MICROSCOPIC MEDIUM EFFECTS ON A CHEMICAL-REACTION - A THEORETICAL-STUDY OF DECARBOXYLATION CATALYZED BY CYCLODEXTRINS AS AN ENZYME MODEL

The reaction of decarboxylation of phenylcyanoacetate anion catalyzed by cyclodextrins (CDs) was studied theoretically. The reaction field theory developed in our previous study, which makes it possible to investigate the electronic structure of a given molecule in the heterogeneous environment, is applied with the MNDO method. First, the reaction path in vacuo is investigated. The stationary state points and the corresponding molecular structures are found. Subsequently, the activation energies are evaluated for the reaction occurring in some homogeneous dielectrics to simulate normal solvent environments. The calculated values of the activation energy become smaller as the substrate is more deeply penetrated into the a polar cavity of CD, which is consistent with the fact that there action of decarboxylation proceeds faster in the beta-CD cavity. One of the most interesting findings is that there action can be effectively accelerated even if the reaction center, the carboxyl group of the substrate, is not surrounded by the low dielectric medium formed by the CD cavity but exposed to the bulk water. According to Arrhenius' theory, both calculated and observed activation energies in the CD-catalyzed reaction correspond to 10(2)-10(6) times acceleration effects in comparison with the aqueous solution case. Its efficiency is comparable to enzymic catalysis. On the basis of these results, the role of active sites of enzymes is discussed.