HIGH-PRESSURE NEAR-INFRARED RAMAN-SPECTROSCOPY OF BACTERIORHODOPSIN LIGHT TO DARK-ADAPTATION

Near-infrared (NIR) Raman spectroscopy is employed as an in situ probe of the chromophore conformation to study the light to dark-adaptation process in bacteriorhodopsin (bR) at variable pressure and temperature in the absence of undesired photoreactions. in dark-adapted bR deconvolution of the ethylenic mode into bands assigned to the all-trans (1526 cm(-1)) and 13-cis (1534 cm(-1)) isomers yields a 13-cis to all-trans ratio equal to 1 at ambient pressure (Schulte et al., 1995, Appl. Spectrosc. 49:80-83). Detailed spectroscopic evidence is presented that at high pressure the equilibrium is shifted toward the 13-cis isomers acid that the light to dark adaptation kinetics is accelerated. The change in isomeric composition with temperature and pressure as well as the kinetics support a two-state model with activation volumes of -16 ml/mol for the transition of 13-cis to all-trans and -22 ml/mol for the reverse process. These compare with a conformational volume difference of 6.6 ml/mol, which may be attributed to the ionization of one or two residues or the formation of three hydrogen bonds.