Multi-scale analysis of the toraymyxin adsorption cartridge. Part I: molecular interaction of polymyxin B with endotoxins.

Endotoxins or lipopolysaccharides are the main constituents of the outer leaflet of Gram-negative bacteria membrane and play a central role in the pathogenesis of the septic shock. Polymyxin B has both antibacterial and antiendotoxin capability; indeed it is able to destroy the bacterial outer membrane and bind endotoxin neutralizing its toxic effects. Cartridges containing polymyxin B-immobilized fibers (Toraymyxin PMX-F, Toray Industries, Japan) are used in extracorporeal hemoperfusion to remove circulating endotoxin. The aim of this study is the characterization of the polymyxin B-endotoxin system at the molecular level, thus providing quantitative evaluation of the binding forces exerted in the molecular complex. Polymyxin B was interfaced with five molecular models of lipopolysaccharides differing in their structure and molecular mechanics simulations were performed at different intermolecular distances aimed at calculating the interaction energies of the complex. Binding forces were calculated by fitting interaction energies data. Results show that in the short range the polymyxin B-endotoxin complex is mediated by hydrophobic forces and in the long range the complex is driven by ionic forces only. From a mechanical standpoint, polymyxin B-endotoxin complex is characterized by maximum binding forces ranging between 1.39 nN to 3.79 nN. The knowledge of the binding force behavior at different intermolecular distances allows further investigations at higher scale level (Part II).