Raman spectroscopic study of the interaction of poly-L-lysine with dipalmitoylphosphatidylglycerol bilayers

The interaction of the basic polypeptide poly-L-lysine with the negatively charged phospholipid dipalmitoylphosphatidylglycerol was studied using Raman spectroscopy. The nature of the interaction appeared to depend on the molar ratio of the constituents. At up to one lysine group per lipid molecule, the bilayer was stabilized by the polypeptide that underwent a conformational transition toward an ordered alpha-helical structure, in which the electrostatic interactions were probably maximal. The stabilization of the bilayer was detected by an increase in both the temperature of the thermotropic transition of the lipid and the interchain vibrational coupling of the methylene C-H vibrations. At higher poly-L-lysine concentration, hydrophobic interactions must have been involved to explain the binding of excess polypeptide. There seemed to be a penetration of poly-L-lysine in the bilayer that increased with the polypeptide concentration. Under these conditions, the chain-packing lattice gradually changed from hexagonal to either orthorhombic or monoclinic symmetry. We believe that this change of structure is associated with the interdigitation of the acyl chains.