GAS-PHASE REACTIONS OF DOUBLY-CHARGED ALKALINE-EARTH AND TRANSITION METAL(II)-LIGAND COMPLEXES GENERATED BY ELECTROSPRAY-IONIZATION

Doubly charged metal(II)-complexes of alpha 1-3, alpha 1-6 mannotriose and the conserved trimannosyl core pentasaccharide as well as doubly charged complexes of Co(II), Mn(II), Ca(II) and Sr(II) with acetonitri le generated by electrospray ionization were studied by low energy col lision induced dissociation (CID). Two main fragmentation pathways wer e observed for the metal(II)-oligosaccharide complexes. Regardless of the coordinating metal, loss of a neutral dehydrohexose residue (162 D a) from the doubly charged precursor ion is observed, forming a doubly charged product ion. However, if the oligosaccharide is coordinated t o Co(II) or Mn(II), loss of a dehydroxyhexose cation is also observed. Investigation of the low mass region of the mass spectra of the metal coordinated oligosaccharides revealed intense signals corresponding t o [metal(II) + (CH3CN)(n)](2+) (where n = 1-6) species which were bein g formed by the metal(II) ions and the acetonitrile present in the sam ple. Analysis of these metal(II)-acetonitrile complexes provided furth er insight into the processes occurring upon low energy CID of doubly charged metal complexes. The metal(II)-acetonitrile system showed neut ral loss and ligand cleavage as observed with the oligosaccharide comp lexes, as well as a series of six different dissociation mechanisms, m ost notable among them reduction from [metal(II) + (CH3CN)(n)](2+) to the bare [metal(I)](+) species by electron transfer. Depending on the metal and collision gas chosen, one observes electron transfer from th e ligand to the metal, electron transfer from the collision gas to the metal, proton transfer between ligands, heterolytic cleavage of the l igands, reactive collisions and loss of neutral ligands. (C) 1997 Else vier Science B.V.