Thermochemistry and structures of Na+ coordinated mono- and disaccharide stereoisomers

The Na+ affinities of several mono- and disaccharide stereoisomers are dete rmined in the gas phase based on the dissociations of Na+-bound heterodimer s [saccharide + B-i]Na+, where B-i represents a reference base of known Na affinity (kinetic method). The compounds investigated include the pentoses arabinose, xylose, and ribose; the hexoses glucose, galactose, and mannose ; and the disaccharides melibiose, gentiobiose, and lactose. The decomposit ions of [saccharide + B-i]Na+ are assessed as a function of internal energy , to thereby obtain both absolute Na+ affinities as well as relative entrop ies of Na+ attachment. The Na+ affinities measured are consistent with mult identate coordination of sodium ion by the oxygen sites of the saccharides. In general, hexoses bind Na+ stronger than pentoses, suggesting that the h ydroxymethyl substituent equips them with more conformational flexibility a nd larger inductive effects for complexing Na+. The latter properties are f urther enhanced in the disaccharides, which also carry more basic substitue nts; as a result, disaccharides form even stronger bonds to Na+. The entrop ies of Na+ attachment are found to rise in the order pentose < hexose < dis accharide, pointing to an increase in this direction of the rotational flex ibility lost after attachment of Na+. The favored [monosaccharide + Na](+) structures predicted computationally contain pyranose rings in chair or boa t conformations that permit tri- or tetradentate Na+ coordination and hydro gen bonds between the hydroxyl ligands; the most stable disaccharide comple xes are tetradentate and involve chair forms. In the calculated structures, the pyranose O atom and the hydroxymethyl group(s) generally participate i n the Na+ binding, in agreement with the experimental trends. Small changes in the saccharide stereochemistry alter the optimum Na+ coordination possi ble and, therefore, the Na+ affinity; as a result, the latter thermochemica l property is ideally suitable for the distinction of stereoisomeric saccha rides. (Int J Mass Spectrom 189 (1999) 189-204) O 1999 Elsevier Science B.V .