A Study of the Intermediate of Catechol Dioxygenase Model Reaction by Co(III)tetramine Complexes and Catechols

Abstract

Abstract A model system consisting of several cobalt(III)-tetramine complexes and 3,5-di-tert-butyl catechol (DBcat) has been examined to elucidate the reaction mechanism of catechol dioxygenase. The complexes were made to react with DBcat in mixed solvent (H2O:MeOH =1:1) in the presence or in the absence of dioxygen. NMR measurements showed that the complex with DBcat reacts with dioxygen to yield oxidatively cleavaged products of DBcat. As the intermediate species of the reaction, a cobalt(III)-semiquinone(DBsq) radical and a high-spin cobalt(II) complex were detected by ESR measurements regardless of the presence of dioxygen. This suggests that these species are in equilibrium with a high-spin cobalt(II)-DBsq binary complex formed by one-electron transfer from DBcat to the mother complex. The measurement of the magnetic moment of the binary complex isolated from the solution exhibited that there is an antiferromagnetic spin-coupling between the high-spin(S=3/2) cobalt(II) ion and the semiquinone radical. It is proposed that the oxidative cleavage reaction proceed via this electron transferred intermediate.