THE PIGMENT GLUCOSIDES
Many, if not all, of the red, yellow, violet, and blue pigments of plants either exist as, or are derived from, glucosides. These are of three types: the madder, or alizarin, reds are derivatives of various oxy-anthraquinones; most of the soluble yellow pigments are glucosides derived from flavones or xanthones; and the soluble red, blue, and violet pigments of the cell-sap of plants are mostly anthocyan derivatives. The four basic groups, or nuclei, which are present in these different types of compounds are complex groups consisting essentially of two benzene rings linked together through a third ring in which there are either two oxygen atoms in the ring, or one oxygen in the ring and a second attached to the opposite carbon in the (C=O) arrangement, as shown by the following diagrammatic formulas:
The red dyes which were formerly obtained from madder, the powdered roots of Rubia tinctoria, but are now almost wholly artificially synthetized, consist of at least four different glucosides, the organic group of which, in each case, is an hydroxy-derivative of anthraquinone. The most important of these is ruberythric acid, composed of two molecules of glucose linked with one of alizarin (1,2, dioxyanthraquinone). Xanthopurpurin contains 1,3, dioxyanthraquinone, which is isomeric with alizarin; and rubiadin is a monomethyl (the CH3 being in the 4 position), derivative of this compound. Purpurin is a glucoside of 1,2,4, trioxyanthraquinone.
The soluble yellow pigments are generally glucosides of hydroxy-derivatives of xanthone or flavone, known as oxyxanthones or oxyflavones. The sugars which are united to these nuclei vary greatly, so that there are a great variety of yellow, white, or colorless flavone or xanthone pigment compounds. These compounds are almost universally present in plants. For example, one typical set of examinations of the wood, bark, leaves, and flowers of over 240 different species of tropical plants showed that flavone derivatives were present in every sample which was tested, the pigments being usually located in the powdery coating of the epidermis of the tissues.
The following typical examples will serve to illustrate the composition and properties of the glucosides of this type.
Quercitrin, C21H20O11, is found in oak bark, in the leaves of horse-chestnut, and in many other plants, often associated with other pigments. It is a brilliant yellow crystalline powder. Industrially, it ranks next to indigo and alizarin in importance as a natural dye stuff. It is a glucoside of rhamnose with 1,3,3',4', tetraoxyflavonol (i.e., the flavone nucleus with five OH groups replacing the hydrogens in the 1, 3, 5, 3', and 4' positions). Quercetin, C15H10O7, which is the tetraoxyflavonol itself, without any sugar in combination with it, is found in the leaves of several species of tropical plants and in the bark of others. Isoquercitrin, C21H20O12, is derived from the same flavone, but contains glucose instead of rhamnose, as the sugar constituent of the glucoside.
Apiin, C26H20O9, the yellow glucoside found in the leaves of parsley, celery, etc., contains apiose (a pentose sugar of very unusual structure, represented by the formula,
and apigenin, which is a 1,3,4', trioxyflavone.