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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 produceHCN 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:

glucose

N

C

OH

O

N

C

OH

OH

O

OH

OH

CH N

B glucosidase hydroxynitrile lyase

glucose

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. Thismethod 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 grindyour sample. Dont 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. Transferthe 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 frompale 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 asCuAc2; CuAc2 in full would be C12H18CuO6)

Can you come up with a hypotheses for why some samples released cyanide gas

and others didnt? Think about what parts of a plant are more important evolutionarily.

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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 youthink 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 quantitativedetermination 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: 282-284.