THE ANTHOCYANS. 



The red, violet and blue pigments which are present in the blossoms, in 

 many fruits and in some leaves of plants, and which can be extracted with 

 water or aqueous alcohol, are grouped together as the Anthocyans. Accord- 

 ing to Molisch they occur not only dissolved in the juices of the cell, but also 

 in amorphous and crystalline forms. Molisch obtained crystajs by allowing 

 aqueous or acetic acid extracts to evaporate slowly under the cover slip of a 

 microscope slide. The colour of the aqueous solution of these pigments fades 

 on standing from which it would appear that the pigments are very unstable. 



The chemical investigation of these pigments has shown that they are 

 glucosides and that the pigment is related to that of the group 

 of anthoxanthin pigments. The study of these pigments /\/\ 

 was taken up by Willstatter and his pupils in 1913 and | [ | c 

 is still in progress. They have shown that these pigments J CH 



are glucosides and that they are derivatives of the complex \/\/ 

 benzo-pyrylium nucleus. This nucleus differs from the 

 benzo-y-pyrone nucleus in having a CH group in the place of the CO group. 



The properties of this group differ markedly from those of the benzo- 

 pyrone group. The oxygen atom in the pyrone ring can O-C1 



become quadrivalent and form salts with acids ; it is feebly 

 basic ; the salts are unstable and are decomposed by water. 

 The oxygen atom in the benzopyrylium ring can also be quad- 

 rivalent and form salts with acids. It is strongly basic ; the 

 salts are stable and not easily decomposed by water. 



The benzene ring contains hydroxyl groups; their phenolic character 

 allows of the formation of salts with alkali. These properties explain the 

 existence of the red, violet and blue colours ; the red is the acid salt, the blue is 

 the potassium or metallic salt and the violet is the anhydride of the pigment ; 

 thus the colour is due to the quinonoid structure of the molecule. The 



KO 

 HO 0-C1 



apparent instability of the compounds from the disappearance of the colour 

 of the solutions is not confirmed. The colour returns on acidifying or on 

 evaporating the solution. The pigment apparently undergoes an isomerisation. 



More convenient methods of preparation than the older one (involving 

 precipitation by lead acetate) have been discovered by Willstatter and his 

 pupils ; they depend on the formation of oxonium salts which are soluble 

 with difficulty. 



The skins of blue grapes, whortle berries and Alth&a rosa are ex- 

 tracted with cold glacial acetic acid. The extract is precipitated with ether. 

 The syrupy precipitate is washed with ether and dissolved in a warm aqueous 

 solution of picric acid. The picrate crystallises out on cooling. A solution 

 of the hydrochloride is obtained by treating the picrate with methyl alcohol 

 containing hydrochloric acid and is precipitated with a mixture of ether and 

 petroleum ether. The chloride is crystallised from aqueous alcohol con- 

 taining hydrochloric acid. 



The isolation of the anthocyans from other plants depends on the in- 

 solubility of the chlorides. These salts are generally soluble in water or 

 dilute acid, but soluble with difficulty in 7-15 per cent, hydrochloric acid, 



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