''syn''-Propanethial ''S''-oxide (or (''Z'')-propanethial ''S''-oxide), a member of a class of

organosulfur Organosulfur compounds are organic compounds that contain sulfur. They are often associated with foul odors, but many of the sweetest compounds known are organosulfur derivatives, e.g., saccharin. Nature abounds with organosulfur compounds—sulfur ...

compounds known as

thiocarbonyl In organic chemistry, thioketones (; also known as thiones or thiocarbonyls) are organosulfur compounds related to conventional ketones in which the oxygen has been replaced by a sulfur. Instead of a structure of , thioketones have the structu ...

''S''-oxides (formerly " sulfines"), is a volatile liquid that acts as a

lachrymatory agent Tear gas, also known as a lachrymator agent or lachrymator (), sometimes colloquially known as "mace" after the early commercial aerosol, is a chemical weapon that stimulates the nerves of the lacrimal gland in the eye to produce tears. In ad ...

(triggers tearing and stinging on contact with the eyes). The chemical is released from

onions An onion (''Allium cepa'' L., from Latin ''cepa'' meaning "onion"), also known as the bulb onion or common onion, is a vegetable that is the most widely cultivated species of the genus '' Allium''. The shallot is a botanical variety of the on ...

, ''Allium cepa'', as they are sliced. The release is due to the breaking open of the onion cells and their releasing enzymes called

alliinase In enzymology, an alliin lyase () is an enzyme that catalyzes the chemical reaction :an ''S''-alkyl-L-cysteine ''S''-oxide \rightleftharpoons an alkyl sulfenate + 2-aminoacrylate Hence, this enzyme has one substrate, ''S''-alkyl-L-cysteine ...

s, which then break down

amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha a ...

sulfoxides, generating sulfenic acids. A specific sulfenic acid, 1-propenesulfenic acid, formed when onions are cut, is rapidly rearranged by a second enzyme, called the lachrymatory factor synthase or LFS, giving ''syn''-propanethial ''S''-oxide. The gas diffuses through the air and, on contact with the eye, it stimulates sensory neurons creating a stinging, painful sensation. Tears are released from the tear glands to dilute and flush out the irritant. A structurally related lachrymatory compound, ''syn''-butanethial ''S''-oxide, C₄H₈OS, has been found in another genus ''Allium'' plant, ''

Allium siculum ''Allium siculum'' ( syn. ''Nectaroscordum siculum''), known as honey garlic, Sicilian honey lily, Sicilian honey garlic, or Mediterranean bells, is a European and Turkish species of plants genus ''Allium''. It is native to the regions around the ...

''.

References

{{DEFAULTSORT:Propanethial-S-oxide, syn- Organosulfur compounds Lachrymatory agents Pungent flavors Onions Sulfoxides

See also

References