Cyanide is any chemical compound that contains the cyano group (C≡N), consisting of a carbon atom triple-bonded to a nitrogen atom.
Cyanides are produced by certain bacteria, fungi, and algae and are found in a number of foods and plants. Cyanide is found, although in small amounts, in apple seeds, mangoes and bitter almonds. In plants, cyanides are usually bound to sugar molecules in the form of cyanogenic glycosides and defend the plant against herbivores. Cassava roots (also called manioc), an important potato-like food grown in tropical countries (and the base from which tapioca is made), contains cyanogenic glycosides.
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The word "cyanide" was extracted from "ferrocyanide", which proved to be a compound of iron and what is now known as the cyanide ion. Ferrocyanides and ferricyanides were first discovered as Prussian blue, and were so named because Prussian blue contains iron and is blue; ??????? is Greek for "blue".
Many cyanide-containing compounds are highly toxic, but some are not. Prussian blue, with an approximate formula Fe7(CN)18 is the blue of blue prints and is administered orally as an antidote to poisoning by thallium and Caesium-137.
The most dangerous cyanides are hydrogen cyanide (HCN) and salts derived from it, such as potassium cyanide (KCN) and sodium cyanide (NaCN), among others. Some compounds readily release HCN or the cyanide ion, such as trimethylsilyl cyanide (CH3)3SiCN upon contact with water and cyanoacrylates upon pyrolysis.
Cyanide is an inhibitor of the enzyme cytochrome c oxidase (also known as aa3) in the fourth complex of the electron transport chain (found in the membrane of the mitochondria of eukaryotic cells.) It attaches to the iron within this protein. The binding of cyanide to this cytochrome prevents transport of electrons from cytochrome c oxidase to oxygen. As a result, the electron transport chain is disrupted, meaning that the cell can no longer aerobically produce ATP for energy. Tissues that mainly depend on aerobic respiration, such as the central nervous system and the heart, are particularly affected. Antidotes to cyanide poisoning include hydroxocobalamin and sodium nitrite which release the cyanide from the cytochrome system, and rhodanase, which is an enzyme occurring naturally in mammals that combines serum cyanide with thiosulfate, producing comparatively harmless thiocyanate.
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