Differentiating antimicrobial peptides interacting with lipid bilayer: Molecular signatures derived from quartz crystal microbalance with dissipation monitoring.

Many antimicrobial peptides (AMPs) kill bacteria by disrupting the lipid bilayer structure of their inner membrane. However, there is only limited quantitative information in the literature to differentiate between AMPs of differing molecular properties, in terms of how they interact with the membrane. In this study, we have used quartz crystal microbalance with dissipation monitoring (QCM-D) to probe the interactions between a supported bilayer membrane of egg phosphatidylcholine (egg PC) and four structurally different AMPs: alamethicin, chrysophsin-3, indolicidin, and sheep myeloid antimicrobial peptide (SMAP-29). Multiple signatures from the QCM-D measurements were extracted, differentiating the AMPs, that provide information on peptide addition to and lipid removal from the membrane, the dynamics of peptide-membrane interactions and the rates at which the peptide actions are initiated. The mechanistic variations in peptide action were related to the fundamental structural properties of the peptides including the hydrophobicity, hydrophobic moment, and the probability of α-helical secondary structures.

Size dependence of gold nanoparticle interactions with a supported lipid bilayer: A QCM-D study.

Knowledge of nanoparticle (NP)-membrane interactions is important to advances in nanomedicine as well as for determining the safety of NPs to humans and the ecosystem. This study focuses on a unique mechanism of cytotoxicity, cell membrane destabilization, which is principally dependent on the nanoparticle nature of the material rather than on its molecular properties. We investigated the interactions of 2, 5, 10, and 40nm gold NPs with supported lipid bilayer (SLB) of L-α-phosphatidylcholine using quartz crystal microbalance with dissipation monitoring (QCM-D). Gold NPs were tested both in the absence of and in the presence of polymethacrylic acid (PMAA), used to simulate the natural organic matter (NOM) in the environment. In the absence of PMAA, for all NP sizes, we observed only small mass losses (1 to 6ng) from the membrane. This small lipid removal may be a free energy lowering mechanism to relieve stresses induced by the adsorption of NPs, with the changes too small to affect the membrane integrity. In the presence of PMAA, we observed a net mass increase in the case of smaller NPs. We suggest that the increased adhesion between the NP and the bilayer, promoted by PMAA, causes sufficient NP adsorption on the bilayer to overcompensate for any loss of lipid. The most remarkable observation is the significant mass loss (60ng) for the case of 40nm NPs. We attribute this to the lipid bilayer engulfing the NP and leaving the crystal surface. We propose a simple phenomenological model to describe the competition between the particle-bilayer adhesion energy, the bilayer bending energy, and the interfacial energy at bilayer defect edges. The model shows that the larger NPs, which become more adhesive because of the polymer adsorption, are engulfed by the bilayer and leave the crystal surface, causing large mass loss and membrane disruption. The QCM-D measurements thus offer direct evidence that even if NPs are intrinsically not cytotoxic, they can become cytotoxic in the presence of environmental organic matter which modulates the adhesive interactions between the nanoparticle and the membrane.

Antimicrobial peptide alamethicin insertion into lipid bilayer: a QCM-D exploration.

Alamethicin is a 20-amino-acid, α-helical antimicrobial peptide that is believed to kill bacteria through pore formation in the inner membranes. We used quartz crystal microbalance with dissipation monitoring (QCM-D) to explore the interactions of alamethicin with a supported lipid bilayer. Changes in frequency (Δf) and dissipation (ΔD) measured at different overtones as a function of peptide concentration were used to infer peptide-induced changes in the mass and rigidity of the membrane as well as the orientation of the peptide in the bilayer. The measured Δf were positive, corresponding to a net mass loss from the bilayer, with substantial mass losses at 5 µM and 10 µM alamethicin. The measured Δf at various overtones were equal, indicating that the mass change in the membrane was homogeneous at all depths consistent with a vertical peptide insertion. Such an orientation coupled to the net mass loss was in agreement with cylindrical pore formation and the negligibly small ΔD suggested that the peptide walls of the pores stabilized the surrounding lipid organization. Dynamics of the interactions examined through Δf vs. ΔD plots suggested that the peptides initially inserted into the membrane and caused disordering of the lipids. Subsequently, lipids were removed from the bilayer to create pores and alamethicin caused the remaining lipids to reorder and stabilize within the membrane. Based on model calculations, we concluded that the QCM-D data cannot confirm or rule out whether peptide clusters coexist with pores in the bilayer. We have also proposed a way to calculate the peptide-to-lipid ratio (P/L) in the bilayer from QCM-D data and found the calculated P/L as a function of the peptide concentration to be similar to the literature data for vesicle membranes.

Characterization of supported lipid bilayer disruption by chrysophsin-3 using QCM-D.

Antimicrobial peptides (AMPs) are naturally occurring polymers that can kill bacteria by destabilizing their membranes. A quartz crystal microbalance with dissipation monitoring (QCM-D) was used to better understand the action of the AMP chrysophsin-3 on supported lipid bilayers (SLB) of phosphatidylcholine. Interaction of the SLB with chrysophsin-3 at 0.05 µM demonstrated changes in frequency (Δf) and energy dissipation (ΔD) that were near zero, indicating little change in the membrane. At higher concentrations of chyrsophsin-3 (0.25-4 µM), decreases in Δf of up to 7 Hz were measured. These negative frequency changes suggest that mass was being added to the SLB, possibly due to peptide insertion into the membrane. At a chrysophsin-3 concentration of 10 µM, there was a net mass loss, which was attributed to pore formation in the membrane. QCM-D can be used to describe a mechanistic relationship between AMP concentration and interaction with a model cell membrane.