Graduate Thesis Or Dissertation


Binding of bacterial lipopolysaccharide by the cationic amphiphilic peptide WLBU2 at interfaces Public Deposited

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  • Passage of blood through a sorbent device for removal of bacteria and endotoxin by specific binding with immobilized, membrane-active, bactericidal peptides holds promise for treating severe blood infections. Peptide insertion in the target membrane and stable binding is desirable, while membrane disruption and release of degradation products to the circulating blood is not desirable. Here we describe interactions between bacterial endotoxin (lipopolysaccharide, LPS) and the membrane-active, bactericidal peptides WLBU2 and polymyxin B (PmB). Analysis of the interfacial behavior of mixtures of LPS and peptide using air-water interfacial tensiometry and optical waveguide lightmode spectroscopy strongly suggested insertion and stabilization of intact LPS vesicles by WLBU2, while no such peptide-LPS interactions were evident with PmB. Analysis with dynamic light scattering showed in fact that LPS vesicles appear to undergo peptide-induced destabilization in the presence of PmB. Circular dichroism spectra confirmed that WLBU2, which shows disordered structure in aqueous solution and substantially helical structure in membrane-mimetic environments, is stably located within the LPS membrane in peptide-vesicle mixtures. Interactions between LPS and WLBU2 were also evaluated following immobilization of the peptide at uncoated and polyethylene oxide (PEO)-coated hydrophobic surfaces. PEO layers were prepared by radiolytic grafting of selected PEO-polypropylene oxide (PPO)-PEO triblock surfactants to silanized, hydrophobic surfaces. Immobilization of WLBU2 at the PEO layers was achieved by its noncovalent entrapment among the pendant PEO chains and in separate experiments, its covalent coupling to PEO chains that had been end-activated with pyridyl disulfide groups. Analysis of peptide-LPS interactions using a quartz crystal microbalance with dissipation monitoring showed that upon introduction of LPS suspension to a flow cell housing a surface presenting tethered WLBU2, LPS located at the interface in a fashion irreversible to elution. Circular dichroism spectra recorded for suspensions of LPS and (silanized) hydrophobic silica nanoparticles to which WLBU2-triblock constructs had been adsorbed, confirmed that binding of LPS by tethered WLBU2 is mediated through peptide insertion and conformational change within the LPS membrane. LPS capture by tethered WLBU2 was detected in the presence of fibrinogen as well. However, that outcome is best considered tentative, as it was associated with potentially complex interactions between fibrinogen, LPS, and WLBU2, that remain uncharacterized. In summary, the results of this study strongly suggest that presentation of tethered WLBU2 within a sorbent device will enable the capture of endotoxin from suspension without reintroduction of degradation products to the circulating stream. Thus, they provide a rationale for hypotheses to drive further development of perfusion for the treatment of severe blood infections.
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