Fragmentation of protonated thioether conjugates of acrolein using low collision energies

Abstract

The protonated mercapturic acid conjugate of acrolein, S-(3-oxopropyl)-N-acetyl-L-cysteine (I), undergoes facile retro-Michael loss of acrolein in the gas phase. To determine whether extensive loss of acrolein would impede structural characterization of acrolein-peptide adducts, fragmentation reactions of a series of conjugates, formed by 1,4-Michael addition of acrolein to peptides and cysteine derivatives, were investigated at collision cell potentials up to −50 V using a triple quadrupole mass spectrometer. Differences in fragmentation dynamics suggest protonation at the sulfur of the N-acetylcysteine conjugate I facilitates retro-Michael elimination of acrolein with a low activation energy relative to other fragmentations. Analogous fragmentation was eliminated after borohydride reduction of the aldehyde to an alcohol. Retro-Michael fragmentation was not significant for acrolein conjugates of glutathione derivatives, suggesting that proton sequestration occurs in peptides with multiple amide linkages even when the peptide does not contain a basic amino group. An unexpected outcome of these experiments was the observation of a facile gas-phase cleavage of peptides on the N-terminal side of S-(3-oxopropyl)cysteine residues. Such fragmentation behavior may prove useful for locating cysteine residues in peptides.