Arginine activity in the proton-motive photocycle of bacteriorhodopsin: Solid-state NMR studies of the wild-type and D85N proteins

N-15 solid-state NMR (SSNMR) spectra of guanidyl-N-15-labeled bacteriorhodo psin (bR) show perturbation of an arginine residue upon deprotonation of th e retinal Schiff base during the photocycle. At the epsilon position, an up field shift of 4 ppm is observed while the eta nitrogens develop a pair of 'wing' peaks separated by 24 ppm. Proton-driven spin diffusion between the two 'wing' peaks indicates that they arise from a single Arg residue. An un usually asymmetric environment for this residue is indicated by comparison with guanidyl-N-15 chemical shifts in a series of arginine model compounds. The 'wing' peaks are tentatively assigned to Arg-82 on the basis of the SS NMR investigations of the alkaline and neutral dark-adapted forms of the D8 5N bacteriorhodopsin mutant. Another, less asymmetric pair of eta signals, that is not: affected by Schiff base deprotonation or D85 mutation, is tent atively assigned to Arg-134. The results are discussed in relation to exist ing models of bR structure and function.