Equilibrium Composition of Retinal Isomers in Dark-Adapted Bacteriorhodopsin and Effect of High Pressure Probed by Near-Infrared Raman Spectroscopy

The combination of near-infrared Raman spectroscopy and variations in external parameters offers new opportunities for site-specific studies of proteins. Using excitation at 840 nm, we have measured the near-infrared Raman spectrum of dark-adapted bacteriorhodopsin at ambient and high pressure. The C=C ethylenic stretching region shows two resolved bands at 1526 and 1534 cm −1 , corresponding to the all- trans and 13- cis isomers. From deconvolution of these bands we find an isomeric ratio between 13- cis and all- trans retinal equal to 1 at ambient pressure. The Raman spectrum gives direct spectroscopic evidence that the 13- cis component is favored at high pressure, implying that it has a smaller volume. The pressure dependence of the isomeric ratio yields a molar volume of −6.6 mL/mol, which suggests ionization of one or two residues or the formation of three hydrogen bonds.