Cyanogenesis in Plants
Cyanogenesis is the process by which plants produce hydrogen cyanide gas (HCN) to defend themselves against herbivores. HCN is poisonous not only to animals that eat plants, but to the plants themselves. To prevent poisoning themselves, the plants store cyanogenic glycosides1 in a vacuole of the cell and an enzyme that acts to produce HCN in a separate compartment. When the cell is damaged, the compartment walls are breached, allowing the reaction to take place. In this way, HCN is produced only when needed. Equation of Cyanogenesis:
N C glucose N C HC N O
O
glucose
HO
B glucosidase OH OH
HO
hydroxynitrile lyase OH
We can visualize the production of HCN using Feigl-Anger strips. These strips change color in the presence of cyanide gas, from nearly white to blue or purple. This method is a qualitative rather than quantitative one; in other words, we will be noting whether or not HCN is produced, but not how much. Although cyanide is toxic to people, the amounts we will be producing are small enough not to bother us. Procedure: Obtain about 2-3 apple seeds, or a portion of another fruit seed. In order to release HCN, we will have to damage the cells. Using mortar and pestle, carefully grind your sample. Don’t spend too much time grinding. If the sample is too dry, add one or two drops of water. Too much water will mess up the experiment, so only add water is it is absolutely necessary. Also, be careful not to get the Feigl-Anger strips wet. Transfer the sample to a tube. Place one Feigl-Anger strip over the top of the tube and cork in place. Look for color change. A color change signals that cyanide is being released. The strips will turn from pale blue-green to bright blue or purple in the presence of cyanide gas. The color change may take a few minutes, so be patient. While waiting, prepare another sample in a separate beaker. Equation for Fiegl-Anger strip: 2 CuAc2 + C17H22N2 + 2 HCN 2 CuCN + [C17H22N2]Ac + 3 HAc (Note: For simplicity the original article abbreviated copper II ethylacetoacetate as CuAc2; CuAc2 in full would be C12H18CuO6) Can you come up with a hypotheses for why some samples released cyanide gas and others didn’t? Think about what parts of a plant are more important evolutionarily.
According to Extoxnet, an internet source for toxicity of various compounds, almonds, cassava root, sorghum and lima beans all produce hydrogen cyanide. Cassava is an important food in many parts of the world. Based on our experiment, how do you think people can safely eat cassava?
1
cyanogenic means producing cyanide; glycosides are sugar containing compounds
Sources: Brinker, A.M. and Seigler, D.S. 1989. Methods for the detection and quantitative determination of cyanide in plant materials. Phytochemical Bulletin 21(2): 24-31. Feigl, F. and Anger, V. 1966. Replacement of benzidine by copper ethylacetoacetate and tetra base as spot-test reagent for hydrogen cyanide and cyanogen. Analyst, 91: 282284.