Study of the Affinity of Structurally Different Antimicrobial Peptides to Model Membranes and Their Ability to Induce Membrane Permeabilization

Abstract

Antimicrobial peptides are present in the immune system of flora and fauna, and have attracted attention due to their great potential of lytic action against membranes of a wide range of microorganisms. Here we study the mode of action of two structurally different synthetic cationic antimicrobial peptides: gomesin, which adopts a β-hairpin structure due to two disulfide bridges, and the linear peptide esculentin 1b(1-18), which acquires an α-helix structure upon binding to amphiphilic surfaces. Additionally, different analogues of gomesin are also investigated: a linear one, and some other analogues in which specific residues are replaced by alanine. Our focus is to understand the relationship between peptide structure and its mode of action against liposomes composed of different molar ratios of a neutral and a negatively charged phospholipid, POPC/POPG, mimicking both bacterial and erythrocyte membranes. For this purpose, several techniques were employed: fluorescence measurements of the leakage of carboxyfluorescein (CF) entrapped in vesicles, isothermal titration calorimetry (ITC), and turbidity and zeta potential measurements. The fluorescence studies showed that the ability of all peptides to promote CF leakage from vesicles increases with the POPG molar ratio. The interaction of gomesin and its analogues with POPC/POPG membranes gives rise to exothermic peaks, whose magnitude increases with the POPG molar ratio. Turbidity measurements showed that the binding of these antimicrobial peptides to the membrane surface is accompanied by vesicle aggregation. According to zeta potential measurements, vesicle aggregation seems to start when the membrane surface charge is neutralized by the peptide binding. Financial support: FAPESP and INCT-FCx.