Gg. Kochendoerfer et al., RETINAL ANALOG STUDY OF THE ROLE OF STERIC INTERACTIONS IN THE EXCITED-STATE ISOMERIZATION DYNAMICS OF RHODOPSIN, Biochemistry, 35(50), 1996, pp. 16230-16240
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.