Sequence Permutation of Positive Charges in a Model Peptide Antibiotic Produces Differing Enthalpic and Entropic Contributions to the Lipid-Peptide Binding Affinity

Abstract

With rising disease rates and decreasing effectiveness of conventional antibiotics, there is an immediate need for new antibiotics. One promising solution is through cationic antimicrobial peptides, which act by perturbing bacterial membranes. The interaction of model peptides with membranes can provide valuable information about the role of helical structure and peptide charge distribution on peptide-membrane interactions. We are investigating model peptide antibiotics composed primarily of the hydrophobic dialkylated amino acid Aib (α-aminoisobutyric acid), which imparts a strong 310-helical bias due to steric hindrance at the α-carbon. Cationic lysine residues are placed in adjacent locations in the center of the helix (KK45) or one full turn apart (KK36). Large unilamellar vesicles (LUVs) composed of DMPC or DMPG were used as zwitterionic or anionic membrane models, respectively. NMR data indicates that in a membrane mimicing solvent (DMSO), KK45 adopts a kinked helical stucture, while KK36 adopts a canonical helical structure. We have previously seen that KK45 binding to anionic micells is more favorable than binding of KK36 to anionic micelles. However, unlike binding to micelles, NMR data suggests that binding of peptides to DMPG vesicles is more favorable for KK36 than for KK45. Here we present thermodynamic data characterizing the peptide-LUV interactions using isothermal titration calorimetry (ITC). Preliminary data suggests that binding to DMPG vesicles is exothermic, while binding to DMPC vesicles is endothermic. Complete characterization of the enthalpy of binding, entropy of binding, and thermodynamic binding constant will allow us to elucidate the relationship between peptide structure and the favorability of membrane binding.