Evaluation of the role of multiple hydrogen bonding in offering stability to negative ion adducts in electrospray mass spectrometry.

An experimental approach, electrospray mass spectrometry (ES-MS), and a theoretical approach employing computer modeling, have been used to characterize the interaction between small inorganic anions and neutral analyte molecules that form anionic adduct species in negative mode ES mass spectrometry. Certain anionic adducts of small saccharides (e.g., alpha-D-glucose, sucrose) have shown exceptional stability in ES mass spectra even when internal energies are raised at high "cone" voltages. Computer modeling studies reveal that multiple hydrogen bonding strengthens the interaction between these neutral molecules and the attaching anion. The equilibrium structures and stabilization energies of these anionic adducts have been evaluated by semi-empirical, ab initio, and density functional theory (DFT) methods. Chloride anion is found to be capable of forming "bridging" hydrogen bonds between monosaccharide rings of polysaccharides resulting in the stabilization of chloride adducts, thus reducing the tendency for the glycosidic bond to decompose. Moreover, the tendency for various hydroxyl hydrogens on saccharide molecules to dissociate in the form of HA (A-, anion) during decomposition of anionic adducts, thereby forming [M - H]-, has also been evaluated by computer modeling.