Anthocyanins, the other pigments in the plant kingdom
Anthocyanins are water soluble natural pigments found in the vacuoles of cells of virtually all plant tissues (leaves, flowers, fruits, stems and roots). From a chemical standpoint, anthocyanins are glycosides  of the anthocyanidins . These, in turn, are polyphenolic compounds derived from cation flavylium  and belong to a higher class of molecules called flavonoids (see Scheme 1). Some common anthocyanidins are shown in Scheme 2.
|Scheme 1 . (a) Cation flavylium : General structure and numbering; (B) pelargonidin (anthocyanidin); (C) pelargonidin-3-O-glucoside (anthocyanin).|
|Scheme 2 . Molecular structure of some common anthocyanidins.|
- In photosynthetic tissues (leaves and stems), anthocyanins provide protection against ultraviolet radiation, due to its ability to absorb not only the frequencies of red and blue in the visible radiation but also UV radiation.
- In the flowers , represent an adaptation to attract insects with its striking red and purple tones, in order to enable them pollination.
- In fruits , their bright colors represent a warning for animals favoring dispersal of their seeds.
- In trees with red leaves atypical, anthocyanins are attributed a function of camouflage against herbivores who are attracted by the green color of plants, but they can not perceive light red.
The broth of Lombard is a pH indicator
The anthocyanins at acidic pH (typically pH <4 ) adopt a structure type oxonium (cation flavylium )  and have colors in the range of deep red due to the extended conjugation between the two aromatic fragments which allows light absorption visible with a variable wavelength ( 480 - 550 cm -1 ), depending on the substituents of the rings. A pH around 4.5 , however, these molecules undergo nucleophilic attack on C2 by a water molecule, and adopt a configuration type carbinol pseudobase lacking in color due to the absence of conjugation between the monocyclic fragment and the rest of the molecule, which prevents the absorption of visible light. Above pH 5 reacquire intense colors in shades of blue, green and yellow, due to the predominance of neutral or anionic conformations with strong conjugation (see Scheme 3).
|Scheme 3 . Molecular transformations function of pH for cyanidin .|
|Figure 1a . Lombarda . (Photo by Rode Kool).|
|Scheme 4 (a). cyanidin ; (B) crisantemina (cyanidin-3-O-glucoside); (C) cyanidin-3,5-O-diglucoside.|
Hydrangeas on demand: blue, pink or white
|Figure 2 . hydrangeas blue, pink and white. ( Photo : G. Espino ).|
|Scheme 5 . acid-base equilibria that explain the mobilization of species of Al 3+ complexed water soluble cationic at acidic pH.|
|Scheme 6. (a) Delphinidin (anthocyanidin) (b) mirtilina (3-O-glycoside of delphinidin); (C) Tulipanina (3-O-rutinoside of definidina) present in the flowers of various species. |
As a result of this, the color of hydrangeas can be controlled by changing the pH of the soil, provided there cations Al 3+ in it. So as to transform into pink hydrangea blue hydrangea alkalize the floor must, usually using lime ( CaO which reacts with water to form Ca (OH) 2 , and therefore anion OH - see Scheme 7, equations (1 ) and (2)). This change in pH causes immobilisation of cations Al 3+ , preventing complex formation with mirtilina , and causing the desired color change. Conversely, to obtain blue hydrangea from pink flowers is necessary to acidify the soil, which is usually done by adding Al 2 (SO 4 ) 3 , since aqueous solutions of this salt are acidic due to the hydrolysis of the metal cation (see Scheme 7, equation (3)). Aluminum sulfate has the added advantage of increasing the concentration of cation Al 3+ available.
|Scheme 7 . Equilibria hydration of lime , dissociation of Ca (OH) 2 and hydrolysis of the cation Al 3+ .|
Glycosides of cyanidin also contribute to the reddish tones of many fruits such as blackberries, strawberries, cherries, red apples, red grapes and wine, and also some flowers like red roses , but also provide a color a blue flower cornflower or tile ( centaura cyanus ) (see Figures 3a and 3b). As in other colors anthocyanin cyanidin glycosides and also depend on the pH and the presence or not of metal. In the particular case flower tile blue color is due to the formation of a supermolecular complex protocianina (the anthocyanin cyanidin ) with iron, magnesium and calcium, whereas the red roses protocianina is free.
|Figure 3a . Red rose . Photo by Magnus Manske.|
|3b . Tile ( Centaurea cyanus ). Photo taken by Böhringer Friedrich.|