title: On the viability of heterolytic peptide N-C(α) bond cleavage in electron capture and transfer dissociation mass spectrometry.
abstract: 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-Cα 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-Cα 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-Alax-Lys-OH(2+), x = 3-5). Transition state barrier enthalpies for the cleavage of N-Cα 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-Cα 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-Cα 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.
authors.affiliation: Biomolecular Mass Spectrometry Laboratory, Ecole Polytechnique Fédérale de Lausanne , 1015 Lausanne, Switzerland.
authors.forename: Matthew D
authors.initials: MD
authors.lastname: Wodrich
authors.forename: Konstantin O
authors.initials: KO
authors.lastname: Zhurov
authors.forename: Clémence
authors.initials: C
authors.lastname: Corminboeuf
authors.forename: Yury O
authors.initials: YO
authors.lastname: Tsybin
mesh_terms: D004583:Electrons; D013058:Mass Spectrometry; D015394:Molecular Structure; D010455:Peptides / Q000737:chemistry*
chemical_list: D010455:Peptides