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.
SALVIANIN, SALVIN, and SALVININ
(Willstatter and Bolton, Annalen, 1916, 412, 113).
Of these three compounds salvianin is the colouring matter that occurs in the flowers, salvin and salvinin being hydrolytic products obtained from it, which are diglucosides of pelargonidin.
Salvianin.
As this compound has not yet been obtained in a crystalline condition despite the formation of a finely crystalline picrate the data concerning it must be looked upon as somewhat incomplete and preliminary; Willstatter promises further investigation. (It should be noted, in this connection, that descriptions given by Willstatter and his collaborators in their later papers are not so complete as those of the earlier ones, this being due to the abnormal conditions under which the work was carried out.)
In Salvia splendens (dry petals) the pigment is present to the extent of about 6 per cent., and is not accompanied by any nonglucoside anthocyan. The crude colouring matter was obtained by Willstatter and Bolton, as acetate, by extracting the petals (fresh) with ten times their weight of glacial acetic acid for some days, and precipitating the extract thus obtained by means of ether 1 kg. of petals yields 20 gr. crude product. The substance thus obtained was purified by either of two methods, but in no case could the compound be obtained in a crystalline condition except in the form of the picrate. One method consisted in adding a saturated solution of picric acid to a solution of the crude acetate in 0.05 per cent. HCl, whereupon the picrate separated, and by treatment with methyl-alcoholic hydrochloric acid was converted into the chloride, and this was purified by fractional precipitation from methyl-alcoholic hydrochloric acid by means of ether. The other, which is of interest in view of the fact that it takes advantage of the abnormal distribution number of this pigment, depended upon the fractional extraction of the pigment from its aqueous acid solution by means of amyl alcohol. This is tedious and requires large bulks of solvents for quite small quantities of pigment.
Salvianin chloride.
This as purified consists of a solidified oil of brownish-red colour with a metallic lustre. After drying in high vacuum analyses gave C = 54.12 per cent.; H = 4.64 per cent.; Cl = 4.40 per cent.; and hydrolysis showed that the compound yielded pelargonidin chloride (C15H11O5Cl. H2O), 35.5 per cent. to 37.6 per cent.; glucose, 29.5 per cent, to 33.4 per cent.; and malonic acid, 24.6 per cent, to 26.7 per cent. It would appear therefore that salvianin is a complex body which yields malonic acid as well as glucose on hydrolysis, and hence resembles delphinin; moreover, salvianin is remarkable in that it seems very probable that the glucose molecules are not there as such, but in the form of a derivative containing a smaller number of OH groups and of the composition C6H10O5. Polarimetric evidence points to this derived sugar being first liberated when salvianin is hydrolysed, this passing into glucose as the result of further hydrolysis. In addition to this peculiarity salvianin shows a very marked difference from all other known diglucosides, except salvin (see below), in respect of its distribution number. Of the other known disaccharides of the anthocyan series, only the rhamnoglucosides have a distribution number approaching that of the normal monoglucosides, viz. ca. 10, all the diglucosides having numbers about 1-2, but in the case of salvianin the number is as high as 50 (cf. salvin).
Salvianin chloride is about 2/3; as intense a colouring matter as salvin chloride; it is very easily soluble in very dilute hydrochloric acid (0.5 per cent. HCl), and fairly so in stronger acid (100 c.c. of 4 per cent. HCl dissolves 0.13 gr. at 20°C.), and its solutions in aqueous acid are rather more red than those of pelargonin chloride, whilst those in alcohol are less so. The solid salt changes to an oil on the addition of water, but only forms a solution if little water is used, and on addition of more water the pigment is reprecipitated in the form of red flakes of a basic salt. With very much water a colourless solution of the pseudo-base is formed from which, on addition of acid, the colour is reproduced.
The salt is very readily soluble in ethyl or methyl alcohol if acid be present, forming yellow-red solutions with fluorescence (not so strong as pelargonin); these on addition of alkali give a pure blue colour, whereas an aqueous acid solution of the pigment with caustic soda gives violet rapidly passing to yellow, with sodium carbonate a violet, and with sodium acetate a violet-red colour.
In the presence of hydrochloric acid salvianin is gradually converted into a second diglucoside of pelargonidin - salvin - which can be obtained in fine crystals, and further to a third - salvinin. Both of these products are best obtained by allowing a solution of purified salvianin in dilute hydrochloric acid to stand in a desiccator containing concentrated hydrochloric acid which is kept at 30 per cent, strength by addition of fresh concentrated acid from time to time. In this way oily drops are at first precipitated, followed by a second fraction in the form of deep red tablet-like prisms (salvin chloride), whilst the mother liquors contain salvinin chloride.
