Enzymatic synthesis of novel water-soluble indigoid compounds

Abstract This work was aimed at expanding the diversity of indigoid compounds with novel characteristics by employing biological catalysts. A total of 16 novel indigoid compounds were synthesized using indole derivatives as substrates containing either aminomethyl-, hydroxymethyl-, carboxaldehyde or carboxyl groups at positions 4, 5, 6 or 7 of indole ring. Two different monooxygenase systems – a flavin dependent monooxygenase Hind8 and the mutant G109Q of multicomponent soluble diiron monooxygenase PML were employed to achieve the conversion of all those substrates. Characterization of purified indigo dicarboxylic acids revealed that the produced indigoids were soluble in water (solubility increased more than 1000-fold compared to indigo), also in methanol and DMSO. A bioconversion of indole-7-carboxylic acid resulted in the mixture of indigo-7,7′-dicarboxylic acid and indirubin-7,7′-dicarboxylic acid. Both of those products were separated from each another and purified thus providing the basis for the purification of water-soluble indigoids of similar structure. These findings show that (1) biological catalysts can be an easy approach for synthesizing novel indigoids with important characteristics and (2) introduction of carboxyl group can increase the water-solubility of indigo and indirubin significantly, thus providing a new tool in search for bioactive indigoids.