Solid-State ¹³C NMR Studies of Retinal in Bacteriorhodopsin

Solid-state ¹³C magic-angle sample spinning (MASS) NMR has been used to study lyophilized dark-adapted purple membrane containing ¹³C-labeled retinals. C-10-, C-11-, and C-12-labeled derivatives each showed two lines, assigned to the coexisting 13-cis and all-trans isomers. The isotropic chemical shifts, particularly of C-11, indicate that the Schiff base is protonated. Shift anisotropies are also similar to those of model compounds, indicating that this part of the chromophore is rigid and immobile and possesses the same degree of in-plane bending as crystalline retinal derivatives. Purple membrane samples labeled on the C-19- and C-20-methyl groups both give single lines from the retinal, upfield shifted by 2.1 and 1.0 ppm, respectively, from model compounds. In all cases, high-quality spectra were obtained from ~50-mg samples in modest signal-averaging times. These results suggest that it is now practical to exploit the enormous potential of MASS NMR for structural studies of ¹³C-labeled membrane proteins.