Structural study on the free lipid A isolated from lipopolysaccharide of Porphyromonas gingivalis

The chemical structure of lipid A isolated from Porphyromonas gingivalis lipopolysaccharide was elucidated by compositional analysis, mass spectrometry, and nuclear magnetic resonance spectroscopy. The hydrophilic backbone of free lipid A was found to consisted of beta(1,6)-linked D-glucosamine disaccharide 1-phosphate. (R)-3-Hydroxy-15-methylhexadecanoic acid and (R)-3-hydroxyhexadecanoic acid are attached at positions 2 and 3 of the reducing terminal residue, respectively, and positions 2' and 3' of the nonreducing terminal unit are acylated with (R)-3-O-(hexadecanoyl)-15-methylhexadecanoic acid and (R)-3-hydroxy-13-methyltetradecanoic acid, respectively. The hydroxyl group at position 4' is partially replaced by another phosphate group, and the hydroxyl groups at positions 4 and 6' are unsubstituted. Considerable heterogeneity in the fatty acid chain length and the degree of acylation and phosphorylation was detected by liquid secondary ion-mass spectrometry (LSI-MS). A significant pseudomolecular ion of lipid A at m/z 1,769.6 [M-H]- corresponding to a diphosphorylated GlcN backbone bearing five acyl groups described above was detected in the negative mode of LSI-MS. Predominant ions, however, were observed at m/z 1,434.9 [M-H]- and m/z 1,449.0 [M-H]-, each representing monophosphoryl lipid A lacking (R)-3-hydroxyhexadecanoic and (R)-3-hydroxy-13-methyltetradecanoic acids, respectively. The presence of mono- and diphosphorylated lipid A species was also confirmed by LSI-MS of de-O-acylated lipid A (m/z 955.3 and 1,035.2, respectively).