Stabilities of protonated water-ammonia clusters.

Branching ratios of water and ammonia evaporation have been measured for spontaneous evaporation from protonated mixed clusters H(HO)(NH) in the size range 0 ≤ n ≤ 11 and 0 ≤ m ≤ 7. Mixed clusters evaporate water except for clusters containing six or more ammonia molecules, indicating the formation of a stable core of one ammonium ion surrounded by four ammonia molecules and a second shell consisting predominantly of water. We relate evaporative branching ratios to free energy differences between the products of competing channels and determine the free energy differences for clusters with up to seven ammonia molecules.

Fragmentation of anthracene C₁₄H₁₀, acridine C₁₃H₉N and phenazine C₁₂H₈N₂ ions in collisions with atoms.

We report experimental total, absolute, fragmentation cross sections for anthracene C14H10, acridine C13H9N, and phenazine C12H8N2 ions colliding with He at center-of-mass energies close to 100 eV. In addition, we report results for the same ions colliding with Ne, Ar, and Xe at higher energies. The total fragmentation cross sections for these three ions are the same within error bars for a given target.

Unimolecular dissociation of anthracene and acridine cations: the importance of isomerization barriers for the C2H2 loss and HCN loss channels.

The loss of C(2)H(2) is a low activation energy dissociation channel for anthracene (C(14)H(10)) and acridine (C(13)H(9)N) cations. For the latter ion another prominent fragmentation pathway is the loss of HCN. We have studied these two dissociation channels by collision induced dissociation experiments of 50 keV anthracene cations and protonated acridine, both produced by electrospray ionization, in collisions with a neutral xenon target.

Electron-capture induced dissociation of doubly charged dipeptides: on the neutral losses and N-Cα bond cleavages.

Electron capture by doubly charged peptide cations leads to neutral losses in addition to N-C(α) bond cleavages that give c and z fragments. In this work we discuss the influence of amino acid sequence on hydrogen versus ammonia loss and the propensity for subsequent partial side-chain cleavage after ammonia loss to give w fragment ions. Experiments were done on two series of doubly protonated dipeptides, [XK+2H](2+) and [XR+2H](2+), where X is one of the twenty common amino acid residues, excluding aspartic acid (D), and K and R are lysine and arginine, respectively.

Dipole-guided electron capture causes abnormal dissociations of phosphorylated pentapeptides.

Electron transfer and capture mass spectra of a series of doubly charged ions that were phosphorylated pentapeptides of a tryptic type (pS,A,A,A,R) showed conspicuous differences in dissociations of charge-reduced ions. Electron transfer from both gaseous cesium atoms at 100 keV kinetic energies and fluoranthene anion radicals in an ion trap resulted in the loss of a hydrogen atom, ammonia, and backbone cleavages forming complete series of sequence z ions. Elimination of phosphoric acid was negligible.