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

As a step towards understanding the mechanism of the biological activity of cyclic antimicrobial peptides, the biophysical properties and conformation s 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(VKLKVdYPLKVKLd YP)] and [D-Lys](4)GS14 [cyclo(VKLdKVdYPLKVKLdYP)] (D-amino acid residues a re 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, bu t aggregated at concentrations > 50 mum. The affinity of peptides for self- assembly and interaction with hydrophobic surfaces was related to their fre e energy of intermolecular interaction. The effects of variations in salt a nd organic solvent (trifluoroethanol) concentration and temperature on pept ide conformation were also examined. Similar to GS, GS10 proved to have a s table and rather rigid conformation in different environments and over a br oad range of temperatures, whereas GS12, GS14 and [D-Lys](4)GS14 had more f lexible conformations. Despite its conformational similarity to GS10, GS14 had unique physicochemical properties due to its tendency to aggregate at r elatively low concentrations. The biophysical data explain the direct relat ion between structure, amphipathicity and hydrophobicity of the cyclic pept ides and their hemolytic activity. However, this relation with the antimicr obial activity of the peptides is of a more complex nature due to the diver sity in membrane structures of microorganisms.