Green route to a blue dye | January 15, 2018 Issue - Vol. 96 Issue 3 | Chemical & Engineering News
Volume 96 Issue 3 | p. 9 | Concentrates
Issue Date: January 15, 2018

Green route to a blue dye

Bioengineered process for making indigo removes need for reducing agents
Department: Science & Technology
News Channels: Organic SCENE, Biological SCENE, Organic SCENE, Biological SCENE
Keywords: Biotechnology, biological chemistry, indigo, fermentation, bioengineering
Indigo produced by bacteria is used to dye a cotton scarf.
Credit: Nat. Chem. Biol.
Photo of a scarf that's been dyed with indigo produced by engineered bacteria.
Indigo produced by bacteria is used to dye a cotton scarf.
Credit: Nat. Chem. Biol.

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.

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ISSN 0009-2347
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