Green route to a blue dye

The industrial process for making and using indigo—the dye that gives blue denim its distinctive color—has two strikes against it in terms of environmental friendliness. First, the synthesis involves hazardous chemicals, including aniline, formaldehyde, and strong bases. Second, because indigo isn’t water soluble, the synthesized compound must be treated with reducing agents before it can be used as a dye. A team led by John E. Dueber of the University of California, Berkeley, has devised a microbial fermentation that results in a more sustainable process for using indigo (Nat. Chem. Biol. 2018, DOI: 10.1038/nchembio.2552). They engineered Escherichia coli bacteria to produce indoxyl—a precursor to indigo—from tryptophan. They also engineered the bacteria to produce a glycosyltransferase, an enzyme that adds glucose to indoxyl as a protecting group to stabilize it. The glycosylated compound, indican, is sufficiently stable for long-term storage. When the dye is needed, the glucose can be enzymatically removed with a β-glucosidase to regenerate indoxyl, which spontaneously dimerizes, yielding the reduced form of indigo, the form that crystallizes directly onto cotton fibers. The researchers dyed cotton fabric by spraying it with an indican solution, dipping it in a β-glucosidase solution, and oxidizing the dye compound in air.