Versatile interactions of the antimicrobial peptide novispirin with detergents and lipids

Novispirin G-10 is an 18-residue designed cationic peptide derived from the N-terminal part of an antimicrobial peptide from sheep. This derivative is more specific for bacteria than the parent peptide. We have analyzed Novispirin's interactions with various amphipathic molecules and find that a remarkably wide variety of conditions induce alpha-helical structure. Optimal structure induction by lipids occurs when the vesicles contain 40-80% anionic lipid, while pure anionic lipid vesicles induce aggregation. SDS also forms aggregates with Novispirin at submicellar concentrations but induces alpha-helical structures above the cmc. Both types of aggregates contain significant amounts of beta-sheet structure, highlighting the peptide's structural versatility. The cationic detergent LTAC has a relatively strong affinity for the cationic peptide despite the peptide's net positive charge of +7 at physiological pH and total lack of negatively charged side chains. Zwitterionic and nonionic detergents induce alpha-helical structures at several hundred millimolar detergent. We have solved the peptide structure in SDS and LTAB by NMR and find subtle differences compared to the structure in TFE, which we ascribe to the interaction with an amphiphilic environment. Novispirin is largely buried in the SDS-micelle, whereas it does not enter the LTAC-micelle but merely forms a dynamic equilibrium between surface-bound and nonbound Novispirin. Thus, electrostatic repulsion can be overruled by relatively high-detergent concentrations or by deprotonating a single critical side chain, despite the fact that Novispirin's ability to bind to amphiphiles and form alpha-helical structure is sensitive to the electrostatics of the amphiphilic environment. This emphasizes the versatility of cationic antimicrobial peptides' interactions with amphiphiles.