Reactions of N-acetylcysteine adducts of aromatic (di)isocyanates with functional groups of organic molecules

Abstract

Glutathione thiocarbamate conjugates of isocyanates play a key role in transport and final reactions of isocyanates in the human body by transcarbamoylation. N-acetylcysteine is the simplest model for thiocarbamate reactions. Therefore, transcarbamoylation of Nacetylcysteine adducts of p-tolylisocyanate (pTI-AcCys) and 4,4’-diisocyanatodiphenyl- methane (MDI(AcCys)2) with N-acetylcysteine methyl ester (thiolysis), morpholine (aminolysis), methoxyethanol (alcoholysis), and water (hydrolysis) has been studied in aqueous phosphate buffer solution and in dimethylacetamide (DMAc). Expected reaction products have been synthesised as reference compounds for HPLC-analysis. Concentrations of adducts and of reaction products were monitored by HPLC. Reaction rates and activation energies were determined for pTI in both media, reactions of MDI(AcCys)2 were run at one temperature only. Formation of insoluble reaction products and side reactions due to hydrolysis prevented in depth kinetic analysis of the reactions. Two regimes of reaction rate were observed in aqueous buffer, clear second order kinetics resulted in DMAc. In aqueous buffer (pH 7.4) a reactivity thiolysis > aminolysis > hydrolysis was found, while in DMAc aminolysis was faster than thiolysis. This can be explained by formation of thiolate at pH 7.4, which is not possible in anhydrous DMAc. Reactions of (MDI(AcCys)2) are by a factor of 2 to 4 faster than those of pTI-AcCys. p-Toluidine (pTA) was found in the aqueous system due to hydrolysis, while no 4,4’-methylene dianiline (MDA) could be detected. Under physiological conditions hydrolysis should compete with thiolysis under homogeneous conditions while ureas and carbamates should be much more stable against hydrolysis. No free isocyanate groups could be detected in any of the reactions. In conclusion the isocyanate moiety in thiocarbamates is readily transferred to sulfhydryl- and amino groups but not to aliphatic hydroxy groups. Under physiological conditions hydrolysis competes with these transcarbamoylation reactions. Formation of free isocyanate groups in analytical quantities was shown to be highly unlikely.