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

Abstract

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 μg mL−1 (IC50 values in the range of 0.02–2 μg mL−1). Most of the synthesized peptides showed no cytotoxic effects in an MTT assay up to the highest test concentration of 200 μg 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.

Small cationic antimicrobial peptides delocalize peripheral membrane proteins

“…The latter can be difficult to isolate and are complicated to synthesize chemically, but short cationic peptides are generated readily by solid-phase peptide synthesis and are easily accessible for chemical derivatization (5)(6)(7)(8). They are characterized by positively charged and by hydrophobic amino acids (9,10). Previous mechanistic studies in vitro have examined the interactions of the peptides with membranes or membrane extracts (11)(12)(13)(14)(15), but their effects on bacterial physiology have been largely underexplored.…”

Improvement of the Efficacy of Linear Undecapeptides against Plant-Pathogenic Bacteria by Incorporation of d -Amino Acids

“…In particular, incorporation of D-amino acids is an approach used to protect peptides against enzymatic hydrolysis, since only a few enzymes are known to digest amide bonds involving D-configuration (32). This strategy has been used to improve the biological activity profiles of synthetic antimicrobial peptides, not only increasing the resistance to proteolytic enzymes but also reducing the hemolytic activity while maintaining the antimicrobial activity (23,28,42,44,47,48,54).…”

Synthesis, stability and mechanistic studies of potent anticryptococcal hexapeptides

“…Further structural optimization by bio-screening of synthetic combinatorial libraries resulted in the identification of another antifungal peptide Arg-D-Trp-D-Phe-Ile-D-Phe-His-NH 2 that exhibit IC 50 value of 6.8 μM against Candida albicans [43, 44]. A detailed investigation by our group, of the above mentioned peptide resulted in several analogues with far superior antimicrobial activity [45]. In the present study, out of the many bioactive peptides, Orn-D-Trp-Cha-Ile-D-Phe- His(1-Bzl)-NH 2 ( 1 ) and Orn-D-Trp-D-Phe-Phe(4-Me)-D-Phe-His(1-Bzl)-NH 2 ( 2 ) were chosen for further in-depth structural optimization (Fig.…”