Temperature Dependence of the Binding of Endotoxins to the Polycationic Peptides Polymyxin B and Its Nonapeptide

Abstract

The interaction between endotoxins-free lipid A and various lipopolysaccharide (LPS) chemotypes with different sugar chain lengths-and the polycationic peptides polymyxin B and polymyxin nonapeptide has been investigated by isothermal titration calorimetry between 20 and 50°C. The results show a strong dependence of the titration curves on the phase state of the endotoxins. In the gel phase (,30°C for LPS and ,45°C for lipid A), an endothermic reaction is observed, for which the driving force is an entropically driven endotoxin-polymyxin interaction, due to disruption of the ordered water structure and cation assembly in the lipid A backbone and adjacent molecules. In the liquid crystalline phase (.35°C for LPS and .47°C for lipid A) an exothermic reaction takes place, which is mainly due to the strong electrostatic interaction of the polymyxins with the negative charges of the endotoxins, i.e., the entropic change DS is much lower than in the gel phase. For endotoxins with short sugar chains (lipid A, LPS Re, LPS Rc) the stoichiometry of the polymyxin binding corresponds to pure charge neutralization; for the compounds with longer sugar chains (LPS Ra, LPS S-form) this is no longer valid. This can be related to the lower susceptibility of the corresponding bacterial strains to antibiotics.

Lipopolysaccharide, a Key Molecule Involved in the Synergism between Temporins in Inhibiting Bacterial Growth and in Endotoxin Neutralization

“…Several ITC studies dealing with the binding of AMPs to LPS were reported (53)(54)(55). To keep conditions similar to those used in the biological assays, we conducted ITC measurements at a physiological temperature (38°C) at which the species of LPS used are in the liquid crystalline phase.…”

Dual Mechanism of Bacterial Lethality for a Cationic Sequence-Random Copolymer that Mimics Host-Defense Antimicrobial Peptides

“…It has been suggested that ITC analysis of LPS reports on the state of order of the acyl chains in lipid A. 26 The polymer interactions with LPS and LTA result in very different thermodynamic signatures.…”

The anti-inflammatory action of the analgesic kyotorphin neuropeptide derivatives: insights of a lipid-mediated mechanism

“…This exothermic interaction is related to the electrostatic attraction between the negative charges of the LPS/Lipid A (carboxylates and phosphates) and the positive charge of the peptides. An additional, entropically governed reaction between LPS and KTP derivatives, due to the removal of the highly ordered water layer in the backbone region of LPS (Brandenburg et al 2005), is apparently much less important than the enthalpy electrostatic effect (Table 1).…”