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Anthocyanins, the other pigments in the plant kingdom

Article author : G. Espino ( University of Burgos.)

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 [1] of the anthocyanidins . These, in turn, are polyphenolic compounds derived from cation flavylium  [2] 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.

 These natural products have very different functions in plants:
  1. 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.
  2. In the  flowers , represent an adaptation to attract insects with its striking red and purple tones, in order to enable them pollination.
  3. In  fruits , their bright colors represent a warning for animals favoring  dispersal  of their seeds.
  4. 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 anthocyanins are very common in nature. Thus, for example, are present in many wild fruits like blueberries, blackberries, raspberries, strawberries, blueberries, elderberries and gooseberries, and also in other colors of red fruits like cherries, plums, red grapes (and wine) and red apples. Also found in  the flowers and leaves plenty of plants and  tubers and vegetables like beets and cabbage among many other plant products. They have remarkable beneficial health properties such as antioxidant nature and ability in capturing free radicals, and consequently are attributed a certain anti-cancer, and benefits in brain activity and heart functions.

The broth of Lombard is a pH indicator

One of the most unique properties of anthocyanins is its ability to change color depending on the pH, showing a wide range of shades from red to purple or blue to yellow (with increasing pH) so that act as natural pH indicators. This is because its structure experiences a wide range of molecular changes depending on proton concentration.

The anthocyanins at acidic pH (typically pH <4 ) adopt a structure type oxonium (cation flavylium[2] 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 .
In particular, in the cabbage (Figure 1a) have identified 36 different anthocyanins, many of which are glycosides of cyanidin [3] ( cyanidin-3-O-glucoside[1] cyanidin-3,5-O-diglucoside , soforósido cyanidin glucosides and many others, see Scheme 4 ) . The color of the broth extracted to bake a cabbage varies with the concentration of protons from orange to green-yellow, through shades of red, pink, blue and green with increasing pH (see Figure 1b).

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

Another example of the versatility of anthocyanins as natural pigments is the range of colors having the hydrangeas. In this example, however, the key is not the pH of flowers that is identical in both cases, but the presence and movement, or not, of soluble salts of Al 3+  based soil pH. Thus, hydrangeas bloom with blue when the soil pH is acid ( 4.5-5 ), pink flowers occur at higher pHs ( 6-6.5 ), whereas white flowers appear at pHs around 8  (see Figure 2).

Figure 2 . hydrangeas blue, pink and white. ( Photo : G. Espino ).
But how can the soil pH affect the color of the flowers? The answer lies in the composition of clay soils. These soils contain minerals belonging to the family of aluminum phyllosilicates hydrated as kaolinite (Al 2 If 2 O 5 (OH) 4 = Al 2 O 3 · 2 SiO 2 · 2H 2 O)). Such mineral species can release Al 3+ at acidic pHs due to formation of water soluble cationic complexes such as: [Al (OH 2 ) 6 ] 3+ , [Al (OH 2 ) 5 (OH)] 2 + and [Al (OH 2 ) 4 (OH) 2 ] + (see Scheme 5). In  more alkaline soils however, the Al 3+ clays lies immobilized as insoluble species themselves alumninio silicates or oxides or hydroxides of this metal (so simplified equations:  " Al (OH) 3 (H 2 O) 3 ").
Scheme 5 . acid-base equilibria that explain the mobilization of species of  Al 3+  complexed water soluble cationic at acidic pH.

Thus, the color of pink hydrangea is due to the presence of the pigment  mirtilina  (or  delphinidin-3-o-glucoside ), which  in the free state  is pink (see Scheme 6), while the blue hydrangea the mirtilina is comprising a complex with the cation  Al 3+  gives them their characteristic blue color.
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. [4]

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.

References :

(1) http://mylespower.co.uk/2012/04/06/homemade-ph-indicator/

(2) Yurkanis Bruice, P. Organic Chemistry, Bruice, 4th Ed.
(3) Burrows, A .; Holman, J .; Parsons, A .; Pilling, G .; Price, G. Chemistry 3 : Introducing inorganic, organic and physical chemistry. 2009 . 1 st Ed.

[1]  glycoside : is a molecule in which a sugar is attached to another functional group via a link  glycosidicGlucoside : a  glycoside  derivative of  D-glucose  or  dextrose , which in turn is a natural monosaccharide found in plants and whose empirical formula is C 6 H 12 O 6 .
[2]  In fact the cation  flavylium  is associated with an anion that maintains electrical neutrality. If acidification is carried out with HCl, the anion is Cl - . 
[3]  Despite its name cyanidin molecule contains cyanide groups in its structure.
[4]  rutinoside : is a derivative of the glycoside rutinose , which is a disaccharide present in some natural flavonoids.

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