Determining structural changes in model membranes of Gram-negative bacteria during their interaction with antimicrobial peptides: spectroscopic and microscopic study under electrochemical control

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Antimicrobial peptides (AMPs) are small positively charged polypeptides which interact with microbial cell membranes leading to a distortion of the membrane structure and cell death. By contrast, the interaction of AMPs with cell of a host organism affects neither the membrane integrity nor structure. The recognition at the molecular level of changes at lipids and peptides involved in this interaction is crucial to understand the selectivity of peptides to microbes, reactions occurring in the innate immune system and to develop a new generation of antibiotics. Fabrication of models of lipid membranes of pathogens and of host organisms coupled to in situ structure analyzing techniques may help us to understand molecular aspects of this interaction and the specificity of antimicrobial peptides to certain membranes. Since the composition and supramolecular structure of bacterial cell membranes differs significantly from that of mammalian membranes. Highly asymmetric lipid bilayers mimicking closely the composition and structure of Gram-negative bacteria will be produced using Langmuir-Blodgett transfer. Phospholipid rich symmetric and asymmetric bilayers will be used as models of mammalian cell membranes. Cecropin and melittin peptides will be used in this study. Electrostatic interactions play an important role in the maintenance of the structure of bacterial cell membranes as well as in binding of an AMP to the cell surface. In the result of this interaction the electrochemical properties of the membrane change. A deposition of the lipid bilayer on the electrode surface allows in situ monitoring of changes of the electrochemical properties of the membrane due to its interaction with AMPs. In order to determine in situ changes in the membrane structure polarization modulation infrared reflection absorption spectroscopy and scanning force microscopy under electrochemical control will be used. The proposed approach will provide, at sub-molecular level, information on the orientation, conformation and hydration of polar and hydrophobic parts of each lipid layer of model membranes as well as the structure and orientation of the interacting peptide as a function of electric field acting at the membrane. In this way a holistic picture of structural changes in lipid membranes exposed to physiological electric fields will be provided aiming at the elucidation of molecular aspects of antimicrobial peptides interactions with cell membranes.