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 vegetable Cruciferous vegetables are vegetables of the family Brassicaceae (also called Cruciferae) with many genera, species, and cultivars being raised for food production such as cauliflower, cabbage, kale, garden cress, bok choy, broccoli, Brussels spr ...

s, particularly

garden cress Cress (''Lepidium sativum''), sometimes referred to as garden cress (or curly cress) to distinguish it from Cress (disambiguation), similar plants also referred to as cress (from old Germanic ''cresso'' which means sharp, spicy), is a rather fas ...

. 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 ''Brassica'' () is a genus of plants in the cabbage and mustard family (Brassicaceae). The members of the genus are informally known as cruciferous vegetables, cabbages, or mustard plants. Crops from this genus are sometimes called ''cole cro ...

Occurrence

The compound was first reported in 1899, after its isolation from ''

Tropaeolum majus ''Tropaeolum majus'', the garden nasturtium, nasturtium, Indian cress or monks cress, is a species of flowering plant in the family Tropaeolaceae, originating in the Andes from Bolivia north to Colombia. An easily-grown annual or short-lived pere ...

'', 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 Phytolaccaceae is a family (biology), family of flowering plants. Though almost universally recognized by Taxonomy (biology), taxonomists, its circumscription has varied. It is also known as the Pokeweed family. The APG II system, of 2003 (unch ...

Resedaceae Resedaceae is a family of mostly herbaceous dicotyledonous plants comprising 107 known species in 8 to 12 genera: *'' Borthwickia'' - 1 species, sometimes placed in its own family Borthwickiaceae *'' Caylusea'' - 3 species *''Forchhammeria'' - 1 ...

Salvadoraceae Salvadoraceae is a family in the plant order Brassicales,, p. 10 consisting of three genera with a total of 11 known species. They occur in Africa (including Madagascar), Southeast Asia, and on Java, suggesting they are probably found in much of ...

and

Tovariaceae ''Tovaria'' is a genus of herb In general use, herbs are a widely distributed and widespread group of plants, excluding vegetables and other plants consumed for macronutrients, with savory or aromatic properties that are used for flavoring an ...

Structure

The chemical structure of glucotropaeolin was confirmed by

total synthesis Total synthesis is the complete chemical synthesis of a complex molecule, often a natural product, from simple, commercially-available precursors. It usually refers to a process not involving the aid of biological processes, which distinguishes ...

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 The Lossen rearrangement is the conversion of a hydroxamate ester to an isocyanate. Typically O-acyl, sulfonyl, or phosphoryl O-derivative are employed. The isocyanate can be used further to generate ureas in the presence of amines or generate ...

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

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 Myrosinase (, ''thioglucoside glucohydrolase'', ''sinigrinase'', and ''sinigrase'') is a family of enzymes involved in plant defense against herbivores, specifically the mustard oil bomb. The three-dimensional structure has been elucidated and is ...

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 The mustard oil bomb, formerly known as the glucosinolate–myrosinase complex, is a chemical herbivory defense system found in members of the Brassicaceae (or cabbage family). The mustard oil bomb requires the activation of a common plant secondary ...

. 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