Structural Elucidation of Lipopolysaccharides and their Lipid a Component: Application of Soft Ionization Mass Spectrometry

Abstract

Gram negative bacteria express at their surface various amphiphilic macromolecules among which the lipopolysaccharides (LPS) are of special biomedical significance. They represent the endotoxins of Gram negative bacteria eliciting in higher organisms typical pathophysiological effects such as fever, hypotension, dermal skin necrosis (local Shwartzman phenomenon) and irreversible shock and they represent the O-antigens, determining the serospecificity of LPS and of the bacteria containing them.1 Chemically, LPS consist of a polysaccharide or oligosaccharide portion covalently linked to a lipid component, termed lipid A which anchors the lipopolysaccharide in the outer leaflet of the outer cell membrane. The polysaccharide part consists of an O-specific chain which expresses high structural variability and a core oligo-saccharide which is less variable or structurally identical for many different Gram negative bacteria. The chemical structure of lipid A shows remarkable conformity for all members of the Enterobacteriaceae family and many other Gram negative bacteria. It represents the most conservative LPS region and consists of β-D-glucosaminyl-(1–6)-α-D-glucosamine disaccharide which is phosphorylated in positions 4′ of the nonreducing glucosaminyl residue (GlcN II) and in position 1 of the reducing glucosaminyl group (GlcN I). This hydrophilic backbone carries in ester and amide linkage up to seven hydroxylated and nonhydroxylated saturated fatty acid residues with chain lengths typically between ten and sixteen carbon atoms2,3. Lipid A represents the endotoxic principle of LPS being responsible for all pathophysiological effects of endotoxins. It was and still is of particular interest to find out whether the biological activity of lipid A’s of various origin can be attributed to a common chemical composition and whether a minimal endotoxically active structure can be defined. Furthermore, a correlation between the chemical structure, particularly the number and type of fatty acid residues and the degree of phosphorylation on the one hand, and the physical properties of LPS membrane systems on the other hand had been demonstrated. Thus, the thermotropic phase behavior, i. e., the various three-dimensional supramolecular phase states and the state of order adopted under physiological conditions was shown to depend on these chemical parameters.3