spectra and that the spectrum of 11-cis-retinal is quite similar but not identical with that of rhodopsin and similarly for 9-cw-retinal compared with isorhodopsin. In agreement with previous work, the Raman data demonstrate that retinal and opsin are joined by a protonated Schiff base. Due to the fact that the Raman spectra of 11- cis-retinal (solution) and rhodopsin show bands near 998 and 1018 cm^-1, a spectral region previously assigned to C-Me stretching motions, it is suggested that 11-cis-retinal in solution is composed of a mixture of 12-s-trans and 12-s-cis, and that the conformation of rhodopsin is (perhaps distorted) 12-s-trans.