The interaction of bioactive peptides with an immobilized phosphatidylcholine monolayer

The interaction of three bioactive peptides, bombesin, beta-endorphin, and glucagon with a phosphatidylcholine monolayer that was immobilized to porou s silica particles and packed into a stainless steel column cartridge, has been studied using dynamic elution techniques. This immobilized lipid monol ayer provides a biophysical model system with which to study the binding of peptides to a lipid membrane. In particular,the influence of temperature a nd methanol concentration on the affinity of each peptide for the immobiliz ed lipid surface was assessed. For all test peptides, nonlinear retention p lots were observed at all temperatures that contrasted sharply with the sim ple linear plots observed for the small unstructured control molecules N-ac etyltryptophanamide and diphenylalanine. An analysis of the thermodynamics of the interaction of peptides with the immobilized monolayer was also carr ied out. The results revealed that while the peptides interacted with the m onolayer predominantly through hydrophobic interactions, the relative contr ibution of Delta H-assoc(O) and Delta S-assoc(O) to the overall free energy of association was dependent on the temperature and methanol concentration . In particular, it was evident that under most conditions, the binding of the peptides to the immobilized lipid monolayer was enthalpy-driven, i.e., mediated by nonclassical hydrophobic interactions. Significant band-broaden ing and asymmetric and split peaks were also observed for bombesin, beta-en dorphin, and glucagon at different temperatures and methanol concentrations . These changes in affinity and peak shape are consistent with the formatio n of multiple conformational species during the interaction of these peptid es with the lipid monolayer. In addition, the binding behavior of the three test peptides on an n-octylsilica surface that lacked the phospho headgrou ps of the phospholipid was significantly different from that observed with the immobilized phosphatidylcholine surface, indicating a specificity of in teraction between the peptides and the lipid surface. Overall, these experi mental results demonstrate that the biomimetic phosphatidylcholine monolaye r provides a stable and sensitive system with which to explore the molecula r mechanism of peptide conformational changes during membrane interactions.