Electron transfer to protonated beta-alanine N-methylamide in the gas phase: an experimental and computational study of dissociation energetics and mechanisms.

Ammonium radicals derived from protonated beta-alanine N-methyl amide (BANMA) were generated by femtosecond collisional electron transfer to gas-phase cations prepared by chemical ionization and electrospray. Regardless of the mode of precursor ion preparation, the radicals underwent complete dissociation on the time scale of 5.15 micros.

Adenine radicals in the gas phase: an experimental and computational study of hydrogen atom adducts to adenine.

The elusive hydrogen atom adduct to the N-1 position in adenine, which is thought to be the initial intermediate of chemical damage, was specifically generated in the gas phase and characterized by neutralization-reionization mass spectrometry. The N-1 adduct, 1,2-dihydroaden-2-yl radical (1), was generated by femtosecond electron transfer to N-1-protonated adenine that was selectively produced by electrospray ionization of adenine in aqueous-methanol solution. Radical 1 is an intrinsically stable species in the gas phase that undergoes specific loss of the N-1-hydrogen atom to form adenine, but does not isomerize to the more stable C-2 adduct, 1,2-dihydroaden-1-yl radical (5).

Electron-rich radicals by neutralizationreionization mass spectrometry. Generation, dissociations and energetics of the hydrogen atom adduct to acetamide.

Protonated acetamide exists as two planar conformers, the more stable anti-form (anti-1(+)) and the syn-form (syn-1(+)), DeltaG(degree) (298) (anti-->syn) = 10.8 kJ mol(-1). Collisional neutralization of 1(+) produces 1-hydroxy-1-amino-1-ethyl radicals (anti-1 and syn-1) which in part survive for 3.

Toward a general mechanism of electron capture dissociation.

The effects of positive charge on the properties of ammonium and amide radicals were investigated by ab initio and density functional theory calculations with the goal of elucidating the energetics of electron capture dissociation (ECD) of multiply charged peptide ions. The electronic properties of the amide group in N-methylacetamide (NMA) are greatly affected by the presence of a remote charge in the form of a point charge, methylammonium, or guanidinium cations. The common effect of the remote charge is an increase of the electron affinity of the amide group, resulting in exothermic electron capture.

Peptide cation-radicals. A computational study of the competition between peptide N-Calpha bond cleavage and loss of the side chain in the [GlyPhe-NH2 + 2H]+. cation-radical.

Cation-radicals and dications corresponding to hydrogen atom adducts to N-terminus-protonated N(alpha)-glycylphenylalanine amide (Gly-Phe-NH(2)) are studied by combined density functional theory and Møller-Plesset perturbational computations (B3-MP2) as models for electron-capture dissociation of peptide bonds and elimination of side-chain groups in gas-phase peptide ions. Several structures are identified as local energy minima including isomeric aminoketyl cation-radicals, and hydrogen-bonded ion-radicals, and ylid-cation-radical complexes. The hydrogen-bonded complexes are substantially more stable than the classical aminoketyl structures.

Mechanism and energetics of intramolecular hydrogen transfer in amide and peptide radicals and cation-radicals.

Intramolecular hydrogen transfer in five model amide and peptide radicals and cation-radicals was investigated by combined B3LYP-MP2 calculations. Hypervalent ammonium radicals produced by electron capture in protonated peptides undergo competitive elimination of ammonia, H-atom loss, and H-atom migration to neighboring amide carbonyls. The calculated transition state energies for H-atom migration are slightly but uniformly lower than those for H-atom loss.