Characterization of a structural series of lipid A obtained from the lipopolysaccharides of Neisseria gonorrhoeae. Combined laser desorption and fast atom bombardment mass spectral analysis of high performance liquid chromatography-purified dimethyl derivatives.

Monophosphoryl lipid A (MLA) obtained from the lipopolysaccharides of serum-sensitive strains of Neisseria gonorrhoeae was fractionated on a silicic acid column to yield the hexaacyl and pentaacyl MLAs. The dimethyl derivative of the hexaacyl MLA was analyzed by proton nuclear magnetic resonance spectroscopy. The dimethyl esters of hexaacyl and pentaacyl MLAs were further purified by reverse-phase high performance liquid chromatography, and all of the peaks were analyzed by laser desorption mass spectrometry. Considerable structural information was obtained by laser desorption mass spectrometry due to three kinds of specific fragmentations of the sugar at the reducing end. Two major fractions were also analyzed by positive ion fast atom bombardment mass spectrometry. High performance liquid chromatography was able to separate the dimethyl MLA according to number, nature, and position of the fatty acyl groups. Since almost no structural information is available, the mass spectra of the samples were interpreted on the basis of the established structure of a model lipid A (hexaacyl MLA derived from Salmonella minnesota). Thirteen different structures of dimethyl MLA were identified. The four prominent dimethyl MLAs found in the fractionated samples were M1 (Mr = 1463), M2 (Mr = 1479), M3 (Mr = 1661), and M4 (Mr = 1677). These MLAs appear to have a 1'----6 linked glucosamine disaccharide backbone. The most prominent hexaacyl MLA was M3. We propose that it contains hydroxylaurate at the 3- and 3'-positions in ester linkage and lauroxymyristate at the 2- and 2'-positions in amide linkage of the glucosamine disaccharide. The most abundant pentacyl MLA was M2. We propose that it contains hydroxylaurate at the 3- and 3'-positions in ester linkage, lauroxymyristate at the 2'-position in amide linkage, and hydroxymyristate at the 2-position in amide linkage of the disaccharide. The lipid A of N. gonorrhoeae appeared to differ from that of the Salmonella strains by the presence of shorter-chain fatty acids and by the normal fatty acid distribution in the reducing and distal subunits.