Hydrogen atom adducts to the amide bond. Generation and energetics of amide radicals in the gas phase

The 1-hydroxy-1-(N-methyl)aminoethyl radical (1) represents a simple model system for hydrogen atom adducts to the amide bond in gas-phase peptide and protein ions relevant to electron capture dissociation (ECD). Radical 1 was generated in the rarefied gas phase by femtosecond electron transfer to the stable cation prepared by selective O-protonation of N-methylacetamide. The main dissociations of 1 were loss of the hydroxyl hydrogen atom and the N-methyl group in a 1.7:1 ratio, as deduced from product analysis and deuterium labeling. The dissociations that occur on the 4.1 microsecond time scale are driven by large Franck-Condon effects on collisional electron transfer that deposit 93-103 kJ mol(-1) in the nascent radicals. Detailed analysis of the potential energy surface for dissociations of 1 revealed several conformers and isomeric transition states for dissociations of the O-H and N-CH3 bonds. The experimental branching ratio is in quantitative agreement with RRKM calculations within the accuracy of the G2 potential energy surface and favors cleavage of the O-H bond in 1 and loss of H. This finding contrasts previously reported results, as discussed in the paper.