Influence of Proline Residues on the Antibacterial and Synergistic Activities of α-Helical Peptides

Abstract

To investigate the influence of proline residues on the activity of α-helical peptides, variants were synthesized with insertions of proline residues to create peptides without proline, or with one or two prolines. The influence of the proline-induced bends was assessed by circular dichroism in the presence of liposomes, and the ability of the peptides to kill microorganisms, to permeabilize the outer and cytoplasmic membranes of Escherichia coli, to bind to liposomes, to form channels in planar lipid bilayers, and to synergize with conventional antibiotics. Representative peptides adopted α-helical conformations in phosphatidylcholine/phosphatidylglycerol (POPC/POPG, 7:3) liposomes as well as in 60% trifluoroethanol solution, as revealed by circular dichroism (CD) spectroscopy. However, the percent of helicity decreased as the number of proline residues increased. Tryptophan fluorescence spectroscopy showed that all of these peptides inserted into the membranes of liposomes as indicated by a blue shift in the emission maximum and an increase in the fluorescence intensity of the single tryptophan at residue 2. Quenching experiments further prove that the tryptophan residue was no longer accessible to the aqueous quencher KI. The peptide that lacked proline exhibited the highest activity [minimal inhibitory concentrations (MICs) of 0.5−4 μg/mL] against all tested Gram-negative and Gram-positive bacteria, but was hemolytic at 8 μg/mL. The single-proline peptides exhibited intermediate antibacterial activity. Peptides with two proline residues were even less active with moderate MICs only against E. coli. With only one exception from each group, the peptides were nonhemolytic. The ability of the peptides to demonstrate synergy in combination with conventional antibiotics increased as the antibacterial effectiveness decreased. All peptides bound to bacterial lipopolysaccharide and permeabilized the outer membrane of E. coli to similar extents. However, their ability to permeabilize the cytoplasmic membrane of E. coli as assessed by the unmasking of cytoplasmic β-galactosidase decreased substantially as the number of proline residues increased. Correspondingly, increasing the number of proline residues caused a decreased ability to form channels in planar lipid bilayers, and the hemolytic, proline-free peptide tended to cause rapid breakage of planar membranes. Thus, the number of bends created by insertion of proline residues is an important determinant of antimicrobial, hemolytic, and synergistic activity.