A Sustainable Route To Synthesize the Xanthommatin Biochrome via an Electro-catalyzed Oxidation of Tryptophan Metabolites

Xanthommatin (Xa) is a naturally occurring broad-spectrum dye present in arthropods and cephalopods. In these animals, Xa is enzymatically produced via the oxidative cyclization of 3-hydroxykynurenine (3-OHK), a metabolite of tryptophan, but it can also be synthesized in vitro using an iron-based oxidizing agent. While the optoelectronic properties (e.g., high refractive index, redox-sensitive color) of both natural and synthetic forms of Xa have inspired its use in materials, its synthesis faces challenges, in terms of both scalability and purity. We describe a procedure to synthesize Xa via electrochemical oxidation of 3-OHK, which offers both economic and ecological advantages over the traditional method. We used density functional theory calculations to describe the kinetics and thermodynamics of the cyclization of Xa based on the loss of eight electrons and protons from 2 molar equiv of 3-OHK. Our results provide insight into the production of this biochrome in a process that can be scaled up for future materials applications and/or adapted to the production of other phenoxazinones synthesized from ortho-aminophenols.