LIPID-PROTEIN INTERACTIONS IN THE PURPLE MEMBRANE - STRUCTURAL SPECIFICITY WITHIN THE HYDROPHOBIC DOMAIN

In the absence of native-like interactions between bacteriorhodopsin a nd its neighbouring lipids, the pigment chromophore is reversibly titr ated from its purple 570 nm form to a blue-shifted 480 nm form in the moderately alkaline pH range. Quantitation of this acid-base chromopho re equilibrium in vesicles prepared from modified lipid mixtures shows that it is absent under conditions where bacteriorhodopsin is allowed to interact with methyl-substituted alkyl chains. The peculiar homoge neous structure of purple membrane alkyl chain lipids is thus likely t o be an essential requirement for maintenance of the native bacteriorh odopsin structure over a wide pH range.