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Anthocyanidins are common plant pigments, the sugar-free counterparts of anthocyanins. They are based on the
flavylium
Pyrylium is a cation (positive ion) with formula , consisting of a six-membered ring of five carbon atoms, each with one hydrogen atom, and one positively charged oxygen atom. The bonds in the ring are conjugated as in benzene, giving it an arom ...
cation, an oxonium ion
In chemistry, an oxonium ion is any cation containing an oxygen atom that has three bonds and 1+ formal charge. The simplest oxonium ion is the hydronium ion ().
Alkyloxonium
Hydronium is one of a series of oxonium ions with the formula R''n' ...
, with various groups substituted for its hydrogen
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...
atoms. They generally change color from red through purple, blue, and bluish green as a function of pH.
Anthocyanidins are an important subclass of the polymethine
Polymethines are compounds made up from an ''odd'' number of methine groups (CH) bound together by alternating single and double bonds.Kachovski and Dekhtyar, ''Dyes and Pigments'', 22 (1983) 83-97. Compounds made up from an ''even'' number of meth ...
dyes and flavonoids. The flavylium cation is a chromenylium cation with a phenyl
In organic chemistry, the phenyl group, or phenyl ring, is a cyclic group of atoms with the formula C6 H5, and is often represented by the symbol Ph. Phenyl group is closely related to benzene and can be viewed as a benzene ring, minus a hydrogen ...
group substituted in position 2; and chromenylium (also called benzopyrylium) is a bicyclic version of pyrylium. The positive charge can move around the molecule.
At least 31 monomeric anthocyanidins have been properly identified in living organisms, mostly as the core components of anthocyanins. The latter are responsible for the red, purple, blue, or black color of many fruits (like grape
A grape is a fruit, botanically a berry, of the deciduous woody vines of the flowering plant genus '' Vitis''. Grapes are a non- climacteric type of fruit, generally occurring in clusters.
The cultivation of grapes began perhaps 8,000 years a ...
s and blueberries), flowers (like rose
A rose is either a woody perennial flowering plant of the genus ''Rosa'' (), in the family Rosaceae (), or the flower it bears. There are over three hundred species and tens of thousands of cultivars. They form a group of plants that can be ...
s), leaves (like purple cabbage), and even tubers (like radish
The radish (''Raphanus raphanistrum'' subsp. ''sativus'') is an edible root vegetable of the family Brassicaceae that was domesticated in Asia prior to Roman times.
Radishes are grown and consumed throughout the world, being mostly eaten raw ...
es and purple yam
''Dioscorea alata'', also known as purple yam, ube (, ), or greater yam, among many other names, is a species of yam (a tuber). The tubers are usually a vivid violet-purple to bright lavender in color (hence the common name), but some range in ...
s). They are also found in some animals.
Classification
3-Deoxyanthocyanidins such as luteolinidin are a class of anthocyanidins lacking an hydroxyl group on carbon 3. :Natural occurrence
Most plant anthocyanins are based on cyanidin (30%), delphinidin (22%), and pelargonidin (18%), respectively. Altogether 20% of the anthocyanins are based on the three common anthocyanidins (peonidin, malvidin, and petunidin) that are methylated. Around 3%, 3%, and 2% of the anthocyanins or anthocyanidins are respectively labeled as 3-desoxyanthocyanidins, rare methylated anthocyanidins, and 6-hydroxyanthocyanidins, respectively. In bryophytes, anthocyanins are usually based on 3-desoxyanthocyanidins located in the cell wall. A new anthocyanidin, riccionidin A, has been isolated from the liverwort '' Ricciocarpos natans''. It could be derived from 6,7,2′,4′,6′-pentahydroxyflavylium, having undergone ring closure of the 6’ -hydroxyl at the 3-position. Its visible spectrum in methanolic HCl is at 494 nm. This pigment was accompanied by riccionidin B, which most probably is based on two molecules of riccionidin A linked via the 3′- or 5′-positions. Both pigments were also detected in the liverworts ''Marchantia polymorpha'', ''Riccia duplex'', and ''Scapania undulata''.Effect of pH
The stability of anthocyanidins is dependent on pH. At a low pH (acidic conditions), colored anthocyanidins are present, whereas at a higher pH (basic conditions) the colorless chalcones forms are present.
References
External links
* {{Authority control PH indicators Bioindicators