Characterization of Antigenic Oligosaccharides from Gram-Negative Bacteria via Activated Electron Photodetachment Mass Spectrometry.

Lipooligosaccharides (LOS), composed of hydrophilic oligosaccharides and hydrophobic lipid A domains, are found on the outer membranes of Gram-negative bacteria. Here we report the characterization of deacylated LOS of LPS by activated-electron photodetachment mass spectrometry. Collision induced dissociation (CID) of these phosphorylated oligosaccharides produces simple MS/MS spectra with most fragment ions arising from cleavages near the reducing end of the molecule where the phosphate groups are located. In contrast, 193 nm ultraviolet photodissociation (UVPD) generates a wide array of product ions throughout the oligosaccharide including cross-ring fragments that illuminate the branching patterns. However, there are also product ions that are redundant or uninformative, resulting in more congested spectra that complicate interpretation. In this work, a hybrid UVPD-CID approach known as activated-electron photodetachment (a-EPD) affords less congested spectra than UVPD alone and richer fragmentation patterns than CID alone. a-EPD combines UVPD of negatively charged oligosaccharides to yield abundant charge-reduced radical ions which are subsequently interrogated by collisional activation. CID of the charge-reduced precursors results in extensive fragmentation throughout the backbone of the oligosaccharide. This hybridized a-EPD approach was employed to characterize the structure and branching pattern of deacylated LOS of E. coli.