Supramolecular Catalysis of Orthoformate Hydrolysis in Basic Solution: An Enzyme-Like Mechanism

摘要

A water-soluble self-assembled supramolecular host molecule catalyzes the hydrolysis of orthoformates in basic solution. Comparison of the rate constants of the catalyzed and uncatalyzed reactions for hydrolysis displays rate accelerations of up to 3900 for tri-n-propyl orthoformate. Kinetic analysis shows that the mechanism of hydrolysis with the supramolecular host obeys the Michaelis-Menten model. Mechanistic studies, including ~(13)C-labeling experiments, revealed that the resting state of the catalytic system is the neutral substrate encapsulated in the host. Activation parameters for the k_(cat) step of the reaction revealed that upon substrate encapsulation in the assembly, the entropy of activation becomes mote negative in contrast to the uncatalyzed reaction. Furthermore, solvent isotope effects reveal a normal k(H_2O)/k(D_2O) = 1.6, confirming an A-S_e2 mechanism in which proton transfer occurs in the rate-limiting step. This is in contrast with the A1 mechanism of the uncatalyzed reaction in which decomposition of the protonated substrate is rate-limiting.