Gas-phase fragmentation of di- and tripeptides via ion–molecule reactions with ClPCl +

Abstract

The reactivity of the phosphenium ion ClPCl + toward gaseous neutral di- and tripeptides was examined by Fourier transform ion cyclotron resonance mass spectrometry. ClPCl + was found to react with the peptides at collision rate. Extensive fragmentation of the peptides leads to only one (dipeptides) or two (tripeptides) dominant product ions. For each dipeptide, the major product ion has a molecular weight that is 34 u less than that of the dipeptide, i.e. the side chains are retained in the fragment ion. A substantial amount of effort was given to resolve the structure of this product ion, with a focus on the ion ( m/z 98) generated from reaction of Gly–Gly (MW 132) with ClPCl +. The molecular formula of the ion of m/z 98 was determined to be C 4H 6NO by exact mass measurements. The relative energies of a variety of isomeric ions with this elemental composition were examined by molecular orbital calculations in order to identify the most likely structures. Collision-activated dissociation experiments, mechanistic considerations, and comparison of calculated adiabatic recombination energies of isomeric ions to that determined experimentally for the unknown ion, led to the assignment of a six-membered, highly unsaturated ring structure to the product ion of m/z 98 of Gly–Gly. This assignment is supported by the finding that the experimentally determined acidity of the unknown ion is in excellent agreement with that calculated for the proposed six-membered structure. The most likely pathway for the formation of this product ion, as well as those obtained for the other dipeptides, involves an electrophilic attack of ClPCl + at the amide carbonyl oxygen of the dipeptide, followed by a hydride shift, proton transfer, and loss of H 2O and Cl 2PO ·. The very high exothermicity of the initial addition (69 kcal mol −1, AM1) allows the adduct to form high-energy intermediates (highest estimated to lie at 34 kcal mol −1 above the adduct) during the reaction that is exothermic by about 36 kcal mol −1 overall. An analogous mechanism readily rationalizes the formation of the two major product ions observed for tripeptides. These reactions involve the cleavage of each peptide bond, yielding product ions that contain the intact side chains of two adjacent amino acids. Similar observations were made for the BrPBr + reagent ion. The results presented here suggest that ClPCl + (and BrPBr +) may provide useful sequence information for peptides.