Delphinin(CHAPTER VIII. Pyran Group. Delphinidin (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.

*The use of this machine for this purpose in connection with the anthocyans was introduced by Willstatter and Everest, who employed it for the repeated fractionation of the blue corn-flower pigment. It is very doubtful whether without its use their work could have been brought to a successful conclusion.Delphinin, the pigment of wild purple delphinium (Delphinium consolida, Linn.), has been investigated by Willstatter and Mieg (Annalen, 1915, 408, 61). It is present in the flowers to the extent of about 1¾ per cent, of the dry weight, and it has been shown that it occurs as the free colouring matter, neither combined with alkalis as a salt, nor with acids as an oxonium compound. It is very interesting to note that delphinin differs from all the other anthocyanins that have yet been isolated, in that dilute neutral solutions of its salts, or of the free base, have never been observed to pass to the pseudo-base, and it is this stability of the colour base that made it possible for Willstatter and Mieg to isolate it from the flower petals without the use of any chemical reagents, alcohol alone of various concentrations serving this purpose. The process was long and tedious, but consisted in the main of repeated solution of the crude pigment in alcohol of such a concentration that a certain portion of the impurities remain insoluble, separation of these from the liquor - usually by means of a Jouan "Bol" centrifuge* - and reprecipitation of the colour by increasing the concentration of the alcohol. After use had been made of this, the pigment was sufficiently insoluble in water to allow of its treatment for the removal of impurity. Their final product was free from acid and almost free from ash (0.5 per cent, ash, as sulphate), and hence was either the colour base, or an internal salt of it.

Delphinin was, however, best isolated as the chloride. The colouring matter, after one precipitation from dilute alcohol, followed by one purification with water, was dissolved in twenty times its weight of 0.2 per cent, hydrochloric acid, the solution filtered, and 20 per cent, hydrochloric acid added, when the chloride separated out in a sufficiently pure condition for crystallisation. It is stated that an alternative method working - as chloride from the start - is more convenient and rapid; the petals being extracted with ethyl alcoholic hydrochloric acid - 3 kg. with 10 litres of solvent (7.5 litres of 96 per cent. EtOH, 1,5 litres cone. HCl, and i litre of water) (as this caused no swelling of the dry petal powder, much larger batches could be worked than when dilute alcohol was used for extraction, and again filtration was possible without the addition of sand) - and the residue after filtration washed with 3 litres of the same solvent. The intense red extract (9 litres), when mixed with an equal volume of ether, deposited the pigment as a syrup which, after decantation of the liquors, was rubbed with alcohol to dehydrate it, then dissolved in methyl alcohol (1.5 litres), and the pigment precipitated as a powder by the addition of absolute alcohol (3 litres) and ether (2 litres). The product was redissolved in methyl alcohol (1 litre), filtered, and reprecipitated by the addition of absolute alcohol (1½ litres) and ether. After drying the substance was dissolved in hot 2 per cent, hydrochloric acid (250 c.c.), warmed for half an hour on the water-bath, cooled, and the colouring matter precipitated by the addition of 10 per cent, hydrochloric acid (1-3 litres). Following upon this purification, it was redissolved in cold methyl alcohol (400 c.c.) and reprecipitated by addition of absolute alcohol (½ litre) and ether (several times its volume).

The product obtained by either of the above processes was crystallised by adding the finely powdered pure amorphous product (6 gr. to 400 c.c.) to 3 per cent, hydrochloric acid and warming in an open beaker, with frequent stirring, for 1½ hours, when it was completely converted into crystalline aggregates consisting of tablets and prisms, which were collected, washed with 3 per cent, hydrochloric acid, and dried. If larger volumes of hydrochloric acid were used, the size of crystal was larger, but the yield was decreased. In the best cases, about 70 per cent, of the weight of the product taken was thus obtained in the crystalline form.

Delphinin chloride, C₄₁H₃₉O₂₁Cl, as crystalline hydrate appears to contain 12 molecules of water, and on drying in high vacuum at 130°C. passes to a product for which the composition C₄₁H₃₉O₂₁Cl, 2H₂O is given, but owing to the fact that some hydrochloric acid was lost on drying, the analytical figures had to be calculated to the acid-free product. It was not found possible to remove the water thus assumed to remain attached to the molecule. The crystalline substance (12H₂O) consists of prismatic tablets of deep brown-red colour (by transmitted light under microscope bluish-red), having a surface shimmer (mattem Samtglanz); the crystals are soft, and mark paper cherry-red. The dried product (2H₂O), when heated, commences to sinter at 150-160°C., and melts with much decomposition and swelling at 200-203°C.

