i] INTRODUCTORY 13 



molecules, or possibly of a flavone molecule with other aromatic sub- 

 stances present in the plant. 



The problem of anthocyanin formation has quite recently received 

 a most important series of contributions by Willstatter and a number 

 of his co-workers. In 1913 Willstatter (245) first published some 

 results on anthocyanins in general and the anthocyanin of the Corn- 

 flower (Centaurea Cyanus] in particular. Willstatter states that all 

 natural anthocyanins are present in the plant in the condition of gluco- 

 sides, and that many of them, moreover, including the anthocyanin of 

 Centaurea, are very unstable in water solution, and readily change in 

 these circumstances to a colourless isomer. The change can be prevented 

 by adding certain neutral salts, and also acids, to the pigment solution. 

 The explanation, according to Willstatter, of these phenomena lies in 

 the fact that anthocyanin is an oxonium compound having a quinonoid 

 structure and containing tetravalent oxygen. The quinonoid structure is 

 rendered stable by the formation of oxonium salts with acids or with 

 neutral salts, such as sodium nitrate and sodium chloride. The pig- 

 ment itself, in the neutral state, is purple in colour, and has the structure 

 of an inner oxonium salt. With acids it forms red oxonium salts, and 

 with alkalies blue salts, the position of the metal being undetermined. 

 More recently Willstatter (256, 257) has published the results of the 

 isolation and analyses of a number of other anthocyanins in addition 

 to that of Centaurea, i.e. from the flowers of Delphinium, scarlet Pelar- 

 gonium, Mallow and Hollyhock; from Grapes also, and the fruits of 

 the Bilberry and Cranberry. In every case crystalline salts were 

 obtained with hydrochloric acid, and from the analyses of these salts 

 Willstatter obtained a series of percentage formulae for his antho- 

 cyanins. Compared with the percentage formulae of the known 

 flavones, he found that each anthocyanin could be derived theoretically 

 from a flavone by loss of an atom of oxygen, i.e. by reduction. 



The view of anthocyanin formation by reduction from a flavone 

 has received support from a series of phenomena of a very curious 

 and interesting nature, which were first observed by Keeble & Arm- 

 strong (240), and of which an account was published in June, 1913. 

 These observations were concerned with the effect of reducing plant 

 extracts with nascent hydrogen. A number of flowers, among which 

 were the yellow Wallflower, Daffodil, Crocus and Polyanthus, w r ere 

 extracted with alcohol, and the extracts acidified and treated with 

 zinc dust. Filtered from the zinc, and exposed to air, the solution 

 develops a red colour which changes to green on addition of alkali. 



