Ring Substituent Effects on the Thiol Addition and Hydrolysis Reactions of N-Arylmaleimides

Maleimide groups ate used extensively in bioconjugation reactions, but limited kinetic information is available regarding their thiol addition and hydrolysis reactions. We prepared a series of fluorogenic coumarin maleimide derivatives that differ by the substituent on their maleimide C=C bond. Fluorescence-based kinetic studies of the reaction with beta-mercaptoethanol (BME) yielded the second-Order rate constants (k(2)), while pH-rate studies from pH 7 to 9 gave base-catalyzed hydrolysis rate constants (k(OH)). Linear free-energy relationships were studied through the correlation of log k(2), and log k(OH) to both electronic (sigma(+)) and steric (E-s(norm)) parameters of the C=C substituent. These correlations revealed the thiol addition reaction is primarily sensitive to the electronic effects, while steric effects dominate the hydrolysis reaction. These mechanistic studies provide the basis for the design of novel bioconjugation reactants or fluorogenic labeling agents.