Application of fast atom bombardment mass spectrometry and nuclear magnetic resonance on the structural analysis of purified lipid A

A method is described for the determination of the complete structure of lipid A obtained from the lipopolysaccharides of Salmonella strains which can now be applied A samples obtained from other gram-negative bacteria. The lipopolysaccharides were treated under mild acid conditions to yield a crude monophosphoryl lipid A (MLA) mixture which was then fractionated on a silicic column to yield the structural analogs. Each of the purified MLA analogs was methylated with diazomethane and further fractionated by reverse-phase high performance liquid chromatography to yield a higly purified dimethyl MLA. Such a sample was analyzed by chemical means and by modern spectroscopic methods. The molecular size of dimethyl MLA and fatty acid distribution in the reduciong and distal glucosamines were determined bu utilizing positive ion fast atom bombardment mass spectrometry. The location of all of the ester-linked fatty acids and the single phosphate group as well as the anomeric configuration of the two glucosamines were determined by utilizing proton-nuclear magnetic resonance spectroscopy. Chemical degradation studies on MLA and dimethyl MLA using triethylamine also contributed to determining the location of the ester-linked fatty acids.