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Alliin is a
sulfoxide In organic chemistry, a sulfoxide, also called a sulphoxide, is an organosulfur compound containing a sulfinyl () functional group attached to two carbon atoms. It is a polar functional group. Sulfoxides are oxidized derivatives of sulfides. E ...
that is a natural constituent of fresh
garlic Garlic (''Allium sativum'') is a species of bulbous flowering plant in the genus ''Allium''. Its close relatives include the onion, shallot, leek, chive, Allium fistulosum, Welsh onion and Allium chinense, Chinese onion. It is native to South A ...
. It is a derivative of the
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 am ...
cysteine Cysteine (symbol Cys or C; ) is a semiessential proteinogenic amino acid with the formula . The thiol side chain in cysteine often participates in enzymatic reactions as a nucleophile. When present as a deprotonated catalytic residue, sometime ...
. When fresh garlic is chopped or crushed, the enzyme
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 ...
converts alliin into
allicin Allicin is an organosulfur compound obtained from garlic, a species in the family Alliaceae. It was first isolated and studied in the laboratory by Chester J. Cavallito and John Hays Bailey in 1944. When fresh garlic is chopped or crushed, the ...
, which is responsible for the aroma of fresh garlic. Allicin and other thiosulfinates in garlic are unstable and form a number of other compounds, such as diallyl sulfide (DAS),
diallyl disulfide Diallyl disulfide (DADS or 4,5-dithia-1,7-octadiene) is an organosulfur compound derived from garlic and a few other genus ''Allium'' plants. Along with diallyl trisulfide and diallyl tetrasulfide, it is one of the principal components of the dist ...
(DADS) and diallyl trisulfide (DAT), dithiins and
ajoene Ajoene is an organosulfur compound found in garlic (''Allium sativum'') extracts. It is a colorless liquid that contains sulfoxide and disulfide functional groups. The name (and pronunciation) is derived from "ajo", the Spanish word for garlic ...
. Garlic powder is not a source of alliin, nor is fresh garlic upon maceration, since the enzymatic conversion to allicin takes place in the order of seconds. Alliin was the first natural product found to have both carbon- and sulfur-centered stereochemistry.


Chemical synthesis

The first reported synthesis, by Stoll and Seebeck in 1951, begins the alkylation of -cysteine with
allyl bromide Allyl bromide (3-bromopropene) is an organic halide. It is an alkylating agent used in synthesis of polymers, pharmaceuticals, synthetic perfumes and other organic compounds. Physically, allyl bromide is a colorless liquid with an irritating and p ...
to form deoxyalliin. Oxidation of this sulfide with
hydrogen peroxide Hydrogen peroxide is a chemical compound with the formula . In its pure form, it is a very pale blue liquid that is slightly more viscous than water. It is used as an oxidizer, bleaching agent, and antiseptic, usually as a dilute solution (3%†...
gives both
diastereomer In stereochemistry, diastereomers (sometimes called diastereoisomers) are a type of stereoisomer. Diastereomers are defined as non-mirror image, non-identical stereoisomers. Hence, they occur when two or more stereoisomers of a compound have dif ...
s of -alliin, differing in the orientation of the oxygen atom on the
sulfur Sulfur (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formula ...
stereocenter In stereochemistry, a stereocenter of a molecule is an atom (center), axis or plane that is the focus of stereoisomerism; that is, when having at least three different groups bound to the stereocenter, interchanging any two different groups cr ...
. A newer route, reported by Koch and Keusgen in 1998, allows stereospecific oxidation using conditions similar to the
Sharpless asymmetric epoxidation The Sharpless epoxidation reaction is an enantioselective chemical reaction to prepare 2,3-epoxyalcohols from primary and secondary allylic alcohols. The oxidizing agent is ''tert''-butyl hydroperoxide. The method relies on a catalyst formed fro ...
. The chiral catalyst is produced from
diethyl tartrate Diethyl tartrate is an organic compound with the formula (HOCHCO2Et)2 (Et = ethyl). Three stereoisomers exist, R,R-, S,S-, and R,S (=S,R-). They are the ethyl esters of the respective R,R-, S,S-, and R,S-tartaric acids. The R,R- and S,S- isomers ...
and
titanium isopropoxide Titanium isopropoxide, also commonly referred to as titanium tetraisopropoxide or TTIP, is a chemical compound with the formula . This alkoxide of titanium(IV) is used in organic synthesis and materials science. It is a diamagnetic tetrahedral mo ...
.


Medical exploration

Garlic has been used since antiquity as a therapeutic remedy for conditions now associated with oxidative stress (production and accumulation of reactive oxygen species (ROS)). In an in vitro test (1991), garlic powder showed
antioxidant Antioxidants are compounds that inhibit oxidation, a chemical reaction that can produce free radicals. This can lead to polymerization and other chain reactions. They are frequently added to industrial products, such as fuels and lubricant ...
properties, and alliin showed good
hydroxyl In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydroxy ...
radical Radical may refer to: Politics and ideology Politics *Radical politics, the political intent of fundamental societal change *Radicalism (historical), the Radical Movement that began in late 18th century Britain and spread to continental Europe and ...
-scavenging effect. Alliin has also been found to affect immune responses in blood.


References

{{reflist Sulfoxides Sulfur amino acids Nutrition Allyl compounds