GOODNESS-OF-FIT IN COMPLEXES BETWEEN SUBSTRATES AND RIBONUCLEASE MIMICS - EFFECTS ON BINDING, CATALYTIC RATE CONSTANTS, AND REGIOCHEMISTRY

The hydrolysis of 4-tert-butylcatechol cyclic phosphate and of 4-methy lcatechol cyclic phosphate catalyzed by alpha-cyclodextrin-6A,6B-bisim idazolide, and by the corresponding derivatives of beta-cyclodextrin ( beta CD) and gamma-cyclodextrin (gamma CD), was examined, All three ca talysts were able to hydrolyze the substrate derived from 4-methylcate chol, but only the beta CD- and gamma CD-based catalysts could hydroly ze the substrate based on 4-tert-butylcatechol. Saturation kinetics we re observed, from which k(cat) and K-m values were derived. The k(cat) 's showed a bell-shaped dependence on pH, indicating a bifunctional me chanism in which one imidazole acted as a base while the other, proton ated, acted as an acid catalyst. The strongest binding was observed be tween the substrate derived from tert-butylcatechol and the beta CD-ba sed catalyst, and this combination also had the highest k(cat), An Arr henius plot showed that the good fit of the tert-butyl group into the beta CD cavity leads to an entropy advantage for catalysis within the complex, as well as an advantage in the binding. Furthermore, the comb ination with the best binding and catalytic constant also exhibited th e highest regioselectivity in the substrate hydrolysis, being essentia lly completely selective for one mode of hydrolysis. All these effects reflect the result of a tight fit of the substrate into the catalyst binding cavity, leading to a well-defined reaction geometry.