Glucotropaeolin or benzyl glucosinolate is a

glucosinolate Glucosinolates are natural components of many pungent plants such as mustard, cabbage, and horseradish. The pungency of those plants is due to mustard oils produced from glucosinolates when the plant material is chewed, cut, or otherwise damaged. T ...

found in cruciferous vegetables, particularly garden cress. Upon enzymatic activity, it is transformed into

benzyl isothiocyanate Benzyl isothiocyanate (BITC) is an isothiocyanate found in plants of the mustard family. Occurrence It can be found in ''Alliaria petiolata'', pilu oil, and papaya seeds where it is the main product of the glucotropaeolin breakdown by the enzyme ...

, which contributes to the characteristic flavor of these brassicas.

Occurrence

The compound was first reported in 1899, after its isolation from '' Tropaeolum majus'', a nasturtium species. Glucotropaeolin is now known to occur widely in other brassica families including

Caricaceae The Caricaceae are a family of flowering plants in the order Brassicales, found primarily in tropical regions of Central and South America and Africa. They are usually short-lived evergreen pachycaul shrubs or small to medium-sized trees growi ...

, Phytolaccaceae, Resedaceae, Salvadoraceae and Tovariaceae.

Structure

The chemical structure of glucotropaeolin was confirmed by total synthesis in 1957. This showed that it is a glucose derivative with β-D-glucopyranose configuration. At that time it was unclear whether the C=N bond was in the Z form, with sulfur and oxygen substituents on the same side of the double bond, or the alternative E form in which they are on opposite sides. The suggestion was made that the Z form was more likely, based on the known decomposition to

by a mechanism analogous to the Lossen rearrangement. However, when an identical product was obtained by an alternative route in 1963, it was pointed out that the E form would be expected to rearrange in a similar way. The matter was settled by X-ray crystallography and other spectroscopic studies and it is now known that all natural glucosinolates are of Z form.

Synthesis

Biosynthesis

Glucotropaeolin is biosynthesised from the amino acid

phenylalanine Phenylalanine (symbol Phe or F) is an essential α-amino acid with the formula . It can be viewed as a benzyl group substituted for the methyl group of alanine, or a phenyl group in place of a terminal hydrogen of alanine. This essential amino a ...

in a multi-step pathway.

Laboratory synthesis

The first laboratory syntheses served to confirm the compound's structure. Later work allowed many glucosinolates including this benzyl derivative to be made. These processes are more efficient than isolating pure materials from the plants in which they are naturally found.

Function

The natural role of glucosinolates are as plant defense compounds. The enzyme myrosinase removes the glucose group in glucotropaeolin to give an intermediate which spontaneously rearranges to

. This is a reactive material which is toxic to many insect predators and its production is triggered when the plant is damaged. This effect has been called the mustard oil bomb. At concentrations typically found in foods, the glucosinolates are not toxic to humans and can be useful flavor components.

References

{{Cruciferous Biochemistry Glucosinolates Benzyl compounds