Sensitization to p-amino aromatic compounds: Study of the covalent binding of 2,5-dimethyl-p-benzoquinonediimine to a model peptide by electrospray ionization tandem mass spectrometry Original Research Article

To understand the hapten–protein complex formation in the context of skin contact allergy to p-amino aromatic derivatives, 2,5-dimethyl-p-benzoquinonediimine was used as a model compound to study the reactivity of p-benzoquinonediimines, first oxidation intermediates of allergenic p-amino aromatic compounds, toward a model peptide containing naturally occurring and potential reactive amino acids. LC–MS analysis, together with electrospray ionization MS/MS, was used for the determination of amino acid selectivity by studying the chemical modifications induced on the peptide due to covalent binding of the p-benzoquinonediimine. Results reported in this paper indicated that 2,5-dimethyl-p-benzoquinonediimine reacted with the ε-NH2 group of lysine to first form a covalent adduct of the Schiff’s base kind. Besides, an oxido-reduction process started that induced an oxidative deamination of lysine to form a peptidyl α-aminoadipic-δ-semialdehyde, by a mechanism similar to the one known for several enzymatic quinonoid co-factors, followed by an intramolecular cyclization of the peptide. From these results it could be concluded that lysine must be considered as an important amino acid for the hapten–protein complex formation in the case of p-benzoquinonediimines and that, in addition to direct covalent binding, further degradation of the peptide can be produced.