Characterization of complex, heterogeneous lipid A samples using HPLC-MS/MS technique III. Positive-ion mode tandem mass spectrometry to reveal phosphorylation and acylation patterns of lipid A.

In this study, we report the detailed analysis of the fragmentation patterns of positively charged lipid A species based on their tandem mass spectra obtained under low-energy collision-induced dissociation conditions of an electrospray quadrupole time-of-flight mass spectrometer. The tandem mass spectrometry experiments were performed after the separation of the compounds with a reversed-phase high performance liquid chromatography method. We found that both, phosphorylated and nonphosphorylated lipid A molecules can be readily ionized in the positive-ion mode by adduct formation with triethylamine added to the eluent. The tandem mass spectra of the lipid A triethylammonium adduct ions showed several product ions corresponding to inter-ring glycosidic cleavages of the sugar residues, as well as consecutive and competitive eliminations of fatty acids, phosphoric acid, and water following the neutral loss of triethylamine. Characteristic product ions provided direct information on the phosphorylation site(s), also when phosphorylation isomers (ie, containing either a C1 or a C4' phosphate group) were simultaneously present in the sample. Continuous series of high-abundance B-type and low-abundance Y-type inter-ring fragment ions were indicative of the fatty acyl distribution between the nonreducing and reducing ends of the lipid A backbone. The previously reported lipid A structures of Proteus morganii O34 and Escherichia coli O111 bacteria were used as standards. Although, the fragmentation pathways of the differently phosphorylated lipid A species significantly differed in the negative-ion mode, they were very similar in the positive-ion mode. The complementary use of positive-ion and negative-ion mode tandem mass spectrometry was found to be essential for the full structural characterization of the C1-monophosphorylated lipid A species.