On the Viability of Heterolytic Peptide N-Calpha Bond Cleavage in Electron Capture and Transfer Dissociation Mass Spectrometry
While frequently employed as an experimental technique, the mechanistic picture surrounding the gas-phase dissociation of peptides carrying multiple positive charges during electron capture and electron transfer dissociation tandem mass spectrometry remains incomplete. Despite this mechanistic uncertainty, most proposals agree that the peptide backbone N-C-alpha bond located to the C-terminal (right) side of an aminoketyl radical formed in a peptide backbone during the electron capture process is homolytically cleaved. Recently, we introduced the "enol" mechanism, which proposes that a backbone N-C-alpha bond located to the N-terminal (left) side of an aminoketyl radical is cleaved heterolytically. Here, we further validate this mechanism using replica-exchange molecular dynamics to create unbiased representative sets of low-energy conformers for several model tryptic peptide systems (H-Ala(x)-Lys-OH2+, x = 3-5). Transition state barrier enthalpies for the cleavage of N-C-alpha bonds proceeding via the homolytic (right-side) and heterolytic (left-side) pathways, determined by density functional computations, identify the preferred cleavage route for each conformer. These findings support our original hypothesis that heterolytic N-C-alpha cleavage can exist in a competitive balance with homolytic cleavages, independent of the relative energy of the precursor dication species. Smaller peptide systems see decreased heterolytic N-C-alpha cleavage probabilities, likely resulting from an insufficient hydrogen-bonding network needed to stabilize and ultimately annihilate the transition state zwitterion. This observation may explain the early dismissal of left-side cleavage pathways based on computational studies employing small model systems.