Thiocyanates from glucosinolates

Possible mechanisms for the formation of organic thiocyanates from glucos-inolates have been discussed following the observation that extracts of the seed flour of Thlaspi arvense converted [l- C]allylglucosinolate into [3- C]allyl thiocyanate. The and n.m.r. and mass spectra of glucosinolates (e.g. sinigrin) and their derivatives have been recorded to aid in the identification and determination of the vast number of glucosinolates that have been found in plants of the Cruciferae. " ... 

Figure 13.1 Degradation of glucosinolates. Hydrolysis is catalyzed by myrosinases and gives rise to different degradation products dependent on the structure of the glucosinolate side chain and the hydrolysis conditions. (I) isothiocyanates, the major product at pH >7 (II) nitriles, the major product at pH <4 (ID) thiocyanates, produced from 2-propenyl-, benzyl-, and 4-methylthiobutylglucosinolates (IV) oxazolidine-2-thiones, produced from glucosinolates with P-hydroxylated side chains, (V) epithionitriles, produced in the presence of epithiospecifier proteins.

Protein-rich rape seed residues from biodiesel production plants are rich in phenolic compounds, glucocinolates and phytic acid. Phenolic compounds and phytic acid could be used as potent antioxidants in cosmetic and pharmaceutical formulations (Shamsuddin, 1995 Amarowicz and Shahidi, 1994 Wanasundara et al., 1996 Oatway et al., 2001). Derivatives from glucosinolate hydrolysis, including isothiocyanates, thiocyanates and nitriles, could be used as anticarcinogenic agents, biopesticides and flavour compounds (Halkier and Gershenzon, 2006). 

Thiocyanates are apparently only formed from three glucosinolates, benzyl-, allyl-, and 4-methylsulfinylbutyl-glucosinolate.13,14 The ability to produce thiocyanates seems to be restricted to just a few plant species, and, like epithionitrile formation, may require the presence of specific protein factors. 

Few studies have directly compared the effects of different hydrolysis products derived from the same glucosinolate. Such comparative studies with insects are listed in Tables 5.1-3, and comparative studies with other organisms in Table 5.4. In many cases, isothiocyanates seem to have greater biological activity than nitriles, thiocyanates, and oxazolidine-2-thiones. However, comparisons are complicated by differences among the hydrolysis products in physico-chemical properties, such as volatility. If two hydrolysis products differ significantly in volatility, for example,... 

Formation of the thiocyanate (94) is believed to be caused by enzymatic rearrangement (E) of the isothiocyanate (54) although direct formation from the glucosinolate has not been excluded (95). It occurs in homogenized Eruca sativa plants but not seed (95), and in Lepidium sativum seed powder but not T. majus (54, 96). The isothiocyanate is initially (10-15 sec) formed in substantial quantities but immediately decreases to a low level (97, 98). Addition of mustard myrosinase to heated Lepidium seed powder produces only isothiocyanate (56). 

Another goitrogenic compound in cabbage is thiocyanate ion (107) which results from breakdown of indolylmethyl isothiocyanate produced from the corresponding glucosinolate, glucobrassicin (97,106,108) (reaction G). The 5-hydroxymethylindole thus produced can dimerize to 3,3 -diindolylmethane or form a complex (ascorbigen) with ascorbic acid (perhaps a novel type of feedback inhibition of ascorbate-dependent... 

Another economic problem of glucosinolates is the burnt flavor of milk from cattle who have eaten the cruciferous weed Coronopus didy-mus. This is caused by benzyl methyl sulfide or benzyl mercaptan formed in the cow from the glucotropaeolin of the weed (110), presumably via the thiocyanate. Finally, ferrous ion catalyzes the nonenzymatic breakdown (H) of glucosinolates to nitriles and thioamides (111)-... 

The intact glucosides are probably not the active agents, but rather the isothiocyanates, thiocyanates, and nitrile derivatives derived from them. The initial hydrolysis products are thiohydroximate 0-sulfonates, which are followed by rearrangements to give isothiocyanates, and, under some conditions, nitriles and thiocyanates are also formed (Dewick, 1984). In contrast to the intact glucosinolates, most of the hydrolysis products are relatively volatile (Fig. 17.6). 

Glucosinolates are thioglucosides. They contain a C=N-group substituted with sulfate and a residue R in anti position, as well as a sulfur atom connected with a /3-D-glucopyranosyl moiety (Table 44). At hydrolysis they liberate glucose, sulfate and an unstable aglycon. From the latter isothiocyanates (mustard oils), thiocyanates, nitriles, or cyanoepithioalkanes may be formed. 

Aromatic and heterocyclic isothiocyanates are unstable and easily hydrolyse to the corresponding alcohols. For example, 4-hydroxybenzyl isothiocyanate (the degradation product of sinal-bin, which occurs in the seeds of white mustard, Leucosinapis album, Brassicaceae) decomposes in normal table mustard to 4-hydroxybenzyl alcohol and thiocyanate ions (Figure 8.68). Isothiocyanates derived from indolyl glucosinolates behave similarly. 

For example, degradation of the most common glucosinolate, sinigrin, yields, in addition to allyl isothiocyanate (see Section 8.2.9.1.5) and allyl cyanide (but-3-enenitrile, see Section 8.2.7.1.3), allyl thiocyanate (it is not clear whether this substance arises directly from the unstable aglycone or by isomerisation of isothiocyanate) and 2-(cyanomethyl)thiirane, also known as... 

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