The excess thermodynamic properties of liquid (CH4+CD4)

Influence of Acyl Chain Saturation on the Membrane-Binding Activity of a Short Antimicrobial Peptide

Different bacterial types and their living environments can lead to different saturations in the chains of their membrane lipids. Such structural differences may influence the efficacy of antibiotics that target bacterial membranes. In this work, the effects of acyl chain saturation on the binding of an antimicrobial peptide G₄ have been examined as a function of the packing density of lipid monolayers by combining external reflection Fourier transform infrared (ER-FTIR) spectroscopy and neutron reflection (NR) measurements. Langmuir monolayers were formed from 1,2-dipalmitoyl-sn-glycero-3-phospho-(1'-rac-glycerol) (DPPG) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-(1'-rac-glycerol) (POPG), respectively, with the initial surface pressures controlled at 8 and 28 mN/m. A reduction in the order of the acyl chains associated with the increase in the layer thickness upon G₄ binding was revealed from ER-FTIR spectroscopy, with peptide binding reaching equilibration faster in POPG than in DPPG monolayers. Whereas the dynamic DPPG-binding process displayed a steady increase in the amide I band area, the POPG-binding process showed little change in the amide area after the initial period. The peptide amide I area from ER-FTIR spectroscopy could be linearly correlated with the adsorbed G₄ amount from NR, irrespective of time, initial pressure, or chain saturation, with clearly more peptide incorporated into the DPPG monolayer. Furthermore, NR revealed that although the peptide was associated with both POPG and DPPG lipid monolayers, it was more extensively distributed in the latter, showing that acyl chain saturation clearly promoted peptide binding and structural disruption.

Interaction of the Antimicrobial Peptides Rhesus θ-Defensin and Porcine Protegrin-1 with Anionic Phospholipid Monolayers

A combination of Langmuir isotherm, Brewster angle microscopy (BAM), and neutron reflectivity studies have been performed to gain insight into the effects on model bacterial cell membranes of the antimicrobial peptides, Rhesus θ-defensin 1 (RTD-1), and porcine protegrin 1 (PG-1). The peptides were interacted with monolayers spread at the air-water interface and prepared from a 3:1 molar mixture of phosphatidylethanolamine and phosphatidylglycerol used to approximate the cell membranes of Gram positive bacteria. The Langmuir film balance measurements show that both peptides perturb the lipid monolayers causing an increase in surface pressure, and the BAM studies show that each results in the formation of small domains within the lipid films, around 5 μm diameter. The overall change in monolayer surface pressure caused by PG-1, however, is a little more pronounced than that due to RTD-1 (+8.5 mN·m(-1) vs +5.5 mN·m(-1)), and the rate of its initial interaction with the monolayer is a little more rapid than that for RTD-1. The neutron reflectivity studies also show differences for PG-1 and RTD-1, with the model fits to these data showing that the more amphiphilic PG-1 becomes fully embedded within the lipid film-causing an extension of the lipid acyl chains but leaving the thickness of the lipid headgroup layer unaffected-while RTD-1 is seen to insert less deeply-causing the same extension of the lipid acyl chains as PG-1 but also causing a significant increase in thickness of the lipid headgroup layer. The various differing effects of the two peptides on anionic lipid monolayers are discussed in the context of their differing hemolytic activities, and their proposed differing propensities to form transmembrane pores.