Effect of high pressure on the absorption spectrum and isomeric composition of bacteriorhodopsin

The effects of high pressure upon the absorption spectra and isomeric composition of the dark (bRD) and light adapted (bRL) forms of bacteriorhodopsin were examined. Pressure favors the 13-cis form of bacteriorhodopsin (bR13-cis). The equilibrium isomeric composition and absorption spectra of bacteriorhodopsin samples at a given pressure were the same starting from either light or dark adapted bacteriorhodopsin. From the effect of pressure on the equilibrium constant between bRall-trans in equilibrium bR13-cis in the dark, the molar volume change between bRall-trans and bR13-cis was found to be -7.8 +/- 3.2 ml/mol. This volume change suggests a difference in conformation between dark- and light-adapted bacteriorhodopsin, but the magnitude of the change is small, involving only a small number of the protein residues.