Deciphering the structure of isomeric oligosaccharides in a complex mixture by tandem mass spectrometry: Photon activation with vacuum ultra-violet brings unique information and enables definitive structure assignment

Carbohydrates have a wide variety of structures whose complexity and heterogeneity challenge the field of analytical chemistry. Tandem mass spectrometry, with its remarkable sensitivity and high information content, provides key advantages to addressing the structural elucidation of polysaccharides. Yet, classical fragmentation by collision-activated dissociation (CAD) in many cases fails to reach a comprehensive structural determination, especially when isomers have to be differentiated. In this work, for the first time, vacuum ultra-violet (VUV) synchrotron radiation is used as the activation process in tandem mass spectrometry of large oligosaccharides. Compared to low energy CAD (LE-CAD), photon activated dissociation brought more straightforward and valuable structural information. The outstanding feature was that complete series of informative ions were produced, with only minor neutral losses. Moreover, systematic fragmentation rules could be drawn thus facilitating the definitive assignments of fragment identities. As a result, most of the structures present in a complex mixture of oligogalacturonans could be comprehensively resolved, including many isomers differing in the position of methyl groups along the galacturonic acid backbone.     

Synthesis of 3E- And 6E-Monosulfated and 3E,6E-Disulfated Lewis X Pentasaccharides,Candidate Ligands for Human L-Selectin

ABSTRACT

Condensation of perbenzyl fucosyl bromide 4 with the known 2-acetamido derivative 1 and with the 2-phthalimido analog 3 gave, respectively, disaccharides 5 and 6 which, after reductive opening of the benzylidene group were glycosylated using peracetylated galactose trichloroacetimidate to give trisaccharides 10 and 11. After removal of the anomeric p-methoxybenzyl group and subsequent activation as the trichloroacetimidate, condensation onto the known lactose derivative 16 afforded protected pentasaccharides 18 and 17 in 24 and 70% yields, respectively. Compound 17 was transformed into 18 by a conventional method in 77% yield making the phthalimido route overall more attractive. The terminal galactose was then selectively deprotected and sulfated at the 3e, 6e and both 3e,6e positions to give the title compounds after complete deprotection.     

Vacuum Ultraviolet Action Spectroscopy of Polysaccharides

We studied the optical properties of gas-phase polysaccharides (maltose, maltotetraose, and maltohexaose) ions by action spectroscopy using the coupling between a quadrupole ion trap and a vacuum ultraviolet (VUV) beamline at the SOLEIL synchrotron radiation facility (France) in the 7 to 18 eV range. The spectra provide unique benchmarks for evaluation of theoretical data on electronic transitions of model carbohydrates in the VUV range. The effects of the nature of the charge held by polysaccharide ions on the relaxation processes were also explored. Finally the effect of isomerization of polysaccharides (with melezitose and raffinose) on their photofragmentation with VUV photons is presented.