Hydrogen Atom Adducts to the Amide Bond. Generation and Energetics of Amide Radicals in the Gas Phase
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Abstract
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.
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