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

Abstract

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.

Serine effects on collision-induced dissociation and photodissociation of peptide cation radicals of the +●-type

“…Such species are sometimes denoted as z +• -type fragment ions, provided the ammonia molecule was eliminated from the protonated N -terminus, which is typical for peptide ions containing lysine C -terminal residues [36]. However, there is evidence [37] that ammonia eliminated from arginine C -terminated peptide cation-radicals partly originates from the guanidine group, rendering the fragment ion structure uncertain.…”

Electron Transfer Reduction of the Diazirine Ring in Gas-Phase Peptide Ions. On the Peculiar Loss of [NH4O] from Photoleucine

“…However, for further investigations, we used a more convenient technique for the specific generation of such stable, long-lived peptide cation radicals [13]. This technique consists of first generating by electrospray ionization a noncovalent peptide-18-crown-6-ether complex [37][38][39] as a doubly charged gas-phase ion, which is then mass-selected (m/z 368, Figure 2b), chargereduced, and stripped of the crown ether ligand by ETD or in combination with CID. Figure 2b illustrates the specific formation of abundant, long-lived (GL*GGR +2H) +• (m/z 472) ions upon ETD of the CE complex (m/z 368).…”

Ground and Excited-Electronic-State Dissociations of Hydrogen-Rich and Hydrogen-Deficient Tyrosine Peptide Cation Radicals

“…A fundamentally different method of generating peptide cation radicals relies on electron attachment to multiply protonated peptides via capture of a slow free electron [12] or collisional transfer from a suitable neutral [13][14][15][16][17][18] or anionic donor [19]. Peptide cation radicals of this type are called Bhydrogen rich.^They are often unstable and undergo extensive dissociations by multiple channels forming backbone fragment ions and providing sequence information for protein analysis [1].…”