Biophysical approaches to antimicrobial peptide–membrane characterization

Antimicrobial peptides (AMPs) represent a current strategy to develop new antibiotics against multi-resistant pathogens. The potential antibiotic activity of AMPs is related to their amphipathic properties and the presence of positively charged residues, which may interact with the negatively charged bacterial membranes. In contrast, they exhibit lower interaction with the eukaryotic, neutrally charged membranes. This is the primary reason AMPs can distinguish between eukaryotic and prokaryotic membranes. AMPs are usually modified or designed de novo, and their properties can be changed by inserting specific amino acid residues into their sequence. To assist in the rational design of AMPs, it is helpful to explore the biophysical changes they may induce in target cell membranes. Therefore, bacterial and eukaryotic model lipid membranes have been extensively used for this purpose. Parameters such as selective binding, lipid membrane interactions, membrane packing, permeability, hydration, and restructuring facilitate the exploration of peptide regions of interest. These parameters can be studied using various physicochemical techniques, including differential scanning calorimetry, X-ray diffraction, nuclear magnetic resonance, and fluorescence spectroscopy. This review aims to provide a practical guide to the main biophysical techniques used to explore the potential antibiotic activity of AMPs using model membranes, and to examine lipid-peptide interactions in order to define the mechanisms of action of these antimicrobial peptides. These techniques determine whether the peptide interacts specifically with bacterial membranes, the preferred bacterial target of a given AMP, the binding affinities of AMPs, potential pore formation and its geometry, and the impact of these interactions on both bacterial and eukaryotic membranes.