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 qu antum yield experiments. The resonance Raman spectra and CD spectra of 13-demethylrhodopsin indicate that its chromophore, an analog in whic h the nonbonded interaction between the 10-H and the 13-CH3 groups is removed, is less distorted in the C-10...C-13 region than the native c hromophore. The reduced torsional and hydrogen-out-of-plane resonance Raman intensities further indicate that the excited state potential en ergy surface has a much shallower slope along the isomerization coordi nate. This is consistent with the decrease in quantum yield from 0.67 in rhodopsin to 0.47 in 13-demethylrhodopsin. The resonance Raman inte nsities show that the steric twist is reintroduced by addition of a me thyl group at the C-10 position. However, the quantum yield of 10-meth yl-13-demethylrhodopsin is found to be only 0.35. This is attributed t o nonisomorphous protein-analog interactions, The nonbonded interactio n between the 10-hydrogen and the 13-methyl group in 11-cis-retinal ma kes this isomer particularly effective as the light-sensing chromophor e in all visual pigments.