Retinal analog study of the role of steric interactions in the excited state isomerization dynamics of rhodopsin.

The role of intramolecular steric interactions in the isomerization of the 11-cis-retinal chromophore in the photoreceptor protein rhodopsin is examined with resonance Raman and CD spectroscopy combined with quantum yield experiments. The resonance Raman spectra and CD spectra of 13-demethylrhodopsin indicate that its chromophore, an analog in which the nonbonded interaction between the 10-H and the 13-CH3 groups is removed, is less distorted in the C10...C13 region than the native chromophore. The reduced torsional and hydrogen-out-of-plane resonance Raman intensities further indicate that the excited state potential energy surface has a much shallower slope along the isomerization coordinate. This is consistent with the decrease in quantum yield from 0.67 in rhodopsin to 0.47 in 13-demethylrhodopsin. The resonance Raman intensities show that the steric twist is reintroduced by addition of a methyl group at the C10 position. However, the quantum yield of 10-methyl-13-demethylrhodopsin is found to be only 0.35. This is attributed to nonisomorphous protein-analog interactions. The nonbonded interaction between the 10-hydrogen and the 13-methyl group in 11-cis-retinal makes this isomer particularly effective as the light-sensing chromophore in all visual pigments.