Influence of metal ion and coordination geometry on the gas phase dissociation and stereochemical differentiation of N-glycosides

A series of metal cationized N-glycoside complexes is studied by tandem mass spectrometry. The complexes are of the form [M(dien)(hex)(n) - H](+), [M(dien)(hex)(n)Cl](+), or [M(dien)(hex)(2)](2+), where M = Co2+, Cu2+, Ni2+, or Zn2+ and n = 1 or 2. The presence of a metal as the charge carrier promotes more extensive cleavage of the carbon backbone of the saccharide moiety compared to the protonated analogues, thus providing greater structural information. Results obtained for the four coordinate singly charged deprotonated complexes differentiate [M(dien)(Glc) - H](+) versus [M(dien)(Gal) - H](+) for a given metal. Such differentiation is not possible for the corresponding five coordinate complexes, indicating that the coordination number of the metal plays an important role. Tandem mass spectra for both of these four and five coordinate complexes indicate that the geometry around the metal dictates the observed dissociation patterns. Furthermore, the metals Zn and Cu promote unique modes of dissociation not observed for the Ni and Co complexes. Finally, the chlorinated and doubly charged complexes give rise to very few dissociation pathways by MS/MS. It is proposed that this lack of product ions is due to the absence of lone pair electrons on the metal alkoxide that drive the dissociation. (C) 2000 Elsevier Science B.V.