Lysine Position Affects Binding of Aib-Rich Model Antibiotics to Lipid Vesicles

Abstract

Due to the increased development of drug resistance and decrease of antibiotic drug effectiveness, there is an immediate need for new antimicrobials. A possible, promising solution is in the form of charged peptide-based molecules, which can function as antibiotics by interacting with and disturbing bacterial membranes. We are exploring these interactions using model peptides composed of hydrophobic, branched amino acid Aib (α-aminoisobutyric acid), and large unilamellar vesicles (LUVs) composed of DMPG and DMPC. These LUVs model bacterial (negatively charged) and non-bacterial (neutral) cell membranes, respectively. Aib naturally occurs in antibiotics used by some bacteria and biases peptides to adopt a helical structure. We present results using two model peptides in which two positively charged lysine molecules were placed in either adjacent positions of the helix (KK45), or a helical turn apart (KK36). Interactions between the LUVs and peptides are investigated using Isothermal Titration Calorimetry (ITC), resulting in binding enthalpies, entropies, and binding constants. Our initial results indicate that both KK45 and KK36 bind in a multi-stage interaction to the DMPG vesicles (bacterial models), while showing very little affinity when binding to DMPC (non-bacterial models). One stage appears to be enthalpy-driven and is consistent with electrostatic interactions between peptide sidechains and lipid headgroups, while the other stage appears to be entropy-driven and is consistent with hydrophobic interactions. Overall KK45 binds more favorably to DMPG vesicles, which is interesting given that the KK45 helix is kinked, while the KK36 helix is not.