Not to be confused withanthocyanidins, the sugar-free counterparts of anthocyanins.
Purple cauliflower contains anthocyanins.
Anthocyanins (from Ancient Greekἄνθος (ánthos)'flower' and κυάνεος/κυανοῦς (kuáneos/kuanoûs)'dark blue'), also calledanthocyans, arewater-solublevacuolarpigments that, depending on theirpH, may appear red, purple, blue, or black. In 1835, the German pharmacistLudwig Clamor Marquart named a chemical compound that gives flowers a blue color, Anthokyan, in his treatise "Die Farben der Blüthen" (English: The Colors of Flowers). Food plants rich in anthocyanins include the blueberry, raspberry,black rice, and black soybean, among many others that are red, blue, purple, or black. Some of the colors of autumn leaves are derived from anthocyanins.[1][2]
Although approved as food and beveragecolorant in the European Union, anthocyanins are not approved for use as afood additive because they have not been verified as safe when used as food orsupplement ingredients.[4] There is no conclusive evidence that anthocyanins have any effect on human biology or diseases.[4][5][6]
In flowers, the coloration that is provided by anthocyanin accumulation may attract a wide variety of animal pollinators, while in fruits, the same coloration may aid in seed dispersal by attracting herbivorous animals to the potentially-edible fruits bearing these red, blue, or purple colors.
Anthocyanins may have a protective role in plants against extreme temperatures.[7][8] Tomato plants protect against cold stress with anthocyanins countering reactive oxygen species, leading to a lower rate ofcell death in leaves.[7]
Superposition of spectra of chlorophylla andb withoenin (malvidin 3O glucoside), a typicalanthocyanin, showing that, 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 don't absorb at all.
The absorbance pattern responsible for the red color of anthocyanins may be complementary to that of greenchlorophyll in photosynthetically active tissues such as youngQuercus coccifera leaves. It may protect the leaves from attacks by herbivores that may be attracted by green color.[9]
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 theepidermis and peripheral mesophyll cells.
The highest recorded amount appears to be specifically in theseed coat of blacksoybean (Glycine max L. Merr.) containing approximately 2 g per 100 g,[33] in purple cornkernels andhusks, and in the skins and pulp of blackchokeberry (Aronia melanocarpa L.) (see table). Due to critical differences in sample origin, preparation, and extraction methods determining anthocyanin content,[34][35] the values presented in the adjoining table are not directly comparable.
Nature, traditional agriculture methods, and plant breeding have produced various uncommon crops containing anthocyanins, includingblue- or red-flesh potatoes and purple or red broccoli, cabbage, cauliflower, carrots, and corn. Gardentomatoes have been subjected to a breeding program usingintrogression lines ofgenetically modified organisms (but not incorporating them in the final purple tomato) to define the genetic basis of purple coloration in wild species that originally were fromChile and theGalapagos Islands.[36] The variety known as "Indigo Rose" became available commercially to the agricultural industry and home gardeners in 2012.[36] Investing tomatoes with high anthocyanin content doubles theirshelf-life and inhibits growth of a post-harvestmoldpathogen,Botrytis cinerea.[37]
Some tomatoes also have been modified genetically withtranscription factors fromsnapdragons to produce high levels of anthocyanins in the fruits.[38] Anthocyanins also may be found in naturally ripenedolives,[39][40] and are partly responsible for the red and purple colors of some olives.[39]
Content of anthocyanins in the leaves of colorful plant foods such as purple corn, blueberries, orlingonberries, is about ten times higher than in the edible kernels or fruit.[41][42]
The color spectrum of grape berry leaves may be analysed to evaluate the amount of anthocyanins. Fruit maturity, quality, and harvest time may be evaluated on the basis of the spectrum analysis.[43]
Reds and purples of autumn leaves of the Europeanbilberry result from production of anthocyanins.
Thereds, purples, and their blended combinations responsible for autumn foliage are derived from anthocyanins. Unlikecarotenoids, anthocyanins are not present in the leaf throughout the growing season, but are produced actively, toward the end of summer.[2] They develop in late summer in thesap of leaf cells, resulting fromcomplex interactions of factors inside and outside the plant. Their formation depends on the breakdown of sugars in the presence of light as the level ofphosphate in the leaf is reduced.[1] Orange leaves in autumn result from a combination of anthocyanins and carotenoids.
