Abstract

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-<i>trans</i> and 13-<i>cis</i> isomers. From deconvolution of these bands we find an isomeric ratio between 13-<i>cis</i> and all-<i>trans</i> retinal equal to 1 at ambient pressure. The Raman spectrum gives direct spectroscopic evidence that the 13-<i>cis</i> 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.

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