Indigo plants have been used to dye fabrics for thousands of years. Unlike other dyes, indigo is not chemically bonded to textile fibers; rather, it adsorbs to the surface of the wires. This allows the white cores of the fibers to show through to varying degrees after wear. Hence the impossible-to-replicate look of perfectly worn jeans.
But indigo plants yield only a small amount of the dye. It’s not nearly enough to keep up with the huge demand that Levi Strauss unleashed when he invented blue jeans in the 1870s. Now, after more than a century of relying on many toxic chemicals to create a synthetic version, researchers have developed bacteria that will make it.
The demand for blue dye was handled by one of Strauss’ fellow Bavarians – Adolph von Baeyer, of aspirin fame. He found a way to make a synthetic version of indigo, for which he won the Nobel Prize in Chemistry in 1905.
But the industrial process currently used to synthesize indigo is bad for the environment. The raw material used to make it is the petroleum product benzene (Baeyer studied under Kekulé, who first discovered the structure of benzene). Benzene is toxic, flammable, volatile, carcinogenic and neurotoxic.
And the problems don’t stop there. After treating benzene with formaldehyde, hydrogen cyanide and other harmful chemicals, we get indigo. It is insoluble in water and therefore must be treated with a corrosive reducing agent to become an active dye. Finally, the wastewater from most paint factories ends up in rivers because it is such a hassle to purify it.
Researchers at UC Berkeley — across the bay from where Strauss made his first jeans — have found a more sustainable way. Genetically engineered bacteria had already been used to replace the chemical synthesis of indigo; but this new work has also succeeded in avoiding the use of the dangerous reducing agents needed to make the dye active. Results are reported in Nature Chemical Biology.
Instead of using the enzyme from the indigo plants traditionally grown on plantations in India, Europe and America, these forward-thinking bioengineers used that from the Japanese indigo plant, which produces a higher yield. The enzyme uses an amino acid (tryptophan) as a starting material and generates a precursor to indigo, which then keeps it in a protected, inactive state by linking it to a sugar molecule.
To activate the enzyme, the glucose must be removed either enzymatically or by acid treatment. This is a commercial challenge, the authors admit, but is less toxic than the current reliance on reducing agents.
They have this gene in E coli and grew the insects in flasks overnight. The glucose-protected indigo precursor is easily excreted into the broth so they don’t even have to purge it from the bacteria – they just dipped denim samples directly into the growth media. Even after washing, the fabric remained blue.
Nature Chemical Biology2018. DOI: 10.1038/NCHEMBIO.2552 (About DOIs).