Orientation measurements on membrane systems made from lipopolysaccharides and free lipid A by FT-IR spectroscopy

Abstract

Here we report on investigations into the orientational behaviour of hydrated membrane systems made from lipopolysaccharide and its lipid component, free lipid A, using Fourier-transform infrared spectroscopy and applying attenuated total reflectance. For the investigated lipopolysaccharides — extracted from mutants of Salmonella minnesota and Escherichia coli, and differing in the length of the polysaccharide moiety — the dependence of dichroic ratios on temperature for several vibrations of the hydrophilic and hydrophobic portions was measured, from which the order parameter for the lipid assembly can be calculated. In the lower temperature range (<40°C) for all lipopolysaccharide preparations the evaluation of the dichroic ratios clearly shows the existence of a highly ordered phase, i.e. the gel state of the hydrocarbon chains within lamellar structures. For this phase an order parameter S=0.70±0.05 could be calculated which is lower than that of typical phospholipids in the same phase state (S=0.80±0.05 for e.g. phosphatidylethanolamines). For deep rough mutant lipopolysaccharides at higher temperatures (>40°C) a transition into a disordered, isotropic phase can usually be observed for which an order parameter S=0.25±0.05 could be approximated. The other rough mutant lipopolysaccharides at higher temperatures predominantly form lamellar structures. Only in special cases, under the influence of divalent cations like Mg²⁺, could isotropic phases also be observed. Free lipid A preparations over the whole temperature range exhibited no unequivocal orientational behaviour. However, the existence of a pure L _β-phase even at lower temperatures may be excluded for these compounds. The observed structural preferences might be of great importance with respect to the expression of biological activities of lipopolysaccharide and free lipid A systems in vivo and in vitro.