Anthocyanin

Not to be confused with anthocyanidins, the sugar-free counterparts of anthocyanins.

Anthocyanins are glycosides of anthocyanidins, the basic chemical structure of which is shown here.

Anthocyanins give these pansies their dark purple pigmentation.

Anthocyanins (also anthocyans; from Greek: ἀνθός (anthos) = flower + κυανός (kyanos) = blue) are water-soluble vacuolar pigments that may appear red, purple, or blue depending on the pH. They belong to a parent class of molecules called flavonoids synthesized via the phenylpropanoid pathway; they are odorless and nearly flavorless, contributing to taste as a moderately astringent sensation. Anthocyanins occur in all tissues of higher plants, including leaves, stems, roots, flowers, and fruits. Anthoxanthins are clear, white to yellow counterparts of anthocyanins occurring in plants. Anthocyanins are derived from anthocyanidins by adding sugars.

Function

Superposition of spectra of chlorophyll a and b with oenin (malvidin 3O glucoside), a typical anthocyanin, in an acidic solution. While chlorophylls absorb in the blue and yellow/red parts of the visible spectrum, oenin absorbs mainly in the green part of the spectrum, where chlorophylls do not absorb at all.

In flowers, bright-reds and -purples are adaptive for attracting pollinators. In fruits, the colorful skins also attract the attention of animals, which may eat the fruits and disperse the seeds. In photosynthetic tissues (such as leaves and sometimes stems), anthocyanins have been shown to act as a "sunscreen", protecting cells from high-light damage by absorbing blue-green and ultraviolet light, thereby protecting the tissues from photoinhibition, or high-light stress. This has been shown to occur in red juvenile leaves, autumn leaves, and broad-leaf evergreen leaves that turn red during the winter. The red coloration of leaves has been proposed to possibly camouflage leaves from herbivores blind to red wavelengths, or signal unpalatability, since anthocyanin synthesis often coincides with synthesis of unpalatable phenolic compounds.

In addition to their role as light-attenuators, anthocyanins also may be antioxidants. However, it is not clear whether anthocyanins can significantly contribute to scavenging of free radicals produced through metabolic processes in leaves, since they are located in the vacuole and, thus, spatially separated from metabolic reactive oxygen species. Some studies have shown hydrogen peroxide produced in other organelles can be neutralized by vacuolar anthocyanin.

Light absorbance

The absorbance pattern responsible for the red color of anthocyanins may be complementary to that of green chlorophyll in photosynthetically active tissues such as young Quercus coccifera leaves. It may protect the leaves from attacks by plant eaters that may be attracted by green color.

pH

Anthocyanins are generally degraded at higher pH. However, some anthocyanins, such as petanin (petunidin 3-[6-O-(4-O-(E)-p-coumaroyl-O-α-L-rhamnopyranosyl)-β-D-glucopyranoside]-5-O-β-D-glucopyranoside), are resistant to degradation at pH 8 and can be used as a food colorant.

Use as pH indicator

Red cabbage (anthocyanin dye) extract at low pH (left) to high pH (right)

Anthocyanins can be used as pH indicators because their color changes with pH; they are pink in acidic solutions (pH < 7), purple in neutral solutions (pH ~ 7), greenish-yellow in alkaline solutions (pH > 7), and colourless in very alkaline solutions, where the pigment is completely reduced.

Occurrence

Anthocyanins are found in the cell vacuole, mostly in flowers and fruits but also in leaves, stems, and roots. In these parts, they are found predominantly in outer cell layers such as the epidermis and peripheral mesophyll cells.

Most frequently occurring in nature are the glycosides of cyanidin, delphinidin, malvidin, pelargonidin, peonidin, and petunidin. Roughly 2% of all hydrocarbons fixed in photosynthesis are converted into flavonoids and their derivatives such as the anthocyanins. Not all land plants contain anthocyanin; in the Caryophyllales (including cactus, beets, and amaranth), they are replaced by betalains. Anthocyanins and betalains have never been found in the same plant.

Sometimes bred purposely for high anthocyanin quantities, ornamental plants such as sweet peppers may have unusual culinary and aesthetic appeal.

In food

Food source	Anthocyanin content in mg per 100 g
Açaí	320
Blackcurrant	190–270
Aronia	1,480
Eggplant (Aubergine)	750
Blood orange	~200
Marion blackberry	317
Black raspberry	589
Raspberry	365
Wild blueberry	558
Cherry	122
Queen Garnet plum	277
Redcurrant	80–420
Purple corn (Z. mays L.)	1,642
Purple corn leaves	10x more than in kernels
Concord grape	326
Norton grape	888

Plants rich in anthocyanins are Vaccinium species, such as blueberry, cranberry, and bilberry; Rubus berries, including black raspberry, red raspberry, and blackberry; blackcurrant, cherry, eggplant (aubergine) peel, black rice, Concord grape, muscadine grape, red cabbage, and violet petals. Red-fleshed peaches and apples contain anthocyanins. Anthocyanins are less abundant in banana, asparagus, pea, fennel, pear, and potato, and may be totally absent in certain cultivars of green gooseberries.

