Low-temperature solid-state 13C NMR studies of the retinal chromophore in rhodopsin

Magic angle sample spinning (MASS) 13C NMR spectra have been obtained of bovine rhodopsin regenerated with retinal prosthetic groups isotopically enriched with 13C at C-5 and C-14. In order to observe the 13C retinal chromophore resonances, it was necessary to employ low temperatures (-15-----35 degrees C) to restrict rotational diffusion of the protein. The isotropic chemical shift and principal values of the chemical shift tensor of the 13C-5 label indicate that the retinal chromophore is in the twisted 6-s-cis conformation in rhodopsin, in contrast to the planar 6-s-trans conformation found in bacteriorhodopsin. The 13C-14 isotropic shift and shift tensor principal values show that the Schiff base C = N bond is anti. Furthermore, the 13C-14 chemical shift (121.2 ppm) is within the range of values (120-123 ppm) exhibited by protonated (C = N anti) Schiff base model compounds, indicating that the C = N linkage is protonated. Our results are discussed with regard to the mechanism of wavelength regulation in rhodopsin.