Fragmentation of lipopolysaccharide anchors in plasma desorption mass spectrometry

Plasma desorption mass spectrometry (PD-MS) was employed for the structural analysis of lipid A's derived from the lipopolysaccharides (LPS) of Escherichia coli D31m4, Rhodopseudomonas sphaeroides 3TCC 17023, and Rhodopseudomonas capsulata ATCC 23782. The deep rough chemotype LPS (ReLPS) of E. coli D31m4, a lipid A containing two 2-keto-3-deoxyoctonate (KDO) units, was also analyzed. Several forms were examined, including the mono- and di-phosphoryl lipid A's, and lipid A's methylated at the phosphate group. Positive ion PD-MS gave molecular ions, ions corresponding to the loss of ester-linked fatty acyl and glycosidic phosphate groups, oxonium ions formed by the cleavage of the distal sugar, and ions resulting from the two-bond cleavage of the reducing-end sugar. Negative ion PD-MS gave molecular ions and fragmentation products of the phosphate group and fatty acyl anions in the low mass region. The presence of the KDO group on the lipid A structure had little effect on the fragmentation pattern of the rest of the molecule of ReLPS. PD-MS of lipid A has allowed us to determine the molecular weight, the distribution of the fatty acyl groups in both the distal and reducing-end sugars, the nature of the O-linked fatty acyl groups, and the presence of a glycosidic-linked phosphate. In combination with proton nuclear magnetic resonance spectroscopy, PD-MS allows one to determine the complete structure of lipid A.