Impact of the antimicrobial peptide Novicidin on membrane structure and integrity

We have studied the impact of an 18-residue cationic antimicrobial peptide Novicidin (Nc) on the structure and integrity of partially anionic lipid membranes using oriented circular dichroism (OCD), quartz crystal microbalance with dissipation (QCM-D), dual polarization interferometry (DPI), calcein dye leakage and fluorescence spectroscopy. OCD consistently showed that Nc is bound in an α-helical, surface bound state over a range of peptide to lipid (P/L) ratios up to ~1:15. Realignment of Nc at higher P/L ratios correlates to loss of membrane integrity as shown by Laurdan fluorescence spectroscopy and by loss of lipid alignment in DPI analysis. Laurdan generalized polarity shows a decrease in water accessibility or mobility in the hydrophobic/hydrophilic interface of the lipid membrane, consistent with rearrangement of lipid packing. QCM-D studies on the interaction of Nc with lipid membranes emphasize the importance of including the dissipation factor in data analysis, revealing formation of a highly hydrated film after exposure to ≥3 μM Nc. Our findings suggest a carpet mechanism of membrane disruption in which peptide binding first induces leakage at a critical surface concentration, probably through formation of transient pores or transient disruption of the membrane integrity, followed by more extensive membrane disintegration at higher P/L ratios.