The effect of polypeptide-lipid interactions on the ultraviolet spectrum of the olefinic bonds of the lipid.

The effect of associating acidic and basic polypeptides with dilute suspensions of vesicles composed of various unsaturated phospholipids was assessed with regard to optical density and ultraviolet absorption. Associating basic polypeptides with phosphatidylserine, phosphatidylethanolamine, and phosphatidylglycerol vesicles, or acidic polypeptide with phosphatidylcholine vesicles, caused an increase in the optical density of the preparations, with no measurable effect on the intensity of the ultraviolet spectrum of the olefinic bonds of the lipid. Associating basic polypeptides with phosphatidylcholine vesicles, in addition to causing similar increases in optical density, resulted in a large decrease in the intensity of ultraviolet absorption by the olefinic bonds. This implies that the interaction between the basic polypeptides and phosphatidylcholine vesicles results in major alterations in the microenvironment of the olefinic bonds, which would require intimate association of the polypeptide with the ninth carbon region of the acyl chains. These observations support the conclusion, drawn from our earlier studies, that the association of basic polypeptides and liquid crystalline phase phosphatidylcholine vesicles is peculiar in that it involves a major hydrophobic component.