Biomimetic Models for Cysteine Proteases. 3. Acylation of Imidazolium-Thiolate Zwitterions by p-Nitrophenylacetate as a Model for the Acylation Step and Demonstration of Intramolecular General-Base-Catalyzed Delivery of H2O by Imidazole to Thiol Esters as a Model for the Deacylation Step

Article abstract of DOI:10.1021/ja00311a074

As biomimetic models for the cysteine proteases, four imidazole-thiol pairs, 4(5)-(mercaptomethyl)imidazole (1a), 2-(mercaptomethyl)imidazole (2a), 2-(mercaptomethyl)-N-methylimidazole (3a), and 2-(4,5-dimethylimidazol-2-yl)benzenethiol (4a) are studied as a function of pH as to their propensity to attack p-NPA and dinitrophenylacetate (for 4a).All species (except 1a) attack through their thiolate forms and show a plateau region at intermediate pH values which is attributable to attack by the thiolate anion of the zwitterionic forms (ImH⁺-S^-). 1a attacks as its thiolate at high pH and through imidazole N at neutrality.Potentiometric and UV-visible spectrophotometric titrations establish quantitatively the microscopic pK_a values from which are derived the fraction of individual species at any pH.General-base assistance of thiol attack on the acylating agent by the proximal imidazole is not required to explain the result.Deacylation of the corresponding thiol esters 1c-4c is studied as a function of pH, and in all cases, a pleteau region from pH 6.5-7 to 8.5-10 is observed.Solvent deuterium isotope effects from 1.88 (1c) to 3.75 (4c) are observed at neutral pH values.In all cases, the origin of the plateau region stems from a general-base-promoted delivery of H2O to the thiol ester by the proximal imidazole.Trapping experiments with Ellman's reagent suggest that S- --> N-acyl transfer is not an important process for these systems.The relevance of these findings is discussed in terms of the mechanism of action of the cysteine proteases.