N-Cα bond dissociation energies and kinetics in amide and peptide radicals.: Is the dissociation a non-ergodic process?

Dissociations of aminoketyl radicals and cation radicals derived from beta-alanine N-methylamide, N-acetyl-1,2-diaminoethane, N-alpha-acetyl lysine amide, and N-alpha-glycyl glycine amide are investigated by combined density functional theory and Moller-Plesset perturbational calculations with the goal of elucidating the mechanism of electron capture dissociation (ECD) of larger peptide and protein ions. The activation energies for dissociations of N-C bonds in aminoketyl radicals decrease in the series N-CH3 > N-CH2-CH2NH2 much greater than N-CH2CONH2 approximate to N-CH2(CONH2)(CH2)(4)NH2. Transition state theory rate constants for dissociations of N-C. bonds in aminoketyl radicals and cation-radicals indicate an extremely facile reaction that occurs with unimolecular rate constants > 10(5) s(-1) in species thermalized at 298 K in the gas phase. In neutral aminoketyl radicals the N-C, bond cleavage results in fast dissociation. In contrast, N-C. bond cleavage in aminoketyl cation-radicals results in isomerization to ion-molecule complexes that are held together by strong hydrogen bonds. The facile N-C. bond dissociation in thermalized ions indicates that it is unnecessary to invoke the hypothesis of non-ergodic behavior for ECD intermediates.