Gas-phase reactivity of lead(II) ions with D-glucose. Combined electrospray ionization mass spectrometry and theoretical study

The gas-phase reactivity of lead(II) ions toward D-glucose has been studied by means of mass spectrometry and theoretical calculations. Electrospray mass spectra show that this reactivity mainly gives rise to [Pb(Dglucose), - H](+) species (m = 1, 2). These ions are obtained by dissociative proton transfer within the doubly charged complexes, [Pb(D-glucose)(n)](2+) complexes (n = 2-12), observed in significant abundance at low cone voltage. Low-energy MS/MS experiments on [Pb(D-glucose) - H](+) (m/z 387) show that this ion essentially dissociates according to cross-ring cleavages leading mainly to the elimination of C2H4O2 and C4H8O4 neutrals and to a minor extent in the loss of C3H6O3. Use of labeled D-glucose demonstrates that the former implies specifically the elimination of C(1) and C(2) stereocenters, whereas the latter corresponds to at least two distinct fragmentations. This finding clearly suggests that several [Pb(D-glucose)-H](+) structures, characterized by different coordination schemes, coexist in the gas phase. This is confirmed by the density functional theory study carried out on both anomers, because the various coordination sites on the pyranose are close in energy. The most favorable Pb2+/D-glucopyranose association is characterized by the metallic center bonded to the deprotonated hydroxymethyl group and interacting with the hemiacetal oxygen. In such a position, the Pb2+ ion catalyzes the ring-opening process by activation of the two C-O ring bonds. This not only accounts for the fragmentation observed upon collision but also suggests that, besides fragmentation, pyranosic complexes may evolve toward energetically favored acyclic structures.