Effects of polyhydric alcohols on the conformational stability of the purple membrane.

The effect of different-sized polyhydric alcohols on the absorption and circular dichroic spectra of the purple membrane has been studied over the wavelength region 800 to 185 nm. Analysis of both the solution and the film spectra of the membrane revealed that these solvents induce conformational changes in the sole membrane protein, bacteriorhodopsin. Additional evidence supportive of these changes was obtained from protein fluorescence spectral studies. Although the net secondary structure of the bacteriorhodopsin is not observably altered, there is a reversible change in the protein tertiary structure. This change does not result in any significant change in the membrane crystallinity or the alignment of the protein helical polypeptide segments with respect to the membrane plane. However, it is of sufficient extent to change the protein-induced screw sense of the retinyl-chromophore symmetry and the local environments of the protein aromatic residues. The underlying mechanism appears to be a membrane surface-solvent interaction phenomenon since the spectral perturbation caused by these solvents appears to be independent of their effective sizes. Furthermore, partial enzymatic removal of the hydrophilic portion of the bacteriorhodopsins with papain increased the response of the membrane to this perturbation. An interpretation of these results is that polyhydric alcohols enhance hydrophobic interactions in the purple membrane which induces a more compact conformation of the bacteriorhodopsin. A possible molecular mechanism is presented. [ABSTRACT FROM AUTHOR]