Effects of Pore-Forming Peptides (Melittin and Magainin 2) on the Phospholipid Bilayer Interior

Abstract

Antimicrobial peptides (AMPs) are one of the most promising solutions to drug-resistant bacteria. Melittin and magainin 2 are two of the most representative and extensively studied AMPs. In this research, I investigated the interaction of these two AMPs with three models of cell membranes: 80% POPC 20% POPG, 40%POPC 40% POPE and 20% POPG, and 80%POPC 20%POPG plus 30% mole fraction of cholesterol. Time-resolved fluorescence emission and fluorescence anisotropy decays of the fluorescent probe 1,6-diphenyl-1,3,5-hexatriene (DPH) were analyzed to determine the effects of AMPs on the bilayer headgroup packing and changes in the interior of the phospholipid bilayer during the process of pore formation. Time-resolved fluorescence anisotropy of DPH was employed to investigate the dynamics and acyl chain ensemble order in the core of the membrane bilayers. DPH anisotropy decay was interpreted in terms of the Brownian Rotational Diffusion model (BRD). A wide range of molar ratio of peptides to lipids (P/L) 0.33% to 30% was studied and I found that at high P/L> 1/50 DPH fluorescence lifetime decreases with increasing melittin concentration but there was no effect of the magainin 2 on the fluorescence lifetime. The motion of DPH is slowed by adding more melittin as shown by the increasing the rotational correlation lifetime 〈∅〉 of DPH. The anisotropy decay analysis leads to characterization of the order of the phospholipid acyl chains ensemble throughout the depth of the bilayer by calculating the orientational probability distribution function of DPH. I found the effect of peptide concentration on the interior of the membrane bilayers by calculating the orientational distribution function and the rotational correlation time of DPH. Steady-state fluorescence measurements were also used to study the charge density effect on the peptide bound lipid to vesicles, and it was confirmed that melittin binds to the membrane bilayer from the blue shift of the maximal emission wavelength λmax of the emission of the intrinsic tryptophan residue in melittin. from the blue shift of the maximal emission wavelength λmax of the intrinsic tryptophan residue in melittin.