Application of NMR-spectroscopy to retinal proteins

The application of NMR spectroscopy to the retinal proteins bacteriorhodopsin (BR) and rhodopsin is reviewed. H-2-, N-15-, and C-13-solid-state NMR spectroscopy have contributed considerably to understanding the conformation and chemical environment of the protonated retinylidene Schiff base in BR as well as in rhodospin. The data from both pigments clarified the mechanism of the opsin shift which is quite different for rhodopsin and BR. An analysis of the chemical shifts of isotopically labeled aspartic acid, tyrosine, and proline incorporated into BR. provided evidence for the protonation state of Asp and Tyr, and the isomerization state of the Xaa-Pro peptide bond, respectively. Solid-state NMR spectroscopy was also applied to the investigation of the photocycle intermediates of BR, as well as bathorhodopsin and metarhodopsin TI, which are formed after light-activation of rhodopsin. Solution NMR spectroscopy of BR solubilized in detergents or organic solvents, as well as of opsin-derived peptide also applied to the investigation of the two- and three-dimensional structure of BR.