Synthesis of CD3-Labeled 11-cis-Retinals and Application to Solid-State Deuterium NMR Spectroscopy of Rhodopsin#

Efficient synthesis of 11-Z-retinals labeled with 2H at the C5, C9, or C13 methyl groups is described. The 2H-labeled retinals were used to regenerate the visual pigment rhodopsin for structural investigations. Solid-state 2HNMR data provided the orientation of retinal within the rhodopsin binding pocket as well as its conformation. Extension of the approach to other membrane receptors can yield knowledge of their mechanisms of activation as a guide for ligand-based drug design. Site-directed nuclear spin labeling in combination with solid-state NMR spectroscopy can give unique information regarding the conformation and orientation of ligands bound to integral membrane proteins and G protein-coupled receptors such as rhodopsin. Biomolecular applications of NMR typi-cally rely on the availability of suitable isotopically labeled compounds. Effectively, two strategies are possible: either the molecules are uniformly labeled, e.g. with 13C, 15N, or 2H isotopes through bacterial expression in the case of proteins,1–4 or rather organic synthesis is used to introduce isotopic label