CARBON KINETIC ISOTOPE EFFECTS ON THE SPONTANEOUS AND ANTIBODY-CATALYZED DECARBOXYLATION OF 5-NITRO-3-CARBOXYBENZISOXAZOLE

The catalytic antibody 21D8 efficiently catalyzes the decarboxylation of a substituted 3-carboxybenzisoxazole-a simple, unimolecular reaction that is not susceptible to general acid/base catalysis but that is highly sensitive to the microenvironment. The transition-state structure of this decarboxylation reaction has been probed by measuring the carbon kinetic isotope effect for the uncatalyzed decarboxylation in water and for the dioxane-accelerated and antibody-catalyzed reactions. The isotope effect for the decarboxylation of 5-nitro-3-carboxybenzisoxazole is k12/k13 = 1.046 in aqueous buffer at 20-degrees-C and 1.048 for the antibody-catalyzed reaction. With increasing dioxane in the reaction medium, the rate of the spontaneous decarboxylation increases by almost four orders of magnitude, whereas the isotope effect displays only a slight, progressive decrease to k12/k13 = 1.043 in 100% dioxane. Together, these results indicate that the structure of the transition state undergoes very little change in spite of > 10(4)-fold increases in the rate of the reaction caused by solvation of the substrate by organic solvents or the low dielectric environment of the antibody active site.