Alliin is a sulfoxide that is a natural constituent of fresh garlic. 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 a ...

cysteine. 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, th ...

, 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. 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 di ...

s of -alliin, differing in the orientation of the oxygen atom on the sulfur

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 c ...

A newer route, reported by Koch and Keusgen in 1998, allows stereospecific oxidation using conditions similar to the Sharpless asymmetric epoxidation. The chiral catalyst is produced from diethyl tartrate 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 m ...

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 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 hydro ...

radical-scavenging effect. Alliin has also been found to affect immune responses in blood.

References

{{reflist Sulfoxides Sulfur amino acids Nutrition Allyl compounds