Allicin reactions

Disulfides (73) are oxidised by hydrogen peroxide to the corresponding sulfenic acids (45) dialkyl disulfides may also be converted to thiol sulfinates by treatment with perbenzoic acid thus, diallyl disulfide (71) yields allicin (72), an essential component of garlic (Scheme 55). The latter reaction involves a nucleophilic attack of the sulfur atom on the oxygen of the peroxide group, and therefore the more electron-rich sulfur atom is preferentially oxidised. For instance, methyl phenyl disulfide (85) yields the sulfoxide (86) (Scheme 56). 

The active constituents are usually secondary metabolites, derived from biosynthetie pathways present within the plant tissue. Allicin, a sulphur eompound, present in garlie (Allium sativum) is considered to be the active constituent. It is produced from alliin by an enzymatic reaction in response to injury. Allicin due to its noxious smell proteets garlic from attack of pests also. Thus an active constituent has therapeutic as well as protective activity. 

As mentioned above, allicin is not stable and in the presence of water and the oxygen in the air it is transformed to polysulphides which give the strong characteristic smell of the air expired from the lungs— the same reaction takes place in the human body as in the test tube. 

The thiosulfinates (e.g., allicin) arc vray reactive, combining with reducing agents and vmdergoing spontaneous reactiorrs in various solvents and media (Lawson, in Koch and Lawson, 1996, pp. 59-65). The reaction products are more stable than the thiosulfinates but still contain the thioallyl groups (5-allyl) or thiomethyl groups, and are active components in garlic oils. 

S-allyl mercaptocysteine (SAMC) results from the reaction of allicin with cysteine. It is undetected in whole garlic, but as chopped garlic ages, protein hydrolysis yields cysteine which then reacts with allicin to yield SAMC whose amount increases as chopped garlic ages [37]. 

Onions may not make us Hve forever, but some of their components may indeed have medical benefits — lowering cholesterol and blood pressure and perhaps even reducing the risk of cancer. This is why their chemistry has drawn a great deal of attention. We now know that sHcing an onion sets off a cascade of reactions that begins with alliin, a compotmd that occurs naturally in onions and garlic. When the tissues of the onion are disturbed, allicinase — an enZ)one that converts alliin to allicin — is released. This, in turn, breaks down to syn-propanethial-s-oxide, the stuff that makes your eyes water. 

It is also feasible to monitor enzymatic reactions taking place inside biological tissues by bringing the specimens very close to the ion source [84]. Biotransformation of alliin to allicin by allinase in raw garlic cloves could be monitored in vivo by internal EESI-MS. The reactants were extracted by an infused solution running throughout the tissue. The extract was ionized on the edge of the specimen [84]. 

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). 

Miron T, Listowsky I, Wilchek M (2010) Reaction mechanisms of allicin and allyl-mixed disulfides with proteins and small thiol molecules. Eur J Med Chem 45(5) 1912-1918. doi 10.1016/j.ejmech.2010.01.031... 

Galano A, Francisco-Marquez M (2009) Peroxyl radical scavenging activity of garlic 2-propenesulfenic acid vs allicin. J Phys Chem B 113 16077-16081 Galano A, Francisco-Marquez M (2009) Reactions of OOH radical with P-carotene, lycopene, and torulene hydrogen atom transfer and adduct formation mechanisms. J Phys ChemB 113 11338-11345... 

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... 

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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