title: Low-temperature solid-state 13C NMR studies of the retinal chromophore in rhodopsin.
abstract: 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.
mesh_terms: D000818:Animals; D002247:Carbon Isotopes; D002417:Cattle; D005615:Freezing; D009682:Magnetic Resonance Spectroscopy / Q000379:methods; D013058:Mass Spectrometry / Q000379:methods; D011487:Protein Conformation; D012168:Retinal Pigments; D012172:Retinaldehyde; D012176:Retinoids; D012243:Rhodopsin; D014212:Tretinoin
chemical_list: D002247:Carbon Isotopes; D012168:Retinal Pigments; D012176:Retinoids; D014212:Tretinoin; D012243:Rhodopsin; D012172:Retinaldehyde