Synthetic biology could tackle the issues of release of nitrous oxide during manufacturing, dependence on non-renewable petroleum.

A tower in the shape of a giant test tube with flashing lights to simulate bubbling chemicals greeted visitors to DuPont’s “Wonder World of Chemistry” pavilion at the 1939 New York World’s Fair. Inside, they were treated to a display of nylon, the company’s new miracle material “made from coal and air.” In 1935, DuPont chemist Wallace Carothers had combined adipic acid and hexamethylenediamine to make “nylon 6,6” with the “6,6” terminology stemming from each component having six carbon atoms. Both are derived from cyclohexanol, which in turn was made from benzene obtained from the distillation of coal tar. Thenitrogen needed for the synthesis of hexamethylenediamine came from the air, hence “made from coal and air.”

Spectators were amazed as they saw nylon fibre being produced in front of their eyes and then woven into stockings. Next came a tug of war with the stockings to demonstrate the strength of the material. A year after the fair, nylon stockings went on sale to the public with five million being sold the first day! Nylon, touted as the first synthetic material deemed to be better than a natural one, became so popular that hucksters even tried to pawn off silk stockings as nylon.

At the time, there was no concern about the environmental consequences of producing nylon, or about it being made from non-renewable raw materials. Today, the nylon industry is huge, with Nylon 6 also having entered the picture. That was the brainchild of Paul Schleck at IG Farben in Germany who, in 1939, based on Carothers’s work, polymerized a small molecule containing six carbon atoms, caprolactam, into Nylon 6. Some 18 million tons of combined nylons are now produced every year with a commercial value of around $10 billion. Toothbrush bristles, lingerie, swimwear, tents, toys, sutures, fishing nets, artificial turf, airbags, reinforcing cord for tires, automobile parts and “invisible” thread for magic tricks all feature nylon.

The usefulness of nylon is beyond question, but there are questions about the environmental impact of producing the plastic on such an enormous scale. There is the issue of the raw materials being sourced from non-renewable petroleum. Then there is the problem of nitrous oxide being released when cyclohexanol is treated with nitric acid to produce adipic acid. Nitrous oxide is commonly known as “laughing gas,” but its appearance in the atmosphere is no laughing matter. It is a greenhouse gas, 300 times more potent than carbon dioxide and accounts for roughly 10 per cent of the greenhouse effect. True, nitrogen fertilizer and animal manure are far greater sources of nitrous oxide, but nylon production is a significant contributor, with about 30 grams being produced for every kilo of adipic acid.

The adipic acid industry now uses either catalysts or high temperatures to convert nitrous oxide waste to innocuous nitrogen gas, but the ideal would be to produce adipic acid without any formation of nitrous oxide. Enter “synthetic biology.” Besides solving the nitrous oxide problem, it can also tackle the issue of using petroleum to make nylon.

Synthetic biology is the manipulation of microbes such as yeasts, fungi or bacteria with a goal of producing useful chemicals. In the simplest form, naturally occurring microbes are used with the production of carbon dioxide by yeast to make dough rise, a classic example. By the 1970s, scientists had discovered methods to modify the DNA, that is, the genetic code of an organism, by inserting a gene from another organism. Bacillus thuringiensis (Bt) is a soil bacterium that produces a toxin fatal to plant-eating insects. The gene that codes for this toxin has been isolated and inserted into the DNA of food crops such as corn and soybeans, thereby enabling them to produce the toxin to deter insects, reducing the need for synthetic pesticides.

This initial form of recombinant DNA technology relied on the use of naturally existing genes. The next goal was the possible synthesis of genes in the laboratory by linking together nucleotides, small molecules that are the building blocks of DNA. By the 1990s, the required methodologies had been worked out and synthetic genes were being inserted into the DNA of bacteria, essentially converting them into little factories to produce the chemicals encoded by the synthetic genes.

Now getting back to nylon, the target for researchers was a gene that codes for an enzyme allowing glucose to be converted to adipic acid. Inserting this gene into the DNA of a bacterium, E. coli for example, would then allow the genetically modified bacterium to convert glucose into adipic acid. Glucose is readily available from plants such as corn, meaning that adipic acid can be produced from a renewable resource, eliminating the need for petroleum. Furthermore, there is no cyclohexanol oxidation involved, so no production of nitrous oxide.

An American company, Genomatica, has now developed a process using synthetic biology to produce not only adipic acid but also caprolactam. It has already manufactured a ton of Nylon 6, demonstrating that the technology works. It is now a question of scaling up production, which is underway a partnership with France’s Aquafil. This company’s plant in Slovenia has been dedicated to the production of renewably sourced nylon with greatly reduced greenhouse gas emissions.

Nylon production has come a long way since the shortages experienced in the Second World War, when women were asked to give up their nylon stockings so they could be turned into parachutes. Now the challenge is to make those stockings and parachutes in an environmentally friendly, “green” fashion.

Joe Schwarcz is director of McGill University’s Office for Science & Society (mcgill.ca/oss). He hosts The Dr. Joe Show on CJAD Radio 800 AM every Sunday from 3 to 4 p.m.

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