Taking the poisonous bite out of cyanide and avoiding other poisons

Caterpillar on leaf.Unless you’re a falsely accused, innocent bystander at the scene of a crime, coincidences can be fun in life. I was demonstrating the reduction of permanganate ion (MnO4-) by thiosulfate ion (S2O32-) to my students. As the purple permanganate solution removes electrons from the thiosulfate, the brown product MnO2 appears. It can be more dramatic if HSO3- is used because the reaction products go completely colourless, but if bisulfite is not available, as was the case in our school, acidified MnO4- will also do the trick with thiosulfate.

On the same day, in a little black notebook1 that I use to keep track of various science tidbits, I came across the strange story of the eastern tent caterpillar, and I decided to share that story with my class. The caterpillars concentrate cyanoglucosides (specifically prunasin) from the leaves of wild cherry that they devour.

Prunasin structural formula.

Prunasin (notice the cyano group)2.

Enzymes then convert the compounds to cyanide, which the caterpillar spits out at its potential predators. Other defensive compounds such as acetone and benzaldehyde are also included in their saliva. Of course, it begs the question, how does such a caterpillar prevent poisoning itself?

This is where rhodanese — an enzyme found in the mitochondria of many animals — comes in. This enzyme can convert cyanide (CN-) to thiocyanate (SCN-), which is much less toxic. Most animals are susceptible to cyanide poisoning because the rhodanese is not distributed in the mitochondria of all tissues. This is probably not the case with caterpillars.

But where does the coincidence enter the picture? It is the question of where rhodanese gets the sulfur to attach to the cyanide group: from thiosulfate, the same ion (in solution) sitting on our demonstration desk. In other words, the thiosulfate that reduces the permanganate in our demonstration, participates in this reaction to convert cyanide to thiocyanate, hence saving this caterpillar from a toxic fate.

Then the following day there was another connection to the demonstration in the news. The Ottawa Citizen reported that paramedics had to treat 11 students at the Higher Learning Institute for chlorine poisoning. They had acidified potassium permanganate with hydrochloric acid. This was done either deliberately or accidentally, but in either case, the chloride ion was oxidized to chlorine gas by the stronger oxidizing agent, MnO4-. We had kept our demo safe by acidifying the electron-mugger with vinegar. Sulfuric acid would also have been a safe alternative. Students were awed by another coincidence, and by the fact that in chemistry there’s a fine line between a successful demo and a disaster.

Notes and references

  1. Enrico’s black notebook
    [We decided to show this page of equations to highlight how Enrico collects these tidbits in this book. He keeps it close at hand for referencing.]Handwritten notes on caterpillar converting CN- to SCN-.
  2. Photo source: https://en.wikipedia.org/wiki/Eastern_tent_caterpillar
  3. Chlorine News story