N[bond]C(alpha) 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 Møller-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[bond]C bonds in aminoketyl radicals decrease in the series N[bond]CH(3) > N-CH(2)CH(2)NH(2) >> N[bond]CH(2)CONH(2) approximately N[bond]CH(CONH(2))(CH(2))(4)NH(2). Transition state theory rate constants for dissociations of N[bond]C(alpha) 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[bond]C(alpha) bond cleavage results in fast dissociation. In contrast, N[bond]C(alpha) bond cleavage in aminoketyl cation-radicals results in isomerization to ion-molecule complexes that are held together by strong hydrogen bonds. The facile N[bond]C(alpha) bond dissociation in thermalized ions indicates that it is unnecessary to invoke the hypothesis of non-ergodic behavior for ECD intermediates.
Department of Chemistry, Bagley Hall, Box 351700, University of Washington, Seattle, Washington 98195-1700, USA. turecek@chem.uwashington.edu
Frantisek
F
Turecek
D000409:Alanine / Q000031:analogs & derivatives; D000577:Amides / Q000737:chemistry*; D002412:Cations / Q000737:chemistry; D005609:Free Radicals / Q000737:chemistry; D006033:Glycylglycine / Q000031:analogs & derivatives; D008239:Lysine / Q000031:analogs & derivatives; D010455:Peptides / Q000737:chemistry*; D013816:Thermodynamics
D000577:Amides; D002412:Cations; D005609:Free Radicals; D010455:Peptides; D006033:Glycylglycine; D008239:Lysine; D000409:Alanine