We have studied the outcome of collisions between the hydrated nucleotide anion adenosine 5′-monophosphate (AMP) and sodium. Electron capture leads to hydrogen loss as well as water evaporation regardless of the initial number m of water molecules attached to the parent ion (m ⩽ 16). The yield of dianions with microsecond lifetimes increases strongly with m, which is explained from dielectric screening of the two charges by the water nanodroplet. For comparison, collision induced dissociation results in water losses with no or very little damage of the AMP molecule itself.

We have studied fragmentation of water embedded adenosine 5’-monophosphate(AMP) anions after collisions with neutral sodium atoms. At a collision energy of 50 keV,loss of water molecules from the collisionally excited cluster ions is the dominant process andfragmentation of the AMP itself is almost completely prohibited if the number of attachedwater molecules is larger than 13. However, regardless of the initial number of water moleculesattached to the ion, capture of an electron, i.e. formation of a dianion, always leads to loss ofa single hydrogen atom accompanied by evaporation of water molecules. This damaging effectbecomes more important as the size of the water cluster increases, which is just the oppositeto the protective behavior observed for collision induced dissociation (CID) without electrontransfer. For both cases, the loss of water molecules within the experimental time frame isqualitatively well described by means of a common model of an evaporative ensemble. Thesesimulations, however, indicate that characteristically different distributions of internal energyare involved in CID and electron capture induced dissociation.

The fragmentation of doubly protonated AK dipeptide ions has been investigated after collisional electron transfer. Electron capture leads to three dominant channels, H loss, NH₃ loss, and N–C_α bond breakage to give either c⁺ or z⁺ fragment ions. The relative importance of these channels has been explored as a function of ion velocity, the degree of complexation with crown ether, and collision gas. Our results indicate that H loss and NH₃ loss are competing channels whereas the probability of N–C_α bond breakage is more or less constant.

The branching ratio between ammonia loss and NC_α bond cleavage of singly charged microsolvated peptides after electron capture from cesium depends on the solvent molecule attached. Density functional calculations reveal that for [GA+H]⁺(CE) (G=glycine, A=alanine, CE=crown ether), the singly occupied molecular orbital of the neutral radical is located mainly on the amide group (see picture).

The results from an experimental study of bare and microsolvated peptide monocations in high-energy collisions with cesium vapor are reported. Neutral radicals form after electron capture from cesium, which decay by H loss, NH₃ loss, or NC_α bond cleavage into characteristic z^. and c fragments. The neutral fragments are converted into negatively charged species in a second collision with cesium and are identified by means of mass spectrometry. For protonated GA (G=glycine, A=alanine), the branching ratio between NH₃ loss and NC_α bond cleavage is found to strongly depend on the molecule attached (H₂O, CH₃CN, CH₃OH, and 18-crown-6 ether (CE)). Addition of H₂O and CH₃OH increases this ratio whereas CH₃CN and CE decrease it. For protonated AAA ([AAA+H]⁺), a similar effect is observed with methanol, while the ratio between the z₁ and z₂ fragment peaks remains unchanged for the bare and microsolvated species. Density functional theory calculations reveal that in the case of [GA+H]⁺(CE), the singly occupied molecular orbital is located mainly on the amide group in accordance with the experimental results.

We have studied electron capture induced dissociation of a set of doubly protonated pentapeptides, all composed of one lysine (K) and either four glycine (G) or four alanine (A) residues, as a function of the sequence of these building blocks. Thereby the separation of the two charges, sequestered on the N-terminal amino group and the lysine side chain, is varied. The characteristic cleavage of N–C_α bonds is observed for all peptides over the whole backbone length, with the charge carrying fragments always containing K. The resulting fragmentation patterns are very similar if G is replaced by A. In the case of [XKXXX+2H]²⁺ (X=A or G), a distinct feature is observed in the distribution of backbone cleavage fragments and the probability for ammonia loss is drastically reduced. This may be due to an isomer with an amide oxygen as protonation site giving rise to the observed increase in breakage at a specific site in the molecule. For the other peptides, a correlation with the distance between amide oxygen and the charge at the lysine side chain has been found. This may be an indication that it is only the contribution from this site to the charge stabilization of the amide π^* orbitals which determines relative fragment intensities. For comparison, complexes with two crown ether molecules have been studied as well. The crown ether provides a shielding of the charge and prevents the peptide from folding and internal hydrogen bonding, which leads to a more uniform fragmentation behavior.

A novel cryogenic electrostatic storage device consisting of two ion-beam storage rings with a common straight section for studies of interactions between oppositely charged ions at low and well-defined relative velocities is under construction at Stockholm University. Here we consider the prospect of using this new tool to measure cross-sections and rate coefficients for mutual neutralization reactions of importance in interstellar ion chemistry in general and specifically in cosmic pre-biotic ion chemistry.