Mekocyanin.(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.

Mekocyanin is one of the pigments of the poppy (Papaver rhoeas, Linn.) a double purple-scarlet garden variety.

This has been isolated and described by Willstatter and Weil (Annalen, 1916, 412, 231), who state that it occurs in the flowers together with a second pigment which they have not, as yet, completely investigated. The petals used contained a total of about 18 per cent, of their dry weight of pigment.

The colouring matter was isolated in the form of its chloride, for which purpose the whole petals - including the darker central portions - were used, as preliminary experiments showed that extracts of the darker portions at the centre of the petals had the same properties as those of the outer parts. The separation of mekocyanin from the other anthocyan that accompanies it was readily accomplished by taking advantage of the unusual great solubility of the former.

For the isolation of the pigment it is said to be necessary to use fresh petals, as there is great difficulty in purifying the colouring matter after extraction from the dried material. The process used by Willstatter and Weil may be described thus: From some 7-8 kg. of fresh poppy petals about 20 litres of extract were obtained on treatment with glacial acetic acid, and to this some 200 c.c. of 10 per cent, methyl alcoholic hydrochloric acid was added before precipitation of the pigment by means of ether, for which purpose 40 litres were required. After standing for a time the clear liquid was decanted, the residue collected, dissolved in water (650 c.c.) and the solution mixed with absolute alcohol (2 litres), to precipitate the impurities present which are difficultly soluble in alcohol. After removal of the latter, a further quantity of 10 per cent, ethyl alcoholic hydrochloric acid (500 c.c.) was added and the colouring matter then precipitated by addition of ether (3 times total volume of liquid), the product obtained in this way being a syrup from which the mother liquors could be readily decanted. In order to separate the mekocyanin from the other component (less soluble in alcohol), the syrup was twice washed with absolute alcohol (1 litre each time) containing 3 per cent, of hydrochloric acid, whereby some 38 gr. of residue was left which contained but little mekocyanin. It was then precipitated from the alcoholic liquors by means of ether (3 times the volume of the solution), collected and dried.

The next purification consisted in allowing the product to stand with 3 per cent, alcoholic hydrochloric acid, when it first passed into solution, and was then reprecipitated as a spongy mass. In about two days the whole was thus transformed and was then collected (26 gr. dry). On dissolving this product, without drying, in 0.3 per cent, hydrochloric acid (90 c.c.) and gradually adding glacial acetic acid (1300 c.c.) the pigment began to separate as flakes, then in spherical forms (13 gr.), and further precipitation was caused by addition of more glacial acetic acid, this fraction consisting of regular spherical garnet-like grains ("kuglig kantigen granatähnlichen Kornern"). Ether now caused the separation of a further quantity of impure product.

The middle fraction described above, although not distinctly crystalline, was used for characterisation and for analyses, whilst the first and last fractions were used for the preparation of the non-glucosidal compound (cyanidin). Further purification by formation of the crystalline ferrocyanide, reconversion of this into the chloride, and careful precipitation of the latter from aqueous HCl solution, by acetone, yielded the pigment (as chloride), in the form of sharply cut, well-formed needles.

Mekocyanin chloride, C₂₇H₃₁O₁₆Cl, forms a hydrate containing 3 molecules of water - C₂₇H₃₁O₁₆Cl, 3H₂O, from which two are readily removed, the third only on drying in high vacuum at 105°C. The salt is very easily soluble in water, dilute solutions becoming decolorised owing to the formation of pseudo-base, and differs greatly from cyanin chloride and the chlorides of other cyanidin saccharides, except prunicyanin chloride, in that it is very easily soluble in aqueous hydrochloric acid (0.01 per cent, to 10 per cent, or even higher). In ethyl alcohol it is difficultly soluble, but rather more soluble in methyl alcohol, and if the chloride is rubbed with either of these solvents containing a small per cent. HCl, it dissolves and is reprecipitated as a voluminous spongy mass. In acetone the salt is insoluble, but is easily soluble in a mixture of acetone and water - this is equally true of most anthocyanin chlorides, and such a mixture is often to be preferred to a mixture of alcohol and water for the recrystallisation of these oxonium salts. In colour the solutions in aqueous acid or in alcohol of mekocyanin chloride are the same as those of cyanin chloride, but the reactions of the pigment are different. Thus, with sodium carbonate it gives a violet colour with slight bluish tinge - cyanin gives blue, keracyanin red-violet - but with caustic soda a corn-flower-blue is produced. With ferric chloride mekocyanin chloride gives the same colour reactions as cyanin chloride.

With regard to distribution between aqueous acid and amyl alcohol, mekocyanin behaves as a normal diglucoside anthocyan, and on complete hydrolysis it yields cyanidin chloride (2 mol.) and glucose (2 mols.), but, by carefully regulating the conditions of hydrolysis, Willstatter and Weil were able to stop the reaction at an intermediate stage at which a monoglucoside (2 mol.) and glucose (1 mol.) had been produced. The monoglucoside thus isolated was identified by them as chrysanthemin chloride, and from this, by hydrolysis, they obtained cyanidin chloride (2 mol.) and glucose (2 mol.).

Mekocyanin ferrocyanide.

This is a highly crystalline characteristic salt, which is readily prepared from the chloride by treating 2 gr. dissolved in 20 c.c. hot 0.3 per cent, hydrochloric acid, with 2 gr. of finely powdered potassium ferrocyanide (added all at once) and allowing the mixture to stand overnight. The amorphous precipitate that first separates has by that time passed to a mass of fine red-brown needle-shaped crystals (0.9 gr.). The salt is fairly soluble in water, sparingly soluble in alcohol. It can be reconverted into the chloride by treatment with picric acid, filtering off the ferrocyanic acid which separates, and adding ethyl alcoholic hydrochloric acid and ether to the filtrate.