Diallyl thiosulfinate allicin

Bocchini, P., Andalo, C., Pozzi, R., Galletti, G.C. and Antonelli, A. (2001) Determination of diallyl thiosulfinate (allicin) in garlic (Allium sativum L.) by high performance liquid chromatography with a post-column photochemical reactor, Anal. Chim. Acta, 441(1), 37-43. 

The flavor precursors, 5 -alk(en)yl-L-cysteine sulfoxides, themselves do not have any specific flavor. However, when garlic or onion cells are damaged by cutting or crushing and the 5 -alk(en)yl-L-cysteine sulfoxides meet the C-S lyase (alliinase), they are converted into various volatile sulfuric compounds. Stoll and Seebeck (1951) first proposed the production of diallyl thiosulfinate (allicin) from the 5 -allyl-L-cysteine sulfoxide (alliin) by the C-S lyase. Through the reaction catalyzed by the C-S lyase, 5 -alk(en)yl-L-cysteine sulfoxides yield alk(en)yl sulfenic acid and aminoacrylic acid, the latter being spontaneously degraded... 

Attempts by Brodnitz et al. in 1971 to determine allicin by gas chromatography-mass spectrometry (GC-MS) provided evidence for an unusual mode of decomposition (70). They indicated that GC caused diallyl thiosulfinate to dehydrate according to the reaction C6HiqS20 -> C gS2 + H2O, affording a 2.4 1 mixture of two compounds, claimed (incorrectly as we shall see below) to be 3-vinyl-4H-l,2-dithiin (10, equation 1) and 3-vinyl-6H-l,2-dithiin (10a, equation 1) (70). Analytical methods for allicin quantitation that rely on GC detection of compounds 10 and 10a are still used (77), even though serious problems exist with this method and more accurate high-performance liquid chromatographic (HPLC) methods have been developed (see below). 

The volatile oil contains allicin (diallyldi-sulfide-5-oxide diallyl thiosulfinate), allyl-propyl disulfide, diallyl disulfide, and diallyl trisulfide as the major components, with lesser amounts of dimethyl sulfide, dimethyl disulfide dimethyl trisulfide, allylmethyl sulfide, 2,3,4-trithiapentane, bis-2-propenyl... 

At room temperature the thiosulfinates (e.g., allicin) convert to variorrs diallyl and allyl sulfides, etc., which may be accelerated by heating. Pnre allicin itself is an unstable oily liquid, lasting less than 20 hours at room temperature (equivalent to say a half-life of a few hours, as has been reported). The decomposition products include various organic sulfides, and it is also been observed that allicin is very reactive with the amino acid cysteine. 

The transformation products of allicin and odier thiosulfinates present in crushed garlic is greatly accelerated and expanded upon heating such as during steam distillation or cooking. As many as 20 different sulfides have been identified in the steam distillate and include diallyl mono to hexasulfides, allyl methyl mono to hexasulfides dimethyl mono to hexasulfides, and allyl 1-propenyl di and trisulfides [38]. 

The main products of alliin transformation in polar media are sulfides. At room temperature or on heating, allicin is converted by water into diaUyl disulfide, diaUyl trisulfide and diallyl polysulfides, which are the principal components of garlic essential oil and aged garlic (Figure 8.62). Other reaction products are allyl alcohol, sulfur dioxide and propene. Disproportionation of thiosulfinates in aqueous (polar) solutions also leads to disulfides and thiosuhonates of general formula R-SO2-S-R. For example. 

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