Prepared by Assoc Prof Darryl Small (v4, Sept 2012) questions/feedback to <darryl.small@rmit.edu.au>

Molecule of the week Cyanide compounds

This sheet describes a very interesting group of molecules, each of which contains cyanide in some form or other. It has long been known that cyanides are among the most toxic compounds and these should only be handled with the greatest care and precautions (if absolutely necessary). However the purpose here is to contrast three different groups of molecules containing cyanide. These demonstrate the principle that, in different molecular forms, the properties of a chemical group can vary widely.

Cyanide ions as a toxin A variety of simple molecules and ionic salts are able to readily release cyanide either as an

ion (CN–) or a gas (HCN). Both of these are readily absorbed by our bodies and can cause instant death. The cyanide impacts directly on the energy release systems of cells, especially the cytochrome energy transfer pathway which is irreversibly blocked so that ATP is no longer generated. It is significant that cyanide in these forms is toxic to all living things, reflecting the fact that all utilise the same cytochrome pathway for energy capture.

Cyanide as a component of food additives Based on the serious toxicity issues described above, most people would be shocked to know that there are some food additives which contain cyanide [1]. These are sodium ferrocyanide {Na4FeII(CN)6} and potassium ferrocyanide and these are often referred to by their E numbers, which are 535 and 536 respectively. Note here that the E number is also known as the INS number, where INS refers to the International Numbering System. These two additives are approved for use in Australia and New Zealand as anti-caking agents.

We can be quite sure that these additives are safe and that they have already undergone extensive evaluations that demonstrate safety beyond all reasonable doubts. The role of an anti-caking agent is to absorb water preferentially so that the main ingredient or food material remains dry and retains its ability to flow without forming a solid material (a “cake”). Such additives are also referred to as being “hygroscopic” indicating that they readily absorb water from the air. In both of these compounds the cyanide ions are very strongly bound to the iron ion in the form of a stable complex. The cyanide is not released from the complex under any conditions that would ever be found in food processing, storage or our digestive tract.

Naturally occurring cyanide compounds in foods There are several molecules (structures -) found in plants where we can see the –CN structure which has a triple bond between the C and N atoms [2, 3]. In some well documented cases we know that free cyanide can be released from these molecules. Some of these are found in foods, and so there are toxicity issues if these foods form a significant part of a diet.

The best known example of cyanogenic components of foods is the molecule known as amygdalin found in the kernels of

apricots and related plants. This is a cyanogenic glycoside (note the spelling is cyanogenic rather than cyanogenetic) and these compounds are also found in cassava but their concentrations are usually reduced prior to ingestion by appropriate processing.

The structures of the compounds show that there are either one or two glucose units present, and these are in the anomeric configuration. The presence of the glucose will ensure that the molecules are water soluble. This factor is relevant to the removal of the potentially toxic compounds [2] as prolonged soaking in water is often sufficient to remove the cyano-compound.

HOO

OHO

OH

HO

N

HO

O

HO

H

OH

O

amygdalin

dhurrin

HOO

OHO

OH

HO

N

OH

linamarin

HOO

OHO

OH

HO

N

CH3

CH3

Other aspects of cyanide compounds in foods Cyanide compounds have been used as reagent in various traditional food analysis procedures reflecting the reduction/oxidation involvement of the associated Fe atoms. In addition, the form of vitamin B12 in supplements is cyanocobalamin which has a labile Co-CN bond. The levels consumed are so low that release of free cyanide is extremely low and poses no risk to health.

References and further reading [1] ANZFSC standard 1.2.4. [2] The textbooks by Fennema and [3] Coultate both have

interesting sections on cyanogenic glycosides in foods.