292 



MISCELLANEOUS NITROGEN AND SULFUR COMPOUNDS 



Natural indigo contains some other dyes as well as indigotin. The European woad plant 

 (Isatis tinctoria) also contains indican and was at one time an important source of blue 

 dye. 



Another pigment (or class of pigments) based on the indole nucleus is melanin. 

 This pigment is responsible for dark hair and skin color in animals. However, its exist- 

 ence in plants is dubious. The probable structure of melanin is indicated below. It is a 

 polymer based on indole-5, 6-quinone. 



o^,^-^ 



Such compounds may be formed by the action of oxidizing enzymes on tyrosine or dihydrox- 

 yphenylalanine. The term melanin should be restricted to compounds of this type and not 

 used for any dark pigment. Careful investigation of dark pigments of higher plants will 

 be needed to decide whether true melanins are present. Many dark plant pigments are 

 known to be free of nitrogen and formed by oxidation of such phenolic compounds as chlo- 

 rogenic acid (see Chap. 4). However, leaves of some composites contain a dark red pig- 

 ment named intybin which seems to be an indolequinone derivative (55). The red pigments 

 of beets (Beta vulgaris) and several other plants (especially in the Chenopodiaceae) were 

 thought for many years to be related to the anthocyanins although they contain nitrogen. 

 It has now been established that they are in fact indole derivatives. The name "betacyanins" 

 has been proposed for them and "betaxanthins" for the related yellow pigments (56). Betanin, 

 the most thoroughly studied of these compounds, is a glucoside which yields the aglycone 

 betanidin on hydrolysis. A tentative structure of betanidin is as follows (57): 



COO 



betanidin 



HOOC 



COOH 



