THE BIMOLECULAR GAS-PHASE REACTION OF PROTONATED ALKYLDIPEPTIDES WITHACETONYLACETONE

The gas-phase reaction of protonated alkyldipeptides with acetonylacet one has been studied in a Fourier-transform ion cyclotron resonance (F T-ICR) mass spectrometer. The reaction is identified as the gas-phase analogue of Paal-Knorr pyrrole synthesis. Under thermal conditions, th e reaction complex loses a water molecule during a condensation reacti on, which couples the dipeptide to the acetonylacetone molecule via an imine bond. Low energy collisional activation of the long-lived imine product ion induces additional loss of a water molecule to form the p rotonated 2,5-dimethylpyrrole derivative of the dipeptide. Detailed in sight into the mechanism is obtained by a comparison of the reactivity of various alkyldipeptides with model compounds with amino functional groups. The reaction is catalysed by the peptide carbonyl groups, whi ch assist in the protonation of the acetonylacetone carbonyl oxygen at oms, making the acetonylacetone carbonyl carbon atoms susceptible to n ucleophilic attack by the peptide amino group. From both the previousl y studied bimolecular hydrogen-deuterium exchange behaviour and the pr esently studied reaction with acetonylacetone, it follows that the bim olecular reactivity of protonated alkyldipeptides is related to the ex tent of mobility of the proton within the reactive complex. (C) 1997 E lsevier Science B.V.