Anthocyanin

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

Anthocyanins give these pansies their dark purple pigmentation.

Not to be confused with Anthocyanidin, their sugar free counterparts.

Anthocyanins (from Greek: ἀνθός (anthos) = flower + κυανός (kyanos) = blue) are water-soluble vacuolar pigments that may appear red, purple, or blue according to 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 their clear, white to yellow counterparts occurring in plants. Anthocyanins are derivatives of anthocyanidins which include pendant sugars.

Function

Red color in Fuji apples

Anthocyanins provide the purple color of the vertical stripes on stem of this Jalapeño cultivar.

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 UV 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-leaved evergreen leaves that turn red during the winter. It has also been proposed that red coloration of leaves may camouflage leaves from herbivores blind to red wavelengths, or signal unpalatability, since anthocyanin synthesis often coincides with synthesis of unpalatable phenolic compounds.^[1]

In addition to their role as light-attenuators, anthocyanins also act as powerful 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 that hydrogen peroxide produced in other organelles can be neutralized by vacuolar anthocyanin.

Occurrence

Juvenile anthocyanin in new rose growth. The reddish hue disappears as the new leaves mature.

Anatomically, 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 frequent in nature are the glycosides of cyanidin, delphinidin, malvidin, pelargonidin, peonidin and petunidin. Roughly 2% of all hydrocarbons fixated in photosynthesis are converted into flavonoids and their derivatives such as the anthocyanins. There is no less than 10⁹ tons of anthocyanins produced in nature per year. Not all land plants contain anthocyanin; in the Caryophyllales (including cactus, beets, and amaranth), they are replaced by betalains.

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 peel, black rice, Concord grape and muscadine grape, red cabbage and violet petals. Anthocyanins are less abundant in banana, asparagus, pea, fennel, pear and potato, and may be totally absent in certain cultivars of green gooseberries.^[2]

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^[8] and in skins and pulp of black chokeberry (Aronia melanocarpa L.) (table). However, the Amazonian palmberry, açaí, having about 320 mg per 100 g^[9] of which cyanidin-3-glucoside is the most prevalent individual anthocyanin (approximately 10 mg per 100 g),^[10] is also a high-content source for which only a small fraction of total anthocyanins has been determined to date. Due to critical differences in sample origin, preparation and extraction methods determining anthocyanin content,^[11][12] the values presented in the adjoining table are not directly comparable.

Nature, primitive agriculture, and plant breeding have produced various uncommon crops containing anthocyanins, including blue- or red-fleshed potatoes and purple or red broccoli, cabbage, cauliflower, carrots and corn. Tomatoes have been bred conventionally for high anthocyanin content by crossing wild relatives with the common tomato to transfer a gene called the anthocyanin fruit tomato ("aft") gene into a larger and more palatable fruit.^[13]

Tomatoes have also been genetically modified with transcription factors from snapdragons to produce high levels of anthocyanins in the fruits.^[14][15][16] Anthocyanins can also be found in naturally ripened olives,^[17][18] and are partly responsible for the red and purple colors of some olives.^[17]

Autumn leaf color

A selected purple-leaf cultivar of European Beech

Plants with abnormally high anthocyanin quantities are popular as ornamental plants. 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, presumably for photoprotection during nitrogen translocation.

Structure

Anthocyanidins: Flavylium cation derivatives

See Anthocyanidins article.

Selected anthocyanidins and their substitutions

Anthocyanidin	Basic structure	R1	R2	R3	R4	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		−OCH3	−OH	−OH	−OH	−OCH3	−H	−OH
Luteolinidin		−OH	−OH	−H	−H	−OH	−H	−OH
Pelargonidin		−H	−OH	−H	−OH	−OH	−H	−OH
Malvidin		−OCH3	−OH	−OCH3	−OH	−OH	−H	−OH
Peonidin		−OCH3	−OH	−H	−OH	−OH	−H	−OH
Petunidin		−OH	−OH	−OCH3	−OH	−OH	−H	−OH
Rosinidin		−OCH3	−OH	−H	−OH	−OH	−H	−OCH3

Anthocyanins: Glucosides 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^[19] 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 indicator, as they change from red in acids to blue in bases.

