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Gas Chromatography (continued)

  
 Figure 1. The thermal decomposition of allicin (Brodnitz, 1971)

Despite this and other contemporary studies, many authors continued to draw conclusions based on products formed by GC analysis of Allium preparations using injection port temperatures as high as 280°C
Further work on the decomposition of allicin confirmed the formation of two C6H8S2 isomers, 3-vinyl-6H-1,2-dithiin (Figure 1) later being correctly identified as 2-vinyl-4H-1,3-dithiin (Figure 2).


 
 Figure 2. The thermal decomposition of allicin (Block et al, 1986)

Since 1971 a number of papers have appeared using the thioacrolein dimers shown in Figure 2 as GC markers for the presence of allicin, however this technique has been shown to be flawed since other thiosulphinates containing the S-2 propenyl group  (e.g.. RS(O)SCH2CH=CH2) could also give rise to the same dimers.
Further interesting chemistry is associated with the major disulphides from garlic, i.e. diallyl disulphide and 1-propenyl disulphide. Upon heating, diallyl disulphide undergoes a sequence of complex reactions leading both to diallyl polysulphides as well as a series of acyclic and heterocyclic compounds resulting from the generation and reaction of thioacrolein and radical species CH2=CHCH2Sn with precursor diallyl polysulphides. Since most of these compounds have been identified in distilled garlic oil, they may be assumed to arise from the thermal breakdown of diallyl disulphide. A comparison of the results of HPLC and GC analyses however leads to an unusual observation: simply because a product has been distilled does not guarantee that it will survive GC analysis In a comprehensive comparative GC-MS analysis of two garlic essential oils Vernin and Metzger published the analysis shown in Table 1. As will be seen later, all of the compounds identified are breakdown products of the primary flavour compounds (thiosulphinates) of garlic and are typical of the results of GC analysis of both garlic oil and extracts which undergo thermal decomposition during analysis.

Mechanisms of formation of sulphide derivatives

The formation of the thermal degradation compounds found in garlic oils (and in GC analyses of garlic extracts) is explained by Block by a sequence involving,

a) a C-S homolysis of diallyl disulphide followed by a reversible terminal and internal addition of the allyldithio radical to diallyl disulphide (Figure 3),

b) an intramolecular hydrogen atom abstraction fragmentation of the intermediate formed by an internal (Markovnikov) addition of the allyl-dithio radical, giving thioacrolein and the 1-(allyldithio)-2 propyl radical,

c) Diels-Alder self-condensation of thioacrolein acting as an heterodiene and its condensation to allyl mono-, di-, and trisulphides.

In Figure 3 (A) the self-condensation of thioacrolein affords two vinyldithiin isomers, (3) and (4), and the condensation with a third molecule of thioacrolein, the trimers (5) and (6).

Compounds French Garlic (%) Mexican Garlic
(%)
diallyl sulphide
allylmethyl sulphide
dithiacyclopentene +
(E)-propenylmethyl disulphide
dimethyl trisulphide
diallyl disulphide
(Z)- propenyl allyl disulphide
(E)-propenyl allyl disulphide
allylmethyl trisulphide
3-vinyl-1,2-dithiin
2-vinyl-1,3-dithiin
dimethyl tetrasulphide
diallyl trisulphide
(Z)-allyl propenyl trisulphide
(E)-allyl propenyl trisulphide
5-methyl-1,2,3,4-tetrathiacyclohexane
allylmethyl tetrasulphide
diallyl tetrasulphide
(E)-allyl propenyl tetrasulphide
Miscellaneous
1.2
4.2

3.0
0.5
21.8
2.1
6.0
9.0
5.5
2.4
-
24.2
0.4
0.6
1.0
1.2
4.9
0.3
11.8
2.4
4.1

1.4
2.7
17.2
Tr
Tr
19.0
0.2
0.5
3.8
26.4
Tr
Tr
1.0
3.0
8.2
-
10.1

 Table 1. Composition of garlic oils from France and Mexico

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© Mike Watson 2005



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