On adding water to the crystalline chloride it is immediately converted, by hydrolytic dissociation, into, the colour base, which separates in the form of deep violet flakes. Formation of the colourless pseudo-base has never been observed with solutions of delphinin, or of its salts.

The chloride is fairly easily soluble in cold methyl alcohol, but difficultly soluble in absolute ethyl alcohol, even when warm; the alcoholic solutions are intense red, with bluish tinge, and solutions in fairly dilute ethyl alcohol (e.g. 50 per cent.) become much more blue when heated, returning to their original colour when cooled. It is insoluble in amyl alcohol, and is easily soluble in cold hydrochloric acid, up to 0.5 per cent. HCl, but only soluble in hot acid containing 1-2 per cent. HCl. In dilute sulphuric acid it is very little soluble (concentrations varying from 0.03-7 per cent), but in concentrated sulphuric acid it is readily soluble forming an orangered solution.

Acid solutions of the chloride are bluer than similar solutionsof oenin, myrtillin, and althein chlorides, and are much more intense in colour than corresponding solutions of cyanin chloride.

When sodium carbonate is added to an acid solution of delphinin chloride it produces a deep blue coloration, which is stable for hours in the cold, and in absence of excess of alkali, but on warming passes through green to yellow, whereas caustic soda produces the same changes, but more rapidly. Acidification of the yellow solution does not reproduce the anthocyan. Lead acetate gives an indigoblue coloured flocculent precipitate when added to a solution of the salt; alcoholic or aqueous ferric chloride an intense blue solution, and sodium bisulphite forms a very soluble colourless compound, which is decomposed by acids with reproduction of the pigment. Fehling's solution is noticeably reduced if hot.

Delphinin chloride is optically active, and Willstatter and Mieg record the following measurements:-

White light (½ watt Osram lamp 20 cms. distant). Solution in 1 per cent HCl, 1 dcm. tube [a] = - 1231°; ½ dcm. tube [a] = - 1439°; mono-chromatic light (solutions used above had to be diluted to four times its original volume)
[a]_c = - 1364° (± 150°); [a]₆₁₄ = - 2273° ( ± 160°).

The absorption spectrum is described by the same authors as consisting of one band that ends very indistinctly, and which differs from that of cyanin chloride in that it does not reach the blue region. The data given are: Solution in 0.25 per cent. HCl, 1 mol. in 2500 litres-
Column 2.5 mm. 575. 538 - - 515. 495
Column 5.0 mm. 586... 545 - 512. 490

Delphinin chloride is only slowly hydrolysed by boiling dilute hydrochloric acid, but rapidly by 20 per cent, acid, and in view of the fact that the acid, beyond causing hydrolysis, gives rise to amorphous products, it is advantageous that the operation be carried through as rapidly as possible (3 mins.) and the mixture cooled at once. The products of hydrolysis are delphinidin chloride (1 mol.), glucose (2 mols.), and p-hydroxy benzole acid (2 mols.). In order to confirm this result and to see whether the p-hydroxy benzoic acid was a regular constituent of the pigment, the flowers from several sources, and three different harvests, 1911-1913, were examined by these authors, and in every case the same result was obtained. Willstatter and Mieg express the opinion that the p-hydroxy benzoic acid in delphinin is attached to the glucose molecules, and not to the phenolic hydroxyl groups of the pigment complex.

The distribution number of delphinin resembles that of a normal diglucoside.

Delphinin colour base is readily produced by adding water to the chloride, and thus separates as a deep violet flocculent precipitate. It is difficultly soluble in boiling methyl alcohol, and insoluble in ethyl alcohol, but dissolves in very dilute ethyl alcohol, or acetone, forming blue-violet solutions. When a solution in dilute alcohol is kept under a bell-jar over absolute alcohol, whereby the alcohol concentration slowly increases, the colour base is deposited in the form of rosettes of microscopic violet coloured needles.

Delphinin picrate, a difficultly soluble compound, was prepared as a brown-red flocculent precipitate by Willstatter and Mieg, but was not obtained in a crystalline condition.