New Antimicrobial Hexapeptides: Synthesis, Antimicrobial Activities, Cytotoxicity, and Mechanistic Studies

Synthetic antimicrobial peptides have recently emerged as promising candidates against drug-resistant pathogens. We identified a novel hexapeptide, Orn-D-Trp-D-Phe-Ile-D-Phe-His(1-Bzl)-NH2, which exhibits broad-spectrum antifungal and antibacterial activity. A lead optimization was undertaken by conducting a full amino acid scan with various proteinogenic and non-proteinogenic amino acids depending on the hydrophobic or positive-charge character of residues at various positions along the sequence. The hexapeptide was also cyclized to study the correlation between the linear and cyclic structures and their respective antimicrobial activities. The synthesized peptides were found to be active against the fungus Candida albicans and Gram-positive bacteria such as methicillin-resistant Staphylococcus aureus and methicillin-resistant Staphylococcus epidermidis, as well as the Gram-negative bacterium Escherichia coli; MIC values for the most potent structures were in the range of 1-5 mu g mL(-1) (IC50 values in the range of 0.02-2 mu g mL(-1)). Most of the synthesized peptides showed no cytotoxic effects in an MTT assay up to the highest test concentration of 200 mu g mL(-1). A tryptophan fluorescence quenching study was performed in the presence of negatively charged and zwitterionic model membranes, mimicking bacterial and mammalian membranes, respectively. The results of the fluorescence study demonstrate that the tested peptides are selective toward bacterial over mammalian cells; this is associated with a preferential interaction between the peptides and the negatively charged phospholipids of bacterial cells.