Enzymes Replace Cyanide in Eco-Friendly Nitrile Synthesis

If the household cleaner emits a lemon-like odor, it may be due to a nitrile called citronellyl nitrile. These adaptable chemical nitrile groups are also used to make chemical-resistant gloves, superglue, and active pharmaceutical ingredients.

The most common production method has involved chemically reacting some extremely toxic cyanide with specific molecules. Recently, Margit Winkler of the TU Graz Institute of Molecular Biotechnology and Ludmila Martínková of the Czech Academy of Sciences Institute of Microbiology developed a room-temperature, cyanide-free biocatalytic process that uses less energy and generates less hazardous waste. The Czech Science Foundation (GACR) and the Austrian Science Fund (FWF) provided funding for the project.

Two Enzyme-Guided Steps

The research group utilizes enzymes to replace cyanide and integrates three individual reactions into a cascade. The starting materials are carboxylic acids, such as fatty acids from plant oils or lignin, a byproduct of wood processing.

First, sugar, oxygen, and the enzyme carboxylate reductase—which is present in living bacterial cells—are added to the carboxylic acid to transform it into an aldehyde. Due to its high reactivity, this aldehyde is unstable and frequently volatile. This material is captured by allowing the aldehyde to react with the hydroxylamine, converting it into the stable compound oxime.

Since oxime is a "watered" form of nitrile, the last step involves the use of the enzyme aldoxime dehydratase. This enzyme, which is restricted to bacterial cells, draws water out of the oxime. After doing this, the cascade has completed its task and produced the desired amount of nitrile.

This technique can now be used to prepare small amounts of nitrile. This technology shows promise, especially for strong fragrances that require little to produce an effect. However, to make the biocatalytic process economically viable, the production of nitrile in greater quantities still needs optimization work.

The first stage of the process currently uses highly diluted solutions, and producing large quantities would need a large reactor, which is not yet financially feasible. That is why Margit Winkler is already researching ways to improve the effectiveness of the first step. On the other hand, oxime dehydration alone is incredibly effective and has already attained production-ready status technologically.

A Bio-Based Process

I’ve been interested in cyanide chemistry ever since I wrote my master thesis, and as we know from Agatha Christie’s novels, cyanide is highly toxic. It was therefore important for us to find a way to avoid this hazardous substance in production. The fatty acids we use come from vegetable oils. So, we really do have a bio-based production method here. We would now like to improve it even further so that it can be used widely.”

Margit Winkler, TU Graz Institute of Molecular Biotechnology