Xanthorhamnin, C34H42O20, is a very complex glucoside containing two rhamnose and one galactose groups, united with rhamnetin, which is quercitin with the H of the OH in either the 1, or 3, position replaced by a methyl group. There are several similar pigments which differ from xanthorhamnin only in the number or position of the methoxy groups (i.e., the OH groups with a CH3 replacing the H), or in the nature of the sugar which is present in the compound. Rhamnetin itself is found in the fruits of certain species of Rhamnus, and is used in dyeing cotton.
The structural arrangement of the characteristic groups of these flavone pigments will be dealt with more in detail in the chapter dealing with Pigments ([Chapter VIII]).
The best-known yellow pigment which is a xanthone derivative is euxanthic acid, known as "Indian yellow," which is a "paired" compound of glucuronic acid (see [page 42]) and euxanthone. The latter is a 2, 3', dioxyxanthone. The pigment is found in the urine of cattle which have been fed on mango leaves.
The soluble red, blue, and violet pigments are glucosides of various hydroxy-derivatives of the anthocyan nucleus. Their constitution and properties will be discussed in detail in the chapter dealing with the Pigments. These compounds are isomeric with similar flavone and xanthone derivatives, and the transition from one color to the other in plants takes place very easily under the action of oxidizing or reducing enzymes. This accounts for the change of reds and blues to yellows and browns, and vice versa, under changing temperature conditions.
The following red or blue plant pigments, which are anthocyan glucosides, have been isolated and studied (for the structural arrangement of the characteristic groups, see pages 116): from cornflower and roses, cyanin, C28H31O16Cl (2 molecules glucose + cyanidin); from cranberries, idain, C21H21O10Cl (galactose + cyanidin); from geranium, pelargonin, C27H30O15Cl (2 molecules glucose + pelargonidin); from pæony, pæonin, C28H33O16Cl (2 molecules glucose + pæonidin, a monomethyl cyanidin); from blue grapes, œnin, C23H25O12Cl (glucose + œnidin); from whortle berry, myrtillin, C22H23O12Cl (glucose + myrtillidin); from larkspur, delphinin, C41H39O21Cl (2 molecules glucose + 2 molecules p-oxybenzoic acid + delphinidin); and from mallow, malvin, C29H35O17Cl (2 molecules glucose + malvidin).
The blue dye, indigo, is derived from a glucoside of an entirely different type, known as indican. Indican is readily extracted from the leaves of various species of indigo plants. When hydrolyzed, it yields glucose and indoxyl (colorless). Indoxyl is easily oxidized to indigotin (the deep blue dye known as "indigo"). The equations illustrating these changes are as follows:
| (a) C14H17O6N | + | H2O | = | C6H12O6 | + | C8H7ON |
| Indican | Glucose | Indoxyl | ||||
| (b) 2C8H7ON | + | O2 | = | C16H10O2N | + | 2H2O |
| Indoxyl | Indigotin |
The structural relationships of indoxyl and indigotin may be illustrated by the following formulas:
