A MODEL-COMPOUND OF NOVEL COFACTOR TRYPTOPHAN TRYPTOPHYLQUINONE OF BACTERIAL METHYLAMINE DEHYDROGENASES - SYNTHESIS AND PHYSICOCHEMICAL PROPERTIES

The synthesis and physicochemical characterization of the model compou nd (1) of novel cofactor TTQ (tryptophan tryptophylquinone), the activ e center of bacterial methylamine dehydrogenases (MADH), are described . The synthesis of compound 1 3-methyl-4-(3'-methylindol-2'-yl)indole- 6,7-dione] has been accomplished by starting with 2-carbomethoxy-3-met hyl-7-methoxyindole (2) as follows. Friedel-Crafts acylation on 2 with propionyl chloride gave the 4-acylated compound, which was converted into 3-methyl-4-propionyl-7-methoxyindole (5) by ester hydrolysis foll owed by thermal decarboxylation. Construction of the second indole rin g by Fischer indolization with phenylhydrazine hydrochloride, followed by deprotection of the methoxy group, gave 7-hydroxy derivative 7, wh ich was finally converted into model compound 1 by oxidation with Frem y's salt. Another simple pathway mimicking one of the postulated biosy nthetic routes was also investigated; hydroxyskatoles were oxidized in to a 6,7-indolequinone derivative and then coupled with skatole itself using AlCl3 as a catalyst to give 1. Strong resemblances in several s pectroscopic properties and in molecular geometry exist between 1 and TTQ cofactor in MADH. Compound 1 exhibits a pair of reversible waves a scribed to a two-electron redox reaction of the quinoid moiety. At pH 7.0, the formal potential vs NHE is 0.107 V, which is close to that of the native enzyme (0.126 V). The spectroscopic titration and the form al potential measurements were employed to determine the pK(a) values of 1 (10.6-10.9) and its reduced form 1H(2) (10.1). Detailed digital s imulation analysis of the voltammograms has revealed a two-step one-el ectron transfer mechanism via its semiquinone radical (1(.-)), allowin g for the evaluation of the two one-electron redox potentials. The EPR spectroscopic measurements suggest a partial pi-conjugation between t he two indole rings and a nonrigid conformation. Reaction of 1 with fr ee ammonia leads to iminoquinone 13, which is reduced electrochemicall y at a more positive electrode potential than 1. The thermodynamics an d kinetics of the iminoquinone formation are also reported in detail. Generation of a radical species derived from 13 and its stability in c omparison with 1(.-) were also examined electrochemically and by EPR m easurements.