Retinal Analog Study of the Role of Steric Interactions in the Excited State Isomerization Dynamics of Rhodopsin

Abstract

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.

Structural Analysis and Dynamics of Retinal Chromophore in Dark and Meta I States of Rhodopsin from 2H NMR of Aligned Membranes

“…This approach is analogous to solution NMR, where residual dipolar couplings are used with distance restraints from nuclear Overhauser effect (NOE) spectroscopy. A highly distorted structure is found for retinal in the dark state, consistent with vibrational 9,[30][31][32] and circular dichroism (CD) spectroscopy 30,31 , and with bioorganic studies of rhodopsin 33,34 . Despite constraints of the binding cavity the ligand undergoes restricted motion, as shown by the rapidly spinning methyl groups, with significant off-axial fluctuations.…”

Bioactivity of Visual Pigments with Sterically Modified Retinal Analogs

“…It has been shown that although the protein binding cavity is flexible to accommodate structurally modified retinals, it frequently leads to less efficient photoactivation and/ or less stable pigments in the ground state (21,39). This is also supported by the present finding that the stability and quantum efficiencies of 14-Me-Rh and 13-EtRh are lower than those of rhodopsin; the light-dependent PDE activity of 14-MeRh is also smaller.…”

Solid-state 2 H NMR relaxation illuminates functional dynamics of retinal cofactor in membrane activation of rhodopsin

“…Despite that three methyl environments are found for the retinal ligand in the dark state, upon 11-cis to trans isomerization ≈ two methyl sites are evident. Such a loss of retinal strain is consistent with two major planes of unsaturation following light absorption (1,34,(38)(39)(40). Application of transition state theory then allows us to connect the T 1Z results with the energetic parameters for retinal bound to rhodopsin (Table 1).…”