Effect of salt on the interaction of Hal18 with lipid membranes

One of the major obstacles in the development of new antimicrobial peptides as novel antibiotics is salt sensitivity. Hal18, an alpha-helical subunit of Halocidin isolated from Halocynthia aurantium, has been previously shown to maintain its antimicrobial activity in high salt conditions. The alpha-helicity of Hal18 in the presence and absence of salt was demonstrated by circular dichroism spectroscopy, which showed that the peptide was mainly unordered containing beta-strands and beta-turns. However, in the presence of dimyristoylphosphatidylcholine (DMPC) and dimyristoylphosphatidylserine (DMPS) vesicles, Hal18 folded to form alpha-helices (circa 42 %). Furthermore, the structure was not significantly affected by pH or the presence of metal ions. These data were supported by monolayer results showing Hal18 induced stable surface pressure changes in monolayers composed of DMPC (5 mN m(-1)) and DMPS (8.5 mN m(-1)), which again were not effected by the presence of metal ions or pH. It is proposed that the hydrophobic groove within its molecular architecture enables the peptide to form stable associations with lipid membranes. The balance of hydrophobicity along the Hal18 long axis would also support oblique orientation of the peptide at the membrane interface. Hence, this model of membrane interaction would enable the peptide to penetrate deep into the membrane. This concept is supported by lysis data. Overall, it would appear that this peptide is a potential candidate for future AMP design for use in high salt environments.