Salvin chloride.
For this compound Willstatter and Bolton tentatively suggest the formula C27H27O13Cl (i.e. pelargonin chloride - 2H2O), and state the following reasons for considering that salvianin is not merely an impure form of salvin: (1) salvin (crystalline) gives only an oily precipitate with picric acid, whereas salvianin (amorphous) yields a finely crystalline picrate; (2) under suitable conditions salvianin readily forms a basic salt, whereas salvin does not do so when similarly treated; (3) salvianin is more strongly fluorescent than salvin. It is unfortunate that as the chloride loses hydrochloric acid on complete drying, the analytical data have had to be calculated back to the chlorine-free base; when this is done, however, good agreement is obtained. Moreover, the formula, which requires the glucose to be present, as in the case of salvianin (see above), in a derived form having the composition C6H10O5, is supported by the abnormal rotation observations which point to an intermediate sugar hydrolytic product being first produced which by further hydrolysis yields glucose, and also by the exceedingly high distribution number, which for this pigment is no less than 57.
The crystalline pigment is easily soluble in water without precipitation of a basic salt (cf. salvianin), forming an orange-red solution - very dilute aqueous solutions become decolorised owing to pseudobase formation - and in dilute hydrochloric acid yields an orangeyellow liquid. It is very soluble in methyl or ethyl alcohol, and these solutions fluoresce, but less strongly than similar solutions of pelargonin or salvianin chlorides.
With reagents the following reactions are recorded: Aqueous, or aqueous acid solutions of salvin chloride on addition of sodium acetate give a violet flocculent precipitate, whereas sodium carbonate produces a violet coloration, and caustic soda a violet colour that rapidly passes to yellow. Lead acetate produces a violet flocculent precipitate.
On complete hydrolysis salvin chloride yields pelargonidin chloride (1 mol.) and glucose (2 mols.).
Salvinin chloride, C27H31O15Cl, which behaves in every way as a normal diglucoside anthocyan, and which is isomeric with pelargonin chloride, was obtained by Willstatter and Bolton from the acid mother liquors from the preparation of salvin chloride, by saturating these with amyl alcohol, when, on standing for a few days, salvianin chloride separated in fine long needles; or directly from salvianin chloride, by allowing its solution in concentrated aqueous hydrochloric acid to stand for several weeks. Any pelargonidin chloride or salvin chloride present in the product was removed by dissolving the crystals in i per cent, aqueous hydrochloric acid (0-3 gr. in 50 c.c.) and repeatedly shaking with amyl alcohol. After filtration of the aqueous layer and making the acid in it up to 2 per cent. HCl, on standing for about a day the salvinin chloride separated almost quantitatively. It may be recrystallised from warm 2 per cent, hydrochloric acid, from which it separates in needles, or by dissolving in 0-5 per cent, hydrochloric acid and saturating the solution with amyl alcohol. The crystalline compound is a hydrate of the composition C27H31O15Cl, 5H2O, whereas pelargonin chloride crystals, when prepared in a similar manner, have the composition C27H31O15Cl, 4H2O.
Salvinin chloride is fairly soluble in water and in ethyl alcohol, even more so in methyl alcohol, and the solutions are orange in colour at first, but change to violet and, if very dilute, finally become colourless. In cold 1-2 per cent, hydrochloric acid it is difficultly soluble, but easily soluble in warm, and crystallises out from such solutions on cooling in long purple-red needles. In cold 7 per cent, sulphuric acid it is very difficultly soluble, but dissolves easily in the hot acid (7 per cent.), and on cooling clusters of needles separate. No characteristic colour reaction is given with ferric chloride. Picric acid gives a scarlet-red amorphous picrate.
The distribution number of salvinin chloride is as that of normal diglucoside anthocyans, viz. about 1-2. The pure product sinters at 167°C. and melts, with decomposition, at 168°C.
On complete hydrolysis salvinin chloride yields pelargonidin (1 molecule) and glucose (2 molecules), but in the course of the hydrolysis the production of an intermediate monoglucoside was observed by Willstatter and Bolton, which resembled pelargonenin (see above), but was considered by them to be isomeric, not identical with it. The data available on this point are, however, very scanty, and the new substance has not been completely examined.
It is of interest to note that of the pelargonidin derivatives so far examined, salvianin, salvin, pelargonin, pelargonenin, and the monoglucoside obtained from salvinin by partial hydrolysis, show fluorescence when in acid alcoholic solution, whereas callistephin does not.
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