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 3 molecules of malonyl-CoA, a C3 unit from a C2 unit (acetyl-CoA).^[20]
2. These streams meet and are coupled together by the enzyme chalcone synthase (CHS), which forms an intermediate chalcone via a polyketide folding mechanism that is commonly found in plants.
3. The chalcone is subsequently isomerized by the enzyme chalcone isomerase (CHI) to the prototype pigment naringenin.
4. Naringenin is subsequently oxidized by enzymes such as flavanone hydroxylase (FHT or F3H), flavonoid 3' hydroxylase and flavonoid 3' 5'-hydroxylase.
5. These oxidation products are further reduced by the enzyme dihydroflavonol 4-reductase (DFR) to the corresponding colorless^[21] leucoanthocyanidins.
6. It was believed that leucoanthocyanidins are the immediate precursors of the next enzyme, a dioxygenase referred to as anthocyanidin synthase (ANS) or leucoanthocyanidin dioxygenase (LDOX). It was recently shown however that flavan-3-ols, the products of leucoanthocyanidin reductase (LAR), are the true substrates of ANS/LDOX.
7. The resulting, unstable anthocyanidins are further coupled to sugar molecules by enzymes like UDP-3-O-glucosyltransferase^[22] to yield the final relatively stable anthocyanins.

More than five enzymes are thus required to synthesize these pigments, each working in concert. Any even minor disruption in any of the mechanism of these enzymes by either genetic or environmental factors would halt anthocyanin production.

Potential food value

Anthocyanins are considered secondary metabolites as a food additive with E number 163.

Although anthocyanins are powerful antioxidants in vitro,^[23] it is unlikely this antioxidant property is conserved after the plant which produced the anthocyanins 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.^[24][25][26] 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 which become rapidly excreted.^[27]

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.^[27]

Dye-sensitized solar cells

Anthocyanins are being used in organic solar cells because of their ability to absorb light and convert it into electrons.^[28] There are many benefits to using dye-sensitized solar cells instead of the traditional silicon cells, such as abundance of anthocyanins, the projected 90% efficiency, and the ability to bend or print inks used in newer manufacturing processes.^[29]

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.^[30] Laboratory-based evidence was provided for potential health effects against:

• cancer
• aging and neurological diseases
• inflammation
• diabetes
• bacterial infections

Cancer research on anthocyanins is the most advanced, where black raspberry (Rubus occidentalis L.) preparations were first used to inhibit chemically induced cancer of the rat esophagus by 30-60% and of the colon by up to 80%.^[30][31] Effective at both the initiation and promotion/progression stages of tumor development, black raspberries are a practical research tool and a promising therapeutic source, as they contain the richest contents of anthocyanins among native North American Rubus berries.^[4]

Work on laboratory cancer models has shown that black raspberry anthocyanins inhibit promotion and progression of tumor cells by

1. stalling growth of pre-malignant cells
2. accelerating the rate of cell turnover, called apoptosis, effectively making the cancer cells die faster
3. reducing inflammatory mediators that initiate tumor onset
4. inhibiting growth of new blood vessels that nourish tumors, a process called angiogenesis
5. minimizing cancer-induced DNA damage.

On a molecular level, berry anthocyanins were shown to turn off genes involved with proliferation, inflammation and angiogenesis.^[32][33][34], while switching on apoptosis.^[35][36]

In 2007, black raspberry studies entered the next pivotal level of research – the human clinical trial – for which several approved studies are underway to examine anti-cancer effects of black raspberries and cranberries on tumors in the esophagus, prostate and colon.^[37]

Anthocyanins also fluoresce; combined with their antioxidant properties this can be a powerful tool for plant cell research, allowing live cell imaging for extended periods of time without a requirement for other fluorophores.^[38]

See also

• Phenolic compounds in wine

References

External links

• Andersen, O.M. Flavonoids: Chemistry, Biochemistry and Applications. CRC Press, Boca Raton FL 2006.
• Robinson GM, Robinson R (1931). "A survey of anthocyanins. I". The Biochemical Journal 25 (5): 1687–705. PMID 16744735. 
• Anthocyanin biosynthesis
• What do the traditional Maori diet, autumn leaves, and red wine have in common? on the Australian science series Catalyst.
• A discussion of the role of anthocyanins in hydrangea coloration
• Anthocyanins FAQ MadSci Network
• Lengthy discussion of Carnivorous Plants that in places touches on the function of anthocyanins in plants
• An excellent overview that provides useful info about the function & synthesis of anthocyanins. Also here is a modified draft.
• Kids orientented overview about leaves with anthocyanins on page 6, a curriculum supplement of Newspaper in Education (NIE) provided by The Washington Post.