MODEL STUDIES OF TTQ-CONTAINING AMINE DEHYDROGENASES

The reactions of a TTQ model compound 3-methyl-4-(3'-methylindol-2'-yl )indole-6,7-dione] with several amines have been investigated in organ ic media to obtain mechanistic information on the action of quinoprote in methylamine and aromatic amine dehydrogenases. It has been found th at compound 1 acts as an efficient catalyst for the autorecycling oxid ation of benzylamine by molecular oxygen in CH3OH. In order to evaluat e the oxidation mechanism of amines by 1, the product analyses and kin etic studies have been carried out under anaerobic conditions. In the first stage of the reaction of 1 with amines, 1 is converted into an i minoquinone-type adduct (so-called substrate imine), which was isolate d and characterized by using cyclopropylamine as a substrate. The obse rved NOE of the isolated product indicates clearly that the addition p osition of the amine is C-6 of the quinone. The molecular orbital calc ulations suggest that the thermodynamic stability of the carbinolamine intermediate is a major factor to determine such regioselectivity; th e C-6 carbinolamine is more stable than the C-7 counterpart by 2.9 kca l/mol. The reactivity of several primary amines and the electronic eff ect of the p-substituents of benzylamine derivatives in the iminoquino ne formation suggest that the addition step of the amine to the quinon e is rate-determining. When amines having an acidic alpha-proton such as benzylamine derivatives are employed as substrates, formation of th e iminoquinone adduct was followed by rearrangement to the product imi ne. The kinetic analysis has revealed that this rearrangement consists of noncatalyzed and general base-catalyzed processes. Large kinetic i sotope effects of 7.8 and 9.2 were observed for both the noncatalyzed and general base-catalyzed processes, respectively, since these steps involve a proton abstraction from the alpha-position of the substrate. In the reaction with benzhydrylamine, the product imine was isolated quantitatively and well characterized by several spectroscopic data. I n the case of benzylamine, the product imine is further converted into the aminophenol derivative by the imine exchange reaction with excess benzylamine. These results indicate clearly that the amine oxidation by compound 1 proceeds via a transamination mechanism as suggested for the enzymatic oxidation of amines by TTQ cofactor.