1H and 13C MAS NMR evidence for pronounced ligand-protein interactions involving the ionone ring of the retinylidene chromophore in rhodopsin

Rhodopsin is a member of the superfamily of G-protein-coupled receptors. This seven α-helix transmembrane protein is the visual pigment of the vertebrate rod photoreceptor cells that mediate dim light vision. In the active binding site of this protein the ligand or chromophore, 11- cis -retinal, is covalently bound via a protonated Schiff base to lysine residue 296. Here we present the complete ¹ H and ¹³ C assignments of the 11- cis -retinylidene chromophore in its ligand-binding site determined with ultra high field magic angle spinning NMR. Native bovine opsin was regenerated with 99% enriched uniformly ¹³ C-labeled 11- cis -retinal. From the labeled pigment, ¹³ C carbon chemical shifts could be obtained by using two-dimensional radio frequency-driven dipolar recoupling in a solid-state magic angle spinning homonuclear correlation experiment. The ¹ H chemical shifts were assigned by two-dimensional heteronuclear ( ¹ H- ¹³ C) dipolar correlation spectroscopy with phase-modulated Lee–Goldburg homonuclear ¹ H decoupling applied during the t ₁ period. The data indicate nonbonding interactions between the protons of the methyl groups of the retinylidene ionone ring and the protein. These nonbonding interactions are attributed to nearby aromatic acid residues Phe-208, Phe-212, and Trp-265 that are in close contact with, respectively, H-16/H-17 and H-18. Furthermore, binding of the chromophore involves a chiral selection of the ring conformation, resulting in equatorial and axial positions for CH ₃ -16 and CH ₃ -17.