Mass spectrometry techniques employing electron capture and electron transfer dissociation represent powerful approaches for the analysis of biological samples. Despite routine employment in analytical fields, the underlying physical processes dictating peptide fragmentation remain less understood. Among the most accepted mechanisms, the Cornell proposal of McLafferty postulates that the homolytic cleavage of N-C-alpha bonds located in the peptide backbone occurs on the right (C-terminal) side of a hydrogen acceptor carbonyl group. Here, we illustrate that an alternative "enol" mechanism, based on a hetetolytic N-C-alpha bond cleavage located on the left (N-terminal) side of an acceptor carbonyl group, not only is thermodynamically viable but also often represents the energetically preferred cleavage route.