Multiple neutral alkali halide attachments onto oligosaccharides in electrospray ionization mass spectrometry

Oligosaccharides perform essential functions in a variety of biological and agricultural processes. Recent approaches to characterization of these molecules by mass spectrometry have utilized mainly soft-ionization methods such as electrospray ionization (ESI) and thermospray (TS), as well as fast atom bombardment (FAB). The behavior of a series of maltooligosaccharides with alpha-(1-->4) linkages, maltose (G(2)) through maltoheptaose (G(7)), under ESI conditions, has been investigated here, The oligosaccharides were dissolved in N,N-dimethylacetamide containing lithium chloride (DMAc/LiCl) prior to analysis by ESI-MS, A highly unusual feature, evident in all mass spectra obtained using this solvent system, was the presence of multiple 'neutral' salt attachments onto lithium adducts of the sugars. Resultant ions took the form of [G(x) + Li + nLiCl](+), where n may reach a value as high as eight. Compared to LiCl, the propensity for alkali halide attachment using other alkali chlorides or lithium halides was greatly reduced. An investigation of this phenomenon is presented in which the organic and inorganic portions of the employed solvent were systematically varied, and semi-empirical computer modeling was performed to better understand lithium coordination by the sugars. (C) 1997 Elsevier Science B.V.