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. While it was previously established that NH(3) is lost exclusively from the N-terminal ammonium group and not from side-chain ammonium groups, we find here that ammonia can be lost from guanidinium radicals as well. The ratio between H loss and NH(3) loss reveals some information on internal ionic hydrogen bonds and peptide conformation since proton sharing between the N-terminal ammonium group and a basic side chain decreases the probability for NH(3) loss due to a lower recombination energy and as a result reduced capture probability. The abundance of w ions was found to correlate with the reaction energy for their formation; highest yield was found for CK and lowest for AK and HK. The survival rate of charge-reduced species was higher for XR than for XK, which is likely linked to the formation of long-lived C(α) radicals in the latter case. The probability for N-C(α) bond cleavage is smaller on average for XR than for XK which indicates that hydrogen transfer from the ε-ammonium radical to the amide group triggers some of the cleavages, or is a result of the different distances between the amide group and the charges in XR and XK. Finally, our data support the previous concept that charge partitioning between c and z fragments can be explained by competition between the two fragments for the proton.

Here we report on the charge partition between c and z fragments formed after femtosecond collisional electron-transfer from Cs atoms to charge-tagged peptide dications. Peptides chosen for study were Ala-Lys (AK) and Lys-Lys (KK) where one or both of the lysine -amino groups were trimethylated to provide one or two fixed charges. For peptides with only one charge tag, the other charge was obtained by protonation of an amino group. In some experiments the ammonium group was tagged by 18-crown-6-ether (CE). Since recombination energies decrease in the order:, it is possible to change the probability for the transferred electron to end up at either the N-terminal or the C-terminal residue by CE attachment. We find, however, that the individual recombination energies have little influence on the relative ratio between the yield of c and z ions as long as there are no mobile protons that can be transferred between the two fragments. Our results can be accounted for by the Utah-Washington model where the electron is captured into an amide * orbital that weakens the N-C ␣ bond and causes its breakage, followed by proton, electron, or hydrogen transfer between the c and z fragments that stay together as an ion-molecule complex for some time. The data are also in accordance with the notion that an amide group competes with the charged groups for the electron. Electron capture by charged groups results in loss of small neutrals such as hydrogen and ammonia. (J Am Soc

Charge partitioning after electron capture induced dissociation of dipeptide dications is determined by proton mobility in the evanescent ion-molecule complex as the remaining proton has enough time to choose the fragment with the highest proton affinity.

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