Amino-acid and water molecules adsorbed on water clusters in a beam

Water clusters (H2O)(n) and (D2O)(n) (n <= 15) are produced by supersonic expansion and then pick up an additional heavy or light water molecule, respectively, or an amino-acid molecule (glycine or tryptophan). The products are analyzed by electron bombardment ionization mass spectrometry. Ionization proceeds via the well-known loss of an OH or OD group, but these turn out to have a strong predilection to come from the guest, rather than the host, molecule: between 30% and 60% of the time the loss originates in the picked-up molecule, even for large n. In fact, the magnitude of this fraction depends on the guest, but is largely insensitive to the cluster size. The observations suggest that the host clusters are frozen into compact annealed shapes, and the adducts reside on the surface and form an inhomogeneity where dissociative ionization tends to localize. It is also notable that no significant amino-acid fragmentation is observed beyond the hydroxyl group loss, in contrast to other measurements employing electron-impact ionization. (c) American Institute of Physics.