Gram-negative bacterial lipid A analysis by negative electrospray ion trap mass spectrometry: Stepwise dissociations of deprotonated species under low energy CID conditions

After mild acid hydrolysis of the lipopolysaccharide (LPS) from Escherichia coli J5, the recovered underivatized lipid A was analyzed using an electrospray source coupled with an ion trap mass analyzer (ESI-ITMS). Our work deals with the study of these complex endotoxic biomolecules under CID conditions. Sequential MSn experiments with the ion trap instrument allowed the characterization of the substituting side chain. Mono- and diphosphorylated singly or doubly deprotonated lipid A molecules have been studied and provided complementary information. The experimental results obtained, from these negatively charged species, under low energy collision conditions motivated us to propose stepwise fragmentation pathways involving intermediate ion–dipole complexes. In this way charge-driven processes are considered instead of the charge-remote mechanisms usually proposed in the literature.