The Natural Pigments.(CHAPTER VIII. Pyran Group. Anthocyanins and Anthocyanidins.)

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.

As previously noted, it is an interesting fact that but very few fundamental compounds form the basis of the comparatively large number of anthocyan pigments that have now been examined. Thus far, all the known colouring matters of this series are derived from pelargonidin, cyanidin, or delphinidin, though from the most recent investigations there appears reason to hope that pigments of different origin may be isolated as the result of further work.

It is a curious coincidence that, before the investigation by Willstatter and Everest of cyanin and cyanidin was completed, it had been decided by them, as the result of preliminary experiments carried out in connection with the corn-flower work, that the pigments of the scarlet pelargonium and the wild larkspur (Delphinium consolida, L.) should be examined next Work had indeed been commenced on them, and thus the three above-mentioned fundamental pigments were the first to be investigated by Willstatter and his collaborators, to whom we owe most of our present knowledge of these colouring matters.

The experimental difficulties to be overcome in the investigation of these pigments were very great until Willstätter and Everest, in their work on the corn-flower pigment, made clear the essential conditions, and further, introduced a rapid test that would distinguish glucoside from non-glucoside pigments.

All the anthocyan colouring matters occur in plants in the form of glucosides, for the available evidence points to their being in nearly every case entirely in that condition. But a few instances have been definitely established in which a small percentage of sugar-free pigment accompanies the glucoside, and in only one exceptional case a considerable proportion of the total colour present was non-glucosidal.

The pigments, at present examined, occur in nature as monoor diglucosides, and the sugars that have been obtained by their hydrolysis are glucose, galactose, and rhamnose; by far the largest portion of the pigments being in combination with glucose only, a smaller number are rhamno-glucosides, whereas, as yet, galactose has only been isolated from one colouring matter of the series.

There are two very interesting abnormal cases of pigments (delphinin and salvianin) in which the natural colouring matter is not merely a complex containing the anthocyanidin combined with the sugar residues, but also with acid components. Thus delphinin, on hydrolysis, yields delphinidin, glucose, and /-oxybenzoic acid, whereas the pigment salvianin gives pelargonidin, glucose, and malonic acid. There is a further point of interest attached to the latter pigment, in that it does not appear to have the two molecules of glucose that it contains present as such, but in a derived form of the composition C₆H₁₀C₅, which is transformed into glucose on hydrolysis.

The three fundamental compounds forming the base of all the anthocyanins to be described below differ from one another only in the number of OH groups present in the molecule.

Pelargonidin, cyanidin, and delphinidin have been shown to have the structures represented by the formulæ (1), (2), and (3) respectively : [KUVA PUUTTUU] and hence are
3:5:7: trihydroxy-2-p-hydroxyphenyl-i: 4: benzopyranol anhydrochloride,
3:5:7: trihydroxy - 2: mp - dihydroxyphenyl -1:4- benzopyranol anhydrochloride, and
3:5:7: trihydroxy -2: mmp-trihydroxyphenyl-i: 4 - benzopyranol anhydrochloride.