Add time:09/25/2019         Source:sciencedirect.com

Gas-phase dissociative electron attachment (DEA) measurements to N-methylformamide (NMF, HC(O)NHCH3), a simple molecule containing the peptide linkage, were performed. The anion yields as a function of electron energy below 15 eV were obtained using a crossed molecular beam/Quadrupole Mass Spectrometer (QMS) setup. DEA to NMF produces CN−, OCN−, NH2−, O−, HCO−, and NH−, similar to formamide. Isotopic studies with N-methyl-d3-formamide (d3-NMF, HC(O)NHCD3) have confirmed and clarified assignments. Newly observed dissociation channels from NMF result in the formation of CH3NHC(O)−/CH3NC(O)H−, CH3NH−, CO−, and CH3−. The dominant fragmentation pathways involve the formation of CN− and OCN− near 1.5 eV, which are produced from the electron capture by the π* orbital of the CO combined with the scission of the NCH3 bond and simultaneous formation of a CN π bond. The methyl neutral by-product assists in lowering the thermodynamic threshold for CN− and OCN− production as compared with the DEA to formamide. The [Parent-H]− signal at m/z 61 from d3-NMF confirms site-selectivity for formation of this ion and excludes H loss from the methyl group of NMF. Peptide bond (OCNH) dissociation is observed resulting in both CH3NH− and HCO−, but these channels require higher energies (>5 eV) to occur.

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