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

Rhodopsin is a prototype for G protein-coupled receptors (GPCRs) that are implicated in many biological responses in humans. A site-directed 2H NMR approach was used for structural analysis of retinal within its binding cavity in the dark and pre-activated meta I states. Retinal was labeled with 2H at the C5, C9, or C13 methyl groups by total synthesis, and was used to regenerate the opsin apoprotein. Solid-state 2H NMR spectra were acquired for aligned membranes in the low-temperature lipid gel phase versus the tilt angle to the magnetic field. Data reduction assumed a static uniaxial distribution, and gave the retinylidene methyl bond orientations plus the alignment disorder (mosaic spread). The dark-state 2H NMR structure of 11-cis-retinal shows torsional twisting of the polyene chain and the β-ionone ring. The ligand undergoes restricted motion, as evinced by order parameters of ≈ 0.9 for the spinning C–C2H3 groups, with off-axial fluctuations of ≈ 15°. Retinal is accommodated within the rhodopsin binding pocket with a negative pre-twist about the C11 = C12 double bond that explains its rapid photochemistry and the trajectory of 11-cis to trans isomerization. In the cryo-trapped meta I state, the 2H NMR structure shows a reduction of the polyene strain, while torsional twisting of the β-ionone ring is maintained. Distortion of the retinal conformation is interpreted through substituent control of receptor activation. Steric hindrance between trans retinal and Trp265 can trigger formation of the subsequent activated meta II state. Our results are pertinent to quantum and molecular mechanics simulations of ligands bound to GPCRs, and illustrate how 2H NMR can be applied to study their biological mechanisms of action.