Conformation and interaction of the cyclic cationic antimicrobial peptides in lipid bilayers

To investigate the role of peptide–membrane interactions in the biological activity of cyclic cationic peptides, the conformations and interactions of four membrane-active antimicrobial peptides [based on Gramicidin S (GS)] were examined in neutral and negatively charged micelles and phospholipid vesicles, using CD and fluorescence spectroscopy and ultracentrifugation techniques. Moreover, the effects of these peptides on the release of entrapped fluorescent dye from unilamellar vesicles of phosphatidylcholine (PC) and phosphatidylethanolamine/phosphatidylglycerol (PE/PG) were studied. The cyclic peptides include GS10 [Cyclo(VKLdYP)2], GS12 [Cyclo(VKLKdYPKVKLdYP)], GS14 [Cyclo(VKLKVdYPLKVKLdYP)] and [d-Lys]4GS14 [Cyclo(VKLdKVdYPLKVKLdYP)] (underlined residues are d-amino acids), were different in their ring size, structure and amphipathicity, and covered a broad spectrum of hemolytic and antimicrobial activities. Interaction of the peptides with the zwitterionic PC and negatively charged PE/PG vesicles were distinct from each other. The hydrophobic interaction seems to be the dominant factor in the hemolytic activity of the peptides, as well as their interaction with the PC vesicles. A combination of electrostatic and hydrophobic interactions of the peptides induces aggregation and fusion in PE/PG vesicles with different propensities in the order: [d-Lys]4GS14 > GS14 > GS12 > GS10. GS10 and GS14 are apparently located in the deeper levels of the membrane interfaces and closer to the hydrophobic core of the bilayers, whereas GS12 and [d-Lys]4GS14 reside closer to the outer boundary of the interface. Because of differing modes of interaction of the cyclic cationic peptides with lipid bilayers, the mechanism of their biological activity (and its relation to peptide–lipid interaction) proved to be versatile and complex, and dependent on the biophysical properties of both the peptides and membranes.