title: 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.
authors.affiliation: Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, DK-8000 Aarhus C, Denmark.
authors.forename: Camilla Skinnerup
authors.initials: CS
authors.lastname: Jensen
authors.forename: Jean Ann
authors.initials: JA
authors.lastname: Wyer
authors.forename: Jørgen
authors.initials: J
authors.lastname: Houmøller
authors.forename: Preben
authors.initials: P
authors.lastname: Hvelplund
authors.forename: Steen Brøndsted
authors.initials: SB
authors.lastname: Nielsen
mesh_terms: D000595:Amino Acid Sequence; D000641:Ammonia / Q000737:chemistry; D002244:Carbon / Q000737:chemistry; D004151:Dipeptides / Q000737:chemistry*; D004583:Electrons; D019791:Guanidine / Q000737:chemistry; D006859:Hydrogen / Q000737:chemistry; D009584:Nitrogen / Q000737:chemistry; D011522:Protons
chemical_list: D004151:Dipeptides; D011522:Protons; D002244:Carbon; D000641:Ammonia; D006859:Hydrogen; D019791:Guanidine; D009584:Nitrogen