Organic thiocyanates - glucosinolate enzymatic degradation products or artefacts of the isolation procedure?

Abstract

Glucosinolates are abundant in plants of the order Brassicales, and they are degraded by myrosinases into various organic breakdown products: isothiocyanates, thiocyanates, nitriles, etc., depending on their structure, conditions of hydrolysis, the presence of certain protein cofactors. Their most common hydrolysis products are isothiocyanates, while simple nitriles, epithionitriles, and thiocyanates are produced occasionally. Organic thiocyanates are described from a very limited number of Brassicales taxa. Up to now benzyl, (4-hydroxyphenyl)methyl, (4-methoxyphenyl)methyl, 4- methylthiobutyl, and allyl thiocyanates were reported as products of glucosinolates autolysis. The present review summarizes the knowledge on the mechanism of organic thiocyanate formation from the corresponding thioglucosides. The enzymatic formation of organic thiocyanates is believed to be enabled by thiocyanate-forming protein (TFP), but they could be formed via metabolic routes that do not involve TFP. All of the reported thiocyanates are produced from stable (carbo)cationic species that allow an isomerization of an isothiocyanate to thiocyanate, and vice versa. Although the possibility that thiocyanates can be biosynthesized in plats under certain conditions cannot be dismissed, allyl thiocyanate can be a thermal isomerization artefact of the original isothiocyanate that is formed in the heated zones of the gas chromatograph, while other thiocyanates could form in an aqueous medium via heterolytic dissociation to ambident nucleophilic SCN- and its recapture. One should always be aware of this analytical shortcoming when concluding on the presence and quantity of these specific (iso)thiocyanantes in the analyzed sample.