Anthocyanidin

Anthocyanidins set the coloring portion of anthocyanins ( a group of plant pigments ), dar. This group consists of anthocyanidin, known as aglycone, and glycosidically bound sugars. Through the range of naturally formed sugars results in a variety of about 250 anthocyanins.

• 4.1 anthocyanidins
• 4.2 flavones
• 4.3 flavonols
• 4.4 reduction

History

Significant contributions to the structure elucidation of anthocyanins made ​​Willstätter Richard, who received the Nobel Prize for Chemistry in 1915 for his pioneering systematic work in this area. He was able to isolate the anthocyanins from plant extracts as well as from this release by degradation reactions, anthocyanidins and identify. Ultimately, are known only approximately ten anthocyanidins as a natural parent compounds.

Structure

The glycosidic bond in anthocyanins can be cleaved by acid catalysis and so the anthocyanidin be released. Anthocyanidins are multi- hydroxy- 2- Phenylchromenyliumsalze ( flavylium salts ), as a counter ion of Benzopyryliumsalzen act in nature mostly carboxylates of various water-soluble acids in laboratory preparations often chloride. For crystallization, formerly used picrates. By far the most important in this group of plant pigment cyanidin, delphinidin and pelargonidin followed by. Their methyl peonidin, and malvidin petunidin are also found frequently.

Anthocyanidins have always in position 2 to a p-hydroxyphenyl substituents (B- ring ), and in position 3 a hydroxy group. The most important natural anthocyanidins are available in 5 - and 7- hydroxy- position of the A ring.

In addition to the group of anthocyanidins, there is the small group of 3- Deoxyanthocyanidine ( R3 = H). These include

• Apigeninidin (3- Deoxypelargonidin, Gesneridin )
• Luteolinidin (3- Deoxycyanidin ) and as Diosmetinidin methyl (3- Deoxypeonidin )
• Tricetinidin (3- Deoxydelphinidin )
• Columnidin

3- Deoxyanthocyanidine behave in terms of color and acidity as anthocyanidins.

Properties

Anthocyanidins absorb light in the visible range between 450 and 650 nm and therefore appear red, purple or blue. The wavelength range is influenced except by the molecular structure and the pH of the solution. In the acidic environment outweighs the red color in basic are a lot of blues and purples to find.

PH dependence of the color change

The color changes based on chemical reactions.

• At pH values ​​below 3 are colored red and are in the form of Flavyliumkationen ago.
• PH values ​​4-5 lead by hydroxylation to colorless carbinol pseudo- base ( " Leuco bases " Chromenol ).
• At pH values ​​between 7 and 8 of this molecule is deprotonated to Flavenolatanion having a blue color. Here the Π - electrons are delocalized over the entire molecule longest possible distance and therefore excited with lowest light energy.
• PH values ​​from 8 lead in the absence of Glycosidgruppen in the 5-position also Flavenolat dianions, but competes in the alkaline hydrolytic opening of the pyran ring. The molecule is converted irreversibly to a yellow chalcone anion.
• Compared with the pKa values ​​of diphenols (pKa approximately 9.5 and 11.7 ) anthocyanidins are more than 1000 times more acidic. Flavylium salts in water are more than 10 times more acidic than formic acid (pKa 3.8 ).

Synthesis

Robert Robinson 1921 was based on the analysis of the substitution pattern of the flavonoids on the recognition that the flavonoids and thus the anthocyanidins must be constructed biochemically from a C6 and a C6 - C3 unit. 1953 extended Arthur Birch and FW Donovan this theory: The flavonoid biosynthesis must be based on a p -hydroxycinnamic acid and three acetate units, as a Polyketosäure intermediate is formed.

Synthetic anthocyanidins are often not directly, but made ​​about similar flavonoids. Flavones are redox equivalent with anthocyanidins, flavanones with 3- Deoxyanthocyanidinen. Numerous methods are known to reduce flavones, flavanones and flavonols to the corresponding anthocyanidins and 3- Deoxyanthocyanidinen.

Anthocyanidins

Preparative anthocyanidins are accessible by a Knoevenagel condensation of 3,5- dimethoxy salicylaldehydes with α - methoxyacetophenones. The primary product tautomerizes with ring closure to α - flavanol, which loses water upon addition of acid and the methoxy-substituted flavylium salt results ( Robinson annulation ). Methyl groups can be split off gently with hydriodic acid.

Flavones

The Allan -Robinson reaction and their intramolecular variant, the Baker - Venkataraman rearrangement, lead by base-catalyzed condensation reactions to flavones.

Flavonols

In 1908 Karl von Auwers discovered by chance a ring expansion reaction ( Auwers reaction) that leads from Cumaronen to flavonols.

The Algar - Flynn - Oyamada reaction is a base-catalyzed ring closure reaction with hydrogen peroxide. This flavone precursors are oxidized to the flavonol.

Reduction

With classic mild reduction methods specifically flavones to 3 - Deoxyanthocyanidinen or flavonols can be reduced to anthocyanidins. With drastic reduction methods (sodium amalgam in water) but can also be flavanones to red-colored Flavyliumsalzen implement.

Review article

• H. Halbwirth: "The Creation and Physiological Relevance of divergent hydroxylation patterns in the flavonoid pathway ," in Int J Mol Sci. 2010, 11, 595-621 (PMC 2852856 ( free full text ) ).