Anthocyanins are present in approximately 10% of tree species in temperate regions, although in certain areas such asNew England, up to 70% of tree species may produce anthocyanins.[2]
Anthocyanins are approved for use asfood colorants in the European Union, Australia, and New Zealand, havingcolorant code E163.[44][45] In 2013, a panel of scientific experts for theEuropean Food Safety Authority concluded that anthocyanins from various fruits and vegetables have been insufficiently characterized by safety andtoxicology studies to approve their use asfood additives.[4] Extending from a safe history of using red grape skin extract andblackcurrant extracts to color foods produced in Europe, the panel concluded that these extract sources were exceptions to the ruling and were sufficiently shown to be safe.[4]
Anthocyaninextracts are not specifically listed among approved color additives for foods in the United States; however,grape juice, redgrape skin and many fruit and vegetable juices, which are approved for use as colorants, are rich in naturally occurring anthocyanins.[46] No anthocyanin sources are included among approved colorants fordrugs orcosmetics.[47] When esterified with fatty acids, anthocyanins can be used as a lipophilic colorant for foods.[48]
Although anthocyanins have been shown to haveantioxidant propertiesin vitro,[49] there is no evidence for antioxidant effects in humans after consuming foods rich in anthocyanins.[5][50][51] Unlike controlled test-tube conditions, the fate of anthocyaninsin vivo shows they are poorly conserved (less than 5%), with most of what is absorbed existing as chemically modifiedmetabolites that are excreted rapidly.[52] The increase in antioxidant capacity of blood seen after the consumption of anthocyanin-rich foods may not be caused directly by the anthocyanins in the food, but instead by increaseduric acid levels derived frommetabolizingflavonoids (anthocyanin parent compounds) in the food.[52] It is possible that metabolites of ingested anthocyanins are reabsorbed in thegastrointestinal tract from where they may enter the blood for systemic distribution and have effects as smaller molecules.[52]
In a 2010 review ofscientific evidence concerning the possible health benefits of eating foods claimed to have "antioxidant properties" due to anthocyanins, theEuropean Food Safety Authority concluded that 1) there was no basis for a beneficial antioxidant effect from dietary anthocyanins in humans, 2) there was no evidence of acause-and-effect relationship between the consumption of anthocyanin-rich foods and protection ofDNA,proteins, andlipids fromoxidative damage, and 3) there was no evidence generally for consumption of anthocyanin-rich foods having any "antioxidant", "anti-cancer", "anti-aging", or "healthy aging" effects.[5]
The anthocyanins, anthocyanidins with sugar group(s), are mostly 3-glucosides of the anthocyanidins. The anthocyanins are subdivided into thesugar-freeanthocyanidinaglycones and the anthocyanin glycosides.[citation needed] As of 2003, more than 400 anthocyanins had been reported,[53] while later literature in 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 often are used as pH indicators, as they change from red in acids to blue in bases through a process calledhalochromism.
Anthocyanins are thought to be subject tophysiochemical degradationin vivo andin vitro. Structure, pH, temperature, light, oxygen, metal ions, intramolecular association, and intermolecular association with other compounds (copigments, sugars, proteins, degradation products, etc.) generally are known to affect the color and stability of anthocyanins.[54] B-ring hydroxylation status and pH have been shown to mediate the degradation of anthocyanins to their phenolic acid and aldehyde constituents.[55] Indeed, significant portions of ingested anthocyanins are likely to degrade to phenolic acids and aldehydein vivo, following consumption. This characteristic confounds scientific isolation of specific anthocyanin mechanismsin vivo.
Red cabbage extract at low pH (left) to high pH (right)
Anthocyanins generally are degraded at higher pH. However, some anthocyanins, such aspetanin (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 may be used effectively as afood colorant.[56]
Anthocyanins may be used aspH indicators because their color changes with pH; they are red or pink in acidic solutions (pH < 7), purple in neutral solutions (pH ≈ 7), greenish-yellow in alkaline solutions (pH > 7), and colorless in very alkaline solutions, where the pigment is completely reduced.[57]
These streams meet and are coupled together by the enzyme chalcone synthase, which forms an intermediatechalcone-like compound via apolyketide folding mechanism that is commonly found in plants,
The chalcone is subsequently isomerized by the enzyme chalcone isomerase to the prototype pigmentnaringenin,
Naringenin is subsequently oxidized by enzymes such as flavanone hydroxylase, flavonoid 3'-hydroxylase, and flavonoid 3',5'-hydroxylase,
Leucoanthocyanidins once were 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), recently have been shown to be their true substrates,
The resulting unstable anthocyanidins are further coupled to sugar molecules by enzymes such as UDP-3-O-glucosyltransferase,[60] to yield the final relatively-stable anthocyanins.
Thus, more than five enzymes are required to synthesize these pigments, each working in concert. Even a minor disruption in any of the mechanisms 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 the environmental adaptation, disease tolerance, and pest tolerance provided by anthocyanins.
In anthocyanin biosynthetic pathway,L-phenylalanine is converted to naringenin by phenylalanine ammonialyase, cinnamate 4-hydroxylase, 4-coumarate CoA ligase, chalcone synthase, and chalcone isomerase. Then, the next pathway is catalyzed, resulting in the formation of complex aglycone and anthocyanin through composition by flavanone 3-hydroxylase, flavonoid 3'-hydroxylase, dihydroflavonol 4-reductase,anthocyanidin synthase,UDP-glucoside: flavonoid glucosyltransferase, andmethyl transferase.[61]
The phenolic metabolic pathways and enzymes may be studied by mean oftransgenesis of genes. TheArabidopsis regulatory gene in the production of anthocyanin pigment 1 (AtPAP1) may be expressed in other plant species.[62]
Anthocyanins have been used inorganic solar cells because of their ability to convert light energy into electrical energy.[63] The many benefits to usingdye-sensitized solar cells instead of traditionalp-n junction silicon cells, include lower purity requirements and abundance of component materials, as well as the fact that they may be produced on flexible substrates, making them amenable to roll-to-roll printing processes.[64]
Anthocyaninsfluoresce, enabling a tool for plant cell research to allow live cell imaging without a requirement for otherfluorophores.[65] Anthocyanin production may be engineered into genetically modified materials to enable their identification visually.[66]
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