Conformation and other biophysical properties of cyclic antimicrobial peptides in aqueous solutions

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Abstract
As a step towards understanding the mechanism of the biological activity of cyclic antimicrobial peptides, the biophysical properties and conformations of four membrane-active cyclic peptide antibiotics, based on gramicidin S (GS), were examined in aqueous environments. These cyclic peptides, GS10 [cyclo(VKLdYP)2], GS12 [cyclo(VKLKdYPKVKLdYP)], GS14 [cyclo(VKLKVdYPLKVKLdYP)] and [d-Lys]4GS14 [cyclo(VKLdKVdYPLKVKLdYP)] (d-amino acid residues are denoted by d and are underlined) had different ring sizes of 10, 12 and 14 residues, were different in structure and amphipathicity, and covered a broad spectrum of hemolytic and antimicrobial activities. GS10, GS12 and [d-Lys]4GS14 were shown to be monomeric in buffer systems with ionic strength biological environments. GS14 was also monomeric at low concentrations, but aggregated at concentrations > 50 µm. The affinity of peptides for self-assembly and interaction with hydrophobic surfaces was related to their free energy of intermolecular interaction. The effects of variations in salt and organic solvent (trifluoroethanol) concentration and temperature on peptide conformation were also examined. Similar to GS, GS10 proved to have a stable and rather rigid conformation in different environments and over a broad range of temperatures, whereas GS12, GS14 and [d-Lys]4GS14 had more flexible conformations. Despite its conformational similarity to GS10, GS14 had unique physicochemical properties due to its tendency to aggregate at relatively low concentrations. The biophysical data explain the direct relation between structure, amphipathicity and hydrophobicity of the cyclic peptides and their hemolytic activity. However, this relation with the antimicrobial activity of the peptides is of a more complex nature due to the diversity in membrane structures of microorganisms.