Anthocyanins molecular structures

The magnitude of the copigmentation is influenced by pH value, pigment and copigment concentrations, chemical structure of anthocyanin, temperature, and ionic strength of the medium. As to the effect of the solvent, the important issue is the hydrogen-bonded molecular structure of the liquid water, not the polarity of the medium. ... 

The numbers and types of fragments depend on the anthocyanin structure pattern. The aglycone (anthocyanidin) ordinarily is very stable and cannot be broken easily. In most cases, cleavage of the glycosidic groups will occur to generate small amounts of anthocyanidins in addition to the intact anthocyanin molecular ions. 

The flavonols and their glycosides contribute to specific taste characteristics such as bitterness and astringency in berry fruits and their products (Shahidi and Naczk, 1995). The molecular structure of flavonols lacks the conjugated double bonds of the anthocyanins, and they are thereby colorless. They may, however, contribute to discoloration of berry fruits, as they are readily oxidized by O-phenoloxidase in the presence of catechin and chlorogenic acid. Discoloration may also occur as a consequence of complex formation with metallic ions. On the other hand, the flavonol glycoside rutin is known to form complexes with anthocyanins, thus stabilizing the color of these compounds. 

Anthocyanins display their typical color at low pH values, where the molecules are in the oxonium form (Wrolstad, 2000). The oxonium form is more stable, and anthocyanins thus have higher stability in an acid environment. If pH rises to levels above 4.5, the anthocyanin instability is particularly problematic. The detailed molecular structure of the anthocyanins, i.e., acylation and the position of the glycosidic substitution, affects the exact pH at which the color changes occur. Copigmented anthocyanins are less susceptible to degradation. 

Anthocyanins and their combinations with tannins are not very astringent, bnt have a marked bitterness, especially in yonng wines, i.e. when the molecular structures are well defined and not too complex. 

The above transformations result in a reduced anthocyanin content, contrasting with the increase in color. The new condensed pigments formed are more intensely colored than anthocyanins. Other anthocyanin and tannin breakdown reactions may lead to a loss of color, generally accompanied by a tendency towards yellow-orange hues. This is characteristic of the normal development of bottle-aged red wines. The breakdown of anthocyanins involves a loss of molecular structure in the red coloring matter, possibly accompanied by the appearance of a yellowish hue. 

An anthocyanin occurs in solution as a mixture of different secondary structures, a quinonoidal base, a carbinol pseudobase, and a chalcone pseudobase. ° hi addition, different mechanisms for the stabilization of anthocyanins lead to the formation of tertiary structures such as self-association, inter-, and intra-molecular co-pigmentation. ... 

Polyphenoloxidase (PPO, EC 1.14.18.1) is one of the most studied oxidative enzymes because it is involved in the biosynthesis of melanins in animals and in the browning of plants. The enzyme seems to be almost universally distributed in animals, plants, fungi, and bacteria (Sanchez-Ferrer and others 1995) and catalyzes two different reactions in which molecular oxygen is involved the o-hydroxylation of monophenols to o-diphenols (monophenolase activity) and the subsequent oxidation of 0-diphenols to o-quinones (diphenolase activity). Several studies have reported that this enzyme is involved in the degradation of natural phenols with complex structures, such as anthocyanins in strawberries and flavanols present in tea leaves. Several polyphenols... 

Goto, T. and Kondo, T., Structure and molecular stacking of anthocyanins. Flower color variation. Angew. Chem. Int. Ed. Engl. 30, 17, 1991. 

The total number of different anthocyanins reported to be isolated from plants in this review is 539 (Appendix A). This number includes 277 anthocyanins that have been identified later than 1992. Several previously reported anthocyanins have for the first time received complete structural elucidation, and some structures have been revised. The majority of anthocyanins with the most complex structures and highest molecular masses have been reported in the period of this review. 

In solution, the anthocyanins actually might exist in equilibrium with essentially four molecular forms - the flavylium cation, the quinoidal base, the hemiacetal base and chalcone [19]. The relative amounts of the four structure forms depends on both the pH and the difference in structure of the anthocyanins [20-22], Generally, anthocyanins exist primarily as the stable flavylium cation above pH 2. This uniqueness in the chemical structure is one of the important key factors affecting their absorption, metabolism, bioavailability and, consequently, the biological responses of the human body to anthocyanins. 

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