Thiamine-Appended Cyclodextrin Dimer as a Ligase Model

Abstract

Cyclodextrins (CD’s) have been extensively studied as enzyme models and as molecular receptors due to their abilities to bind various hydrophobic compounds into their hydrophobic cavities with substrate selectivity [1,2]. The most successful enzyme model in our studies is imidazole-appended 2,6-dimethyl-s-cyclodextrin [3]. This model caused about 1000-fold acceleration of the hydrolysis of p-nitrophenyl acetate, and k cat for this reaction of this model is 2.67 × 10 -2 s-1, which is over twice as much as that of α-chymotrypsin. Recently we found an artificial enzyme using a CD dimer has cooperative binding ability and allosteric regulation ability for hydrolysis reactions [4]. As the acyl chain length of the substrate, p-nitrophenylalkanoate was longer, the transition state of the hydrolysis reaction was more stabilized by the CD dimer. This new artificial enzyme shows the homotropic allosteric effect with the Hill constant of 1.8 for the hydrolysis reaction of p-nitrophenyl methoxyethoxyethoxyacetate as the substrate. So, CD dimers are expected to be also effective building blocks for enzyme models that catalyze synthetic reactions such as a carbon-carbon bond formation reaction. Thiamine pyrophosphate participates as the essential cofactor in numerous enzymatic reactions involving formation and breaking of carbon-carbon bonds, which are also important for synthetic organic reactions. In this symposium, we present the synthesis and characterization of the thiazolium-appended s-cyclodextrin dimer (1) for the benzoin condensation reaction (Scheme 1).