Study of the gas-phase fragmentation behaviour of sulfonatedpeptides

Abstract

A series of singly and doubly protonated peptides bearing sulfonated residue have been studied, usingboth experiment and molecular modelling, to elucidate fragmentation chemistry of sulfonated peptides.Collision-induced dissociation mass spectra indicate that the sulfo group loss (neutral loss of 80 Da) isthe dominant dissociation channel. Modelling results suggest the proton transfer mechanism, whereupon vibrational excitation, the acidic side chain proton is transferred from the sulfo group hydroxyl tothe ester oxygen resulting in S O bond cleavage and formation of the unmodified hydroxyl containingresidue and SO3. Conformations associated with potential energy profile of the reaction imply the chargeremote nature of the proposed mechanism. The proposed proton transfer mechanism was compared withthe intramolecular nucleophilic substitution (SN2) mechanism, the main pathway suggested for neutralloss of phosphoric acid from phosphopeptides. Both pathways (proton transfer and SN2) are available forsulfonated and phosphorylated peptides ; however, each posttranslational modification favours differentmechanism. The change of the bond dissociation enthalpies and the ability of stabilising the transitionstate structures are demonstrated as main factors responsible for each posttranslational modificationactivating a different pathway