Keracyanin.(CHAPTER VIII. Pyran Group. Cyanidin and its derivates.)

The Natural Organic Colouring Matters
By
Arthur George Perkin, F.R.S., F.R.S.E., F.I.C., professor of colour chemistry and dyeing in the University of Leeds
and
Arthur Ernest Everest, D.Sc., Ph.D., F.I.C., of the Wilton Research Laboratories; Late head of the Department of Coal-tar Colour Chemistry; Technical College, Huddersfield
Longmans, Green and Co.
39 Paternoster Row, London
Fourth Avenue & 30th Street, New York
Bombay, Calcutta, and Madras
1918

Kaikki kuvat (kemialliset kaavat) puuttuvat // None of the illustrations (of chemical formulas) included.

Keracyanin, the colouring matter of the sweet cherry (Prunus avium, Linn.), has been isolated and described by Willstatter and Zollinger (Annalen, 1916, 412, 164).

For the isolation of the colouring matter the fruit was stoned, the skins rapidly separated from the flesh, and extracted by standing in closed vessels with glacial acetic acid. The skins of 150 kg. of cherries thus yielded 15 litres of extract from which the pigment was precipitated in the form of a syrup by means of ether.

The method given for the purification of the pigment is complicated, and stated to be merely of a temporary character, but it sufficed for the isolation of the pigment in pure and crystalline condition, and permitted at least of a preliminary examination of the compound. This involved fractional precipitation of the crude pigment dissolved in methyl alcoholic hydrochloric acid, by means of ether, followed by its conversion into lead salt; the decomposition of this with recovery of the pigment, and finally further fractional precipitation of a solution of the latter in methyl alcoholic hydrochloric acid by means of ether. The colouring matter was finally obtained in deep bronze-brown clusters of crystals.

Keracyanin chloride, C₂₇H₃₁O₁₅Cl, forms crystalline hydrates, of which Willstatter and Zollinger describe two, viz.: C₂₇H₃₁O₁₅Cl, 4H₂O (which loses 3H₂O when dried in vacuum desiccator) and
C₂₇H₃₁O₁₅Cl, 3H₂O
(which loses 1½H₂O in vacuum desiccator), both of which become anhydrous if dried in high vacuum at 105°C. The former hydrate was obtained when 1.3 gr. of the pure chloride were dissolved in 10 c.c. of very dilute hydrochloric acid, and the solution mixed with 20 c.c. of 10 per cent. HCl; on standing for several days, the compound was precipitated in the form of bright red flakes composed of very fine needles. The second hydrate was prepared by taking 0.85 gr. of the pure chloride dissolved in 8 c.c. methyl alcohol, and adding 16 c.c. of 7 per cent, aqueous hydrochloric acid. It crystallised from this solution in short brown-yellow prisms.

Acid aqueous, or alcoholic, solutions of keracyanin chloride closely resemble those of cyanin chloride in colour, but differ in their reaction with sodium carbonate, with which keracyanin gives a red-violet colour. With caustic soda it gives a blue colour like cyanin; whereas ferric chloride, when added to an alcoholic solution of the salt, gives a blue coloration, which on dilution with water becomes blue-violet (stable).

The salt is easily soluble in hot water, decolorisation taking place - due to pseudo-base formation - in dilute solutions, as with other pigments of the series; in methyl alcohol, even when cold, it is very easily soluble, and in ethyl alcohol fairly easily. It is less soluble in acids than mekocyanin chloride and more so than cyanin chloride, and it is interesting to note that whilst it is difficultly soluble in cold hydrochloric acid of concentrations between 0.01 per cent, and 0.1 per cent, it is easily soluble in 1 per cent. HCl. As the concentration increases beyond this the solubility decreases, until 3 per cent. dissolves but little, and in cold 7 per cent. HCl it is very difficultly soluble, though considerably so if the acid is hot.

On hydrolysis keracyanin yields cyanidin chloride, glucose, and rhamnose, approximately in molecular proportions, but, as previous investigators have found in the case of other rhamno-glucosides - e.g. rutin - Willstatter and Zollinger observed, in the analytical data obtained, some variation from the theoretical proportions of the sugars required for a simple rhamno-glucoside of cyanidin, though evidence points to its being such a compound.

It is of interest to note that in common with the other rhamnoglucoside anthocyans - violanin and prunicyanin - keracyanin has a distribution number (viz. 67) approximating to that of a normal monoglucoside.