The highest recorded amount appears to be specifically in the seed coat of black soybean (Glycine max L. Merr.) containing some 2,000 mg per 100 g, in purple corn kernels and husks, and in skins and pulp of black chokeberry (Aronia melanocarpa L.) (table). Due to critical differences in sample origin, preparation and extraction methods determining anthocyanin content, the values presented in the adjoining table are not directly comparable.

Nature, traditional agriculture, and plant breeding have produced various uncommon crops containing anthocyanins, including blue- or red-flesh potatoes and purple or red broccoli, cabbage, cauliflower, carrots, and corn. Garden tomatoes have been subjected to a breeding program using introgression lines of genetically modified organisms (but not incorporating them in the final purple tomato) to define the genetic basis of purple coloration in wild species originally from Chile and the Galapagos Islands. The variety known as "Indigo Rose" became commercially available to the agricultural industry and home gardeners in 2012. Investing tomatoes with high anthocyanin content doubles their shelf-life and inhibits growth of a post-harvest mold pathogen, Botrytis cinerea.

Tomatoes have also been genetically modified with transcription factors from snapdragons to produce high levels of anthocyanins in the fruits. Anthocyanins can also be found in naturally ripened olives, and are partly responsible for the red and purple colors of some olives.

In leaves of plant foods

Content of anthocyanins in the leaves of colorful plant foods, such as purple corn, blueberries or lingonberries, is about 10 times higher than in the edible kernels or fruit.

The color spectrum of grape berry leaves can be analysed to evaluate the amount of anthocianins. Fruit maturity, quality and harvest time can be evaluated on the basis of the spectrum analysis.

Autumn leaf color

A selected purple-leaf cultivar of European beech

The reds, the purples, and their blended combinations that decorate autumn foliage come from anthocyanins. Unlike the carotenoids, these pigments are not present in the leaf throughout the growing season, but are actively produced towards the end of summer. They develop in late summer in the sap of the cells of the leaf, and this development is the result of complex interactions of many influences, both inside and outside the plant. Their formation depends on the breakdown of sugars in the presence of bright light as the level of phosphate in the leaf is reduced.

Anthocyanins are present in about 10% of tree species in temperate regions, although in certain areas such as New England, up to 70% of tree species may produce the pigment.

Many science textbooks incompletely state that autumn coloration (including red) is the result of breakdown of green chlorophyll, which unmasks the already-present orange, yellow, and red pigments (carotenoids, xanthophylls, and anthocyanins, respectively). While this is indeed the case for the carotenoids and xanthophylls (orange and yellow pigments), anthocyanins are not synthesized until the plant has begun breaking down the chlorophyll.

Structure

Flavylium cation derivatives

Conventional breeding was used to produce P20 blue tomatoes.

See Anthocyanidins article.

Selected anthocyanidins and their substitutions

Basic structure	Anthocyanidin	R3′	R4′	R5′	R3	R5	R6	R7
	Aurantinidin	−H	−OH	−H	−OH	−OH	−OH	−OH
	Cyanidin	−OH	−OH	−H	−OH	−OH	−H	−OH
	Delphinidin	−OH	−OH	−OH	−OH	−OH	−H	−OH
	Europinidin	−OCH 3	−OH	−OH	−OH	−OCH 3	−H	−OH
	Pelargonidin	−H	−OH	−H	−OH	−OH	−H	−OH
	Malvidin	−OCH 3	−OH	−OCH 3	−OH	−OH	−H	−OH
	Peonidin	−OCH 3	−OH	−H	−OH	−OH	−H	−OH
	Petunidin	−OH	−OH	−OCH 3	−OH	−OH	−H	−OH
	Rosinidin	−OCH 3	−OH	−H	−OH	−OH	−H	−OCH 3

Glycosides of anthocyanidins

The anthocyanins, anthocyanidins with sugar group(s), are mostly 3-glucosides of the anthocyanidins. The anthocyanins are subdivided into the sugar-free anthocyanidin aglycones and the anthocyanin glycosides. As of 2003, more than 400 anthocyanins had been reported while more recent literature (early 2006), puts the number at more than 550 different anthocyanins. The difference in chemical structure that occurs in response to changes in pH is the reason why anthocyanins are often used as pH indicators, as they change from red in acids to blue in bases.

Stability

Anthocyanins are thought to be subject to physiochemical degradation in vivo and in vitro. Structure, pH, temperature, light, oxygen, metal ions, intramolecular association, and intermolecular association with other compounds (copigments, sugars, proteins, degradation products, etc.) are generally known to affect the color and stability of anthocyanins. B-ring hydroxylation status and pH have been shown to mediate the degradation of anthocyanins to their phenolic acid and aldehyde constituents. Indeed, significant portions of ingested anthocyanins are likely to degrade to phenolic acids and aldehyde in vivo, following consumption. This characteristic confounds scientific isolation of specific anthocyanin mechanisms in vivo.

Biosynthesis

Anthocyanins and carotenoids contribute distinctive pigmentation to blood oranges.

