The chromophore-binding site of bacteriorhodopsin. Resonance Raman and surface-enhanced resonance Raman spectroscopy and quantum chemical study

Surface-enhanced Raman spectra of membrane protein, located in native mem brane, bacteriorhodopsin, adsorbed by silver electrodes and hydrosols have been obtained for the first time. The distance between the retinal Schiff’s base and the external side of purple membrane of Halobacteriim halobiim was shown to be 6–9 A. The possible distribition of the point charges aroind protonated retinal Schiff’s base has been proposed on the basis of the resonance Raman data and quantim chemical CNDO/S-CI calculations. Such a model contains tyrosine residue located near the retinal Schiff’s base and connected with COO^- groipvia hydrogen bond COO^- group acts as a protonated Schiff’s base counterion. The distance between oxygen atoms of COO^- group and retinal Schiff’s base plane is 2.5–3.0A. The hydrogen bond (O-H. . .O^-) length between oxygen atom of OH-group and oxygen atom of COO^- group has been chosen 2.7±0.1Å Tyrosine hydroxyl group is located at 2.8–3.5 A from retinal Schiff’s base plane. It was shown that in contrast to generally accepted Honig and Nakanishi model the spectral properties of Brh570, K610, L550 and M4Ï2 forms of bacteriorhodopsin photocycle as well as observed tyrosine deprotonation and COO^- group protonation during M412 formation can be explained reasonably well by the suggested charge distribution. Furthermore, such a model of bacteriorhodopsin active site microenvironment allows to explain catalyzing of photo-induced protonated retinal Schiff’s base deprotonation observed in our preliminary experiments.