Anthocyanins forms

The positions, numbers, and types of sugars on the anthocyanin molecule influence its bioaccessibility. Indeed, a recent human study reported that the acylation of anthocyaifins resulted in a sigififlcant decrease of anthocyanin recoveries in plasma and urine. In addition, anthocyanins form linkages with aromatic acids, aliphatic acids, and methyl ester derivatives, which can also affect their passage through the intestinal barrier. 

The second method utilizes the fact that anthocyanins form colorless compounds with bisulfite ion (12). Again, the other wine constituents do not interfere, and the variation in color, after a large excess of bisulfite is added, is proportional to the concentration of anthocyanins. 

Fig. 13 Equilibrium distribution of four anthocyanin forms of malvidin-3-glucoside as a function of pH The red flavylium cation (AH+), the blue quinonoidal base (A), the colorless carbinol pseudobase (B), and the colorless chalcone (C). (From Ref. 138.)...

The determination of monomeric anthocyanins is based on the ability of anthocyanins to shift in color from bright red/bluish at pH 1 to nearly colorless at pH 4.5 (Wrolstad, 1976 Wrolstad et al., 1982). In contrast, the polymeric anthocyanin forms retain considerable color at pH 4.5. The absorbency at the absorbance maximum of the sample at pH 4.5 is subtracted from that at pH 1, and the total monomeric anthocyanins are calculated based on the molecular weight and extinction coefficient of the most prevalent anthocyanin. Total anthocyanin indices that also include the polymeric pigments are determined from the absorbency at pH 1, omitting measurement at pH 4.5 (Bums et al., 2000). 

The pH of the medium plays a particularly important role in the equilibrium between these different anthocyanin forms, and consequently in color modification. In strongly acid solution, at a pH below 2, the red cation AH+ is the dominant form. As the pH is increased, a rapid proton loss occurs to yield the red or blue quinoidal base A, usually existing in two forms (Figure 9.3). On standing, a further reaction occurs, i.e., hydration of flavylium cation AH+, to give colorless carbinol pseudobase B. Relative amounts of forms AH+, A, B, and C at equilibrium vary with both pH and the structure of anthocyanins. For the common anthocyanin 3-glycosides... 

Table 13.9. Different types of anthocyanin formed according to aging conditions (same experiment as Table 13.8 continued for 10 months) (Ribereau-Gayon et al., 1983)...

However, all these changes are not sufficient to account for the large color variations attributed to anthocyanins in plants. These are in fact due to two types of mechanisms, namely, (1) conversion of the fiavylium ions to secondary anthocyanins forms in aqueous media and (2) molecular interactions. 

Bate-Smith, E.C., Leuco-anthocyanins. 1. Detection and identification of anthocyanins formed from leuco-anthocyanins in plant tissues, Biochem. J., 1954, 58, 122-125. 

Certain anthocyanins form complexes with metals (e.g., iron, aluminum, magnesium), and the result is an augmentation of the anthocyanin color. At times the complexes involve an anthocyanin, a copigment, and a metal. 

LAPIDOT T, HAREL s, AKivi B, GRANITE R and KANNER J (1999) pH-dependent forms of red wine anthocyanins as antioxidants, JAgric Food Chem, 47, 67-70. 

As frequently mentioned in the literature, anthocyanins co-exist in equilibrium in four different forms. The pH conditions shift this equilibrium toward a variety of structural forms, with the direct consequences of color changes of these pigments. As a rule, at pH above 4, yellow compounds (chalcone form), blue compounds (quinoid base), or colorless compounds (methanol form) are produced. Anthocyanins have the highest stabilities at a pH between 1 and 2 since the flavylium cation is the most stable predominant form. 

The overall anthocyanin analysis is generally conducted using the Giusti and Wrolstad method based on the differences in absorbance of anthocyanins at pH 1 and pH 4.5. Then the pigment content is determined using the coefficient of molar extinction of the predominant anthocyanin. It should be noted that this technique only allows dosing of anthocyanins with a color difference between the two pH values (due to transition to the flavylium cation form). A more global analysis of total anthocyanin content may be conducted by direct spectrophotometry of the... 

Anthocyanins are water-soluble pigments, as acylation of the carbon skeleton by glycosides increases the polarity of these molecules. Furthermore, according to the pH conditions, the heterocyclic oxygen atom may be found in cation form, further increasing the molecule s polarity. 

The hydrolysis of zeaxanthin esters by a carboxyl ester lipase indeed enhanced both the incorporation of zeaxanthin in the micellar phase and uptake of zeaxanthin by Caco-2 cells. As mentioned earher, carotenoids can also be linked to proteins by specific bindings in nature and these carotenoid-protein complexes may slow the digestion process and thus make their assimilation by the human body more difficult than the assimilation of free carotenoids. Anthocyanins are usually found in a glycosylated form that can be acetylated and the linked sugars are mostly glucose, galactose, rhamnose, and arabinose. 

Anthocyanins are poorly absorbed from the gastrointestinal tract and the mechanisms involved remain unclear. These compounds are usually recovered in very small amounts in human serum after oral ingestion (less than 1% of the dose) or in the IN fraction after in vitro digestion (about 5%). ° Unlike other polyphenols, anthocyanins constitute an exception because intact glycosides are recovered in the body (without deglycosylation prior to absorption). - This may be explained by either the instability of the free aglycone form or by a specific mechanism of absorption for anthocyanins. 

In the second pathway, the (methylated or not) anthocyanin glucoside may be absorbed intact and serve as a snbstrate for the UDP-glncose dehydrogenase enzyme (GDH EC 1.1.1.22) that converts the glncose form into the corresponding glucu-ronide form, possibly in both hnman hver and small intestine. 

Wu, X., Cao, G., and Prior, R.L., Absorption and metabolism of anthocyanins in elderly women after consumption of elderberry or blueberry, J. Nutr, 132,1865, 2002. Matsumoto, H. et al.. Orally administered delphinidin 3-rutinoside and cyanidin 3-rutinoside are directly absorbed in rats and humans and appear in the blood as