ANTHOCYANINS

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 their clear, white to yellow counterparts occurring in plants. Anthocyanins are derivatives of anthocyanidins, which include pendant sugars.

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 anthocyanidins, the sugar-free counterparts of anthocyanins.

Function

Superposition of spectra of chlorophyll a and b with oenin (malvidin 3O glucoside), a typical anthocyanin, showing, 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 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 hydrogen peroxide produced in other organelles can be neutralized by vacuolar anthocyanin.

Complementary light absorbance with chlorophylls

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 indicator

 

Red cabbage 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. No less than 10⁹ tons of anthocyanins are produced in nature per year.^{[citation needed]} Not all land plants contain anthocyanin; in the Caryophyllales (including cactus, beets, and amaranth), they are replaced by betalains. However, anthocyanins and betalains have never been found in the same plant.

In food

 

Food source	Anthocyanin content in mg per 100 g
açaí	320
blackcurrant	190-270
chokeberry	1,480
eggplant	750
orange	~200
Marion blackberry	317
black raspberry	589
raspberry	365
wild blueberry	558
cherry	350-400
redcurrant	80-420
red grape	888
red wine	24-35
purple corn	1,642

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, 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. Red-fleshed peaches are rich in anthocyanins.

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 gand in skins and pulp of black chokeberry (Aronia melanocarpa L.) (table). However, the Amazonian palmberry, açaí, having about 320 mg per 100 g of which cyanidin-3-glucoside is the most prevalent individual anthocyanin (approximately 10 mg per 100 g), 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, 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. 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.

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.

Anthocyanins: 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.

Anthocyanins: 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.

Dye-sensitized solar cells

Anthocyanins have been used in organic solar cells because of their strong light harvesting, and their ability to convert of this 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 (and potentially anthocyanins), as well as the fact they can be produced on flexible substrates, making them amenable to roll-to-roll printing processes.

Research on health benefits

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. Laboratory-based evidence was provided to demonstrate potential health effects against:

• cancer
• aging and neurological diseases
• inflammation
• diabetes
• bacterial infections
• fibrocystic disease^[39]

A growing body of evidence suggests anthocyanins and anthocyanidins may possess analgesic properties in addition to neuroprotective and anti-inflammatory activities.

In vitro, anthocyanins possess MAO inhibitory activity for both MAO-A and MAO-B; MAO function is connected to neurodegenerative diseases, depression, and anxiety. The relevance to humans of anthocyanins and MAO activity requires further research, however.

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.

Cancer research

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%. 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.

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

1. stalling growth of premalignant cells
2. accelerating the rate of cell turnover, 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 tumor proliferation, inflammation and angiogenesis, while switching on apoptosis.

In 2007, studies entered the next pivotal level of research – the human clinical trial – for which several approved studies are underway to examine anticancer effects of black raspberries and cranberries on tumors in the esophagus, prostate and colon.