1. Anthocyanin pigments are assembled like all other flavonoids from two different streams of chemical raw materials in the cell:
   • One stream involves the shikimate pathway to produce the amino acid phenylalanine. (see phenylpropanoids)
   • The other stream produces three molecules of malonyl-CoA, a C3 unit from a C2 unit (acetyl-CoA).
2. These streams meet and are coupled together by the enzyme chalcone synthase, which forms an intermediate chalcone-like compound via a polyketide folding mechanism that is commonly found in plants.
3. The chalcone is subsequently isomerized by the enzyme chalcone isomerase to the prototype pigment naringenin.
4. Naringenin is subsequently oxidized by enzymes such as flavanone hydroxylase, flavonoid 3' hydroxylase and flavonoid 3' 5'-hydroxylase.
5. These oxidation products are further reduced by the enzyme dihydroflavonol 4-reductase to the corresponding colorless leucoanthocyanidins.
6. Leucoanthocyanidins were once believed to be the immediate precursors of the next enzyme, a dioxygenase referred to as anthocyanidin synthase or leucoanthocyanidin dioxygenase. Flavan-3-ols, the products of leucoanthocyanidin reductase (LAR), have been recently shown to be their true substrates.
7. The resulting unstable anthocyanidins are further coupled to sugar molecules by enzymes such as UDP-3-O-glucosyltransferase to yield the final relatively stable anthocyanins.

More than five enzymes are thus required to synthesize these pigments, each working in concert. Even a minor disruption in any of the mechanism of these enzymes by either genetic or environmental factors would halt anthocyanin production. While the biological burden of producing anthocyanins is relatively high, plants benefit significantly from environmental adaptation, disease tolerance, and pest tolerance provided by anthocyanins.

In anthocyanin biosynthetic pathway, L-phenylalanine is converted to naringenin by phenylalanine ammonialyase (PAL), cinnamate 4-hydroxylase (C4H), 4-coumarate CoA ligase (4CL), chalcone synthase (CHS) and chalcone isomerase (CHI). And then, the next pathway is catalyzed the formation of complex aglycone and anthocyanin composition by flavanone 3-hydroxylase (F3H), flavonoid 3'-hydroxylase (F3'H), dihydroflavonol 4-reductase (DFR), anthocyanidin synthase (ANS), UDP-glucoside: flavonoid glucosyltransferase (UFGT) and methyl transferase (MT). Among those, UFGT is divided into UF3GT and UF5GT, which are responsible for the glucosylation of anthocyanin to produce stable molecules.

In Arabidopsis thaliana, two glycosyltransferases, UGT79B1 and UGT84A2, are involved in the anthocyanin biosynthetic pathway. The UGT79B1 protein converts cyanidin 3-O-glucoside to cyanidin 3-O-xylosyl(1→2)glucoside. UGT84A2 encodes sinapic acid: UDP-glucosyltransferase.

Genetic analysis

The phenolic metabolic pathways and enzymes can be studied by mean of transgenesis of genes. The Arabidopsis regulatory gene in the production of anthocyanin pigment 1 (AtPAP1) can be expressed in other plant species.

Potential food value

Purple cauliflower contains anthocyanins

Anthocyanins are considered secondary metabolites as a food additive with E number E163 (INS number 163); they are approved for use as a food additive in the EU, Australia and New Zealand.

Although anthocyanins have antioxidant properties in vitro, this antioxidant effect is not conserved after the plant is consumed. As interpreted by the Linus Pauling Institute and European Food Safety Authority, dietary anthocyanins and other flavonoids have little or no direct antioxidant food value following digestion. Unlike controlled test-tube conditions, the fate of anthocyanins in vivo shows they are poorly conserved (less than 5%), with most of what is absorbed existing as chemically modified metabolites that are rapidly excreted.

The increase in antioxidant capacity of blood seen after the consumption of anthocyanin-rich foods may not be caused directly by the anthocyanins, but instead may result from increased uric acid levels derived from metabolism of flavonoids.

Dye-sensitized solar cells

Anthocyanins have been used in organic solar cells because of their ability to convert light energy into electrical energy. The many benefits to using dye-sensitized solar cells instead of traditional pn junction silicon cells include lower purity requirements and abundance of component materials, such as titania, as well as the fact they can be produced on flexible substrates, making them amenable to roll-to-roll printing processes.

Research

General research

Richly concentrated as pigments in berries, anthocyanins were the topics of research presented at a 2007 symposium on health benefits that may result from berry consumption.

Anthocyanins also fluoresce, enabling a tool for plant cell research to allow live cell imaging for extended periods of time without a requirement for other fluorophores.

Cancer research

According to the American Cancer Society, there have been no studies in humans showing that any phytochemical supplement can prevent or treat cancer.

Use as visual markers to mark genetically modified materials

Anthocyanin production can be engineered into genetically modified materials to enable their visual identification.

See also

• Phenolic compounds in wine
• p-Coumaroylated anthocyanin

External links

• Anthocyanins FAQ MadSci Network
• An excellent overview that provides useful info about the function & synthesis of anthocyanins. Also here is a modified draft.