We designed de novo and synthesized two series of five 26-residue amphipathic α-helical cationic antimicrobial peptides (AMPs) with five or six positively charged residues (D-Lys, L-Dab (2,4-diaminobutyric acid) or L-Dap (2,3-diaminopropionic acid)) on the polar face where all other residues are in the D-conformation. Hemolytic activity against human red blood cells was determined using the most stringent conditions for the hemolysis assay, 18h at 37°C, 1% human erythrocytes and peptide concentrations up to 1000 μg/mL (~380 μM). Antimicrobial activity was determined against 7 Acinetobacter baumannii strains, resistant to polymyxin B and colistin (antibiotics of last resort) to show the effect of positively charged residues in two different locations on the polar face (positions 3, 7, 11, 18, 22 and 26 versus positions 3, 7, 14, 15, 22 and 26). All 10 peptides had two D-Lys residues in the center of the non-polar face as "specificity determinants" at positions 13 and 16 which provide specificity for prokaryotic cells over eukaryotic cells. Specificity determinants also maintain excellent antimicrobial activity in the presence of human sera. This study shows that the location and type of positively charged residue (Dab and Dap) on the polar face are critical to obtain the best therapeutic indices.
Keywords: Acinetobacter baumannii; Amphipathic α-helical peptides; Antimicrobial peptides (AMPs), Specificity determinants; Gram-negative pathogen; Hemolytic activity; Polar face positively charged residues (D-Lys, L-Dab and L-Dap).