Anionic lipid membrane electrostatic potential and solution pH can influence cationic peptide adsorption to these bilayers, especially those containing simultaneously acid and basic residues. Here, we investigate the effects of the pH solution on MP1 (IDWKKLLDAAKQIL-NH2) adsorption to anionic (7POPC:3POPG) lipid vesicles in comparison to its analog H-MP1, with histidines substituting lysines. We used the association of adsorption isotherms and constant pH molecular dynamic simulations (CpHMD) to explore the effects of membrane potential and pH on peptides’ adsorption on this lipid membrane. We analyzed the fluorescence and zeta potential adsorption isotherms using the Gouy–Chapman theory. In CpHMD simulations for the peptides in solution and adsorbed on the lipid bilayer, we used the conformations obtained by conventional MD simulations at a μs timescale. Non-equilibrium Monte Carlo simulations provided the protonation states of acidic and basic residues. CpHMD showed average pKa shifts of two to three units, resulting in a higher net charge for the analog than for MP1, strongly modulating the peptide adsorption. The fractions of the protonation of acidic and basic residues and the peptides’ net charges obtained from the analysis of the adsorption isotherms were in reasonable agreement with those from CpHMD. MP1 adsorption was almost insensitive to solution pH. H-MP1 was much more sensitive to partitioning, at acidic pH, with an affinity ten times higher than in neutral ones.

Antimicrobial peptides (AMPs) are part of the innate immune system of many species and are compounds with potential application against the development of resistant bacterial strains promoted by conventional antibiotics. The AMPs are rich in cationic and hydrophobic residues and act directly on the lipidic phase of the cell membranes. The MP1 has a broad-spectrum bactericide activity in both Gram-negative and positive bacteria, not being hemolytic or cytotoxic. H-MP1 is a synthetic analog of MP1 with lysines replaced by histidines so that its net charge could be responsive to changes in solution pH. In the present work, we investigated the effect of the solution pH on the structural properties, in the adsorption and insertion, and on the lytic activity of these peptides in lipid bilayers mimicking the cell membrane of Gram-negative bacteria, using experimental and computational biophysical techniques. The results indicate that the lytic activity of H-MP1 is sensitive to pH, increasing to an acidic environment, matching that of MP1, which is not influenced by solution pH. Molecular Dynamic simulations indicated that the adsorption process of both peptides started by the interaction of the N-terminus with the bilayer, followed by the complete adsorption of the peptide laying parallel to the bilayer plane, inducing an increase in the peptide’s helical content enhancing peptides contact with the bilayer hydrophobic phase.