Small Cationic Peptides: Influence of Charge on Their Antimicrobial Activity

Abstract

The first stage of the action mechanism of small cationic peptides with antimicrobial activity is ruled by electrostatic interactions between the peptide and the pathogen cell membrane. Thus, an increase in its activity could be expected with an increase in the positive charge on the peptide. By contrast, the opposite behavior has been observed when the charge increases to reach a critical value, beyond which the activity falls. This work studies the perturbation effects in a cell membrane model for two small cationic peptides with similar length and morphology but with different cationic charges. The synthesis and antibacterial activity of the two peptides used in this study are described. The thermodynamic study associated with the insertion of these peptides into the membrane and the perturbing effects on the bilayer structure provide valuable insights into the molecular action mechanism associated with the charge of these small cationic peptides.

Figure 1

Edmundson representation obtained for the two peptides, pep+4 and pep+7.

Figure 2

Electrostatic potential-encoded electron density surfaces of the structures of peptides pep+4 (c,d) and pep+7 (a,b). The coloring represents electrostatic potential with red indicating the strongest attraction to a positive point charge and blue indicating the strongest repulsion. The electrostatic potential is the energy of interaction of the positive point charge with the nuclei and electrons of a molecule. It provides a representative measure of overall molecular charge distribution.

Figure 3

Peptide and DPPC phosphorous (P) distribution after 100 ns of simulation time. Left column corresponds to pep+4 and right column to pep+7. Labels (a–c) refer to peptide/phospholipid ratios of 1/32, 1/16, and 1/8, respectively.

Figure 4

Snapshot of the starting configuration of DPPC bilayers in the presence of the two peptides studied in this work. In both cases, the peptides were placed near one leaflet of the lipid bilayer. (Up) DPPC in the presence of 40 pep+4 (yellow) and (down) DPPC in the presence of 40 pep+7 (red). Blue beads correspond to chloride ions used to balance the total charge existing in the system. Water has been removed for clarity.

Figure 5

Simulated deuterium order parameters—S_CD along the DPPC hydrocarbon tail of a DPPC bilayer, for different peptide/phospholipid ratios, (a) pep+4 and (b) pep+7. Circles correspond to the experimental data of a DPPC bilayer in the absence of peptides at 350 K.

Figure 6

Phosphorous distribution across the lipid bilayer for different peptide/phospholipid ratios in the presence of pep+4 (a) and pep+7 (b).

Figure 7

Lateral pressure π(z) of the DPPC bilayer in the absence and presence of peptides for different peptide/phospholipid ratios. (a) Corresponds to pep+4 and (b) to pep+7.

Figure 8

(a) Free-energy profile associated with the insertion of cationic peptides into a DPPC bilayer. (b) Atomic phosphorous distribution across the DPPC bilayer.

Figure 9

ΔG, ΔH, and ΔS associated with the insertion of pep+4 and pep+7 into a DPPC bilayer.