Nuclear Magnetic Resonance Studies on Covalent Modification of Amino Acids Thiol and Amino Residues by Monofunctional Aryl 13C-Isocyanates, Models of Skin and Respiratory Sensitizers: Transformation of Thiocarbamates into Urea Adducts

Abstract

Exposure to aryl isocyanates, intermediates in the manufacture of polyurethanes, provokes lung sensitization and asthma but also occupational allergic contact dermatitis, sensitization occurring from a single accidental exposure. The initial step in the sensitization process is believed to be the covalent binding of the -N triple bond C triple bond O group with nucleophilic residues on proteins. While a wide knowledge exists on the reactivity of skin sensitizers toward amino acids, little is known about respiratory sensitizers such as aryl isocyanates. (13)C-Labeled monofunctional aryl isocyanates were synthesized, and their reactivities toward nucleophilic amino acids, GSH, and a model peptide were studied by (13)C and [(1)H-(13)C] NMR spectroscopy. An acetonitrile/buffer solution was used as a solvent to avoid the hampering of the follow up of the reactivity by the isocyanate hydrolysis competing reaction. The compounds reacted with thiol groups, through the formation of thiocarbamate bonds and with amino groups to form urea derivatives. The reactivity was confirmed with GSH, containing both free amino and thiol groups, and with a model peptide, particularly in the case of the reaction with lysine. The use of (13)C NMR to follow the aryl isocyanates reversible conjugation with thiol groups is also reported. Particularly, it is shown that thiocarbamate adducts can be converted into adducts of the urea kind by reaction with amino groups. These results confirmed the hypothesis by which thiol-containing peptides/proteins may act as carriers of isocyanates for possible reaction at a later time and/or place with other nucleophiles and confirmed the role of lysine as a good competing nucleophilic amino acid. The reactivity of aryl isocyanates with thiol and amino groups needs thus to be considered in their assigned sensitization processes.