Oligosaccharide analysis using anion attachment in negative mode electrospray mass spectrometry

Eleven different anionic species were able to form adducts with neutral oligosaccharides at low cone voltage in negative ion mode electrospray mass spectrometry. Among them, fluoride and acetate have the ability to significantly enhance the absolute abundance of [M − H]− for neutral oliogosaccharides, which otherwise have low tendencies to deprotonate due to the lack of a highly acidic group. Evidence shows that the source of high abundances of [M − H]− for neutral oligosaccharides arises from the decomposition of [M + F]− and [M + Ac]− with neutral losses of HF and HAc, respectively. The chloride adducts have the best stability among all the adduct species investigated, and chloride adducts consistently appeared in higher abundances relative to [M − H]−. In tandem mass spectrometry (ES-MS/MS) experiments, upon collision induced dissociation (CID), F− and Ac− adducts gave purely analyte-related product ions, i.e., no detection of the attaching anion and no incorporation of these anions into decomposition products. Cl− adducts produced both Cl− and analyte-related product ions. For the above three anions, CID of adduct species may be used for structural determination of neutral oligosaccharides because, in each case, structurally-informative fragment ions were produced. In the presence of F− and Ac−, simultaneous detection of acidic and neutral oligosaccharides was achieved, because the problem of the presence of an acidic group that can impede the deprotonation of a neutral oligosaccharide was minimized. The ratio of Cl−:non-Cl-containing product ions obtained in CID spectra of chloride adducts of disaccharides was used to differentiate anomeric configurations of disaccharides. Density functional theory (DFT) was employed to evaluate the optimized structures of chloride adducts of disaccharides, and it was found that chloride anions favor close contact with the hydrogen from the anomeric hydroxyl group. Multiple hydrogen bonding further stabilizes the chloride adduct.