Electron-capture-induced dissociation of microsolvated di- and tripeptide monocations: elucidation of fragmentation channels from measurements of negative ions.

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, NH(3) loss, or N-C(alpha) 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 NH(3) loss and N-C(alpha) bond cleavage is found to strongly depend on the molecule attached (H(2)O, CH(3)CN, CH(3)OH, and 18-crown-6 ether (CE)). Addition of H(2)O and CH(3)OH increases this ratio whereas CH(3)CN and CE decrease it. For protonated AAA ([AAA+H](+)), a similar effect is observed with methanol, while the ratio between the z(1) and z(2) 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.

A new technique for time-resolved daughter ion mass spectrometry on the microsecond to millisecond time scale using an electrostatic ion storage ring.

A new method for time-resolved daughter ion mass spectrometry is presented, based on the electrostatic ion storage ring in Aarhus, ELISA. Ions with high internal energy, e.g., as a result of photoexcitation, dissociate and the yield of neutrals is monitored as a function of time. This gives information on lifetimes in the microsecond to millisecond time range but no information on the fragment masses. To determine the dissociation channels, we have introduced pulsed supplies with switching times of a few microseconds. This allows rapid switching from storage of parent ions to storage of daughter ions, which are dumped into a detector after a number of revolutions in the ring. A fragment mass spectrum is obtained by monitoring the daughter ion signal as a function of the ring voltages. This technique allows identification of the dissociation channels and determination of the time dependent competition between these channels.

On the mechanism of electron-capture-induced dissociation of peptide dications from 15n-labeling and crown-ether complexation.

15N-labeling of di- and tripeptides reveals that electron capture to doubly protonated peptides results almost exclusively in ammonia loss from the N-terminal end, which clearly shows that a significant fraction of electron capture occurs at this end. In accordance with this finding, the competing channel of N-Calpha bond breakage leads to z+* ions and neutral c fragments after electron capture to small dications. In larger peptides that live long enough for internal proton exchanges to occur, c+ ions are also formed and in some cases in dominant yield. Attachment of one or two crown ethers to ammonium groups is likely to reduce the probability of proton transfer, which enhances the formation of z+* relative to c+. The total yield of z+* and c+ is, however, more or less unchanged, which indicates that proton transfer or hydrogen transfer from a NH3 group to the amide group is not required for the N-Calpha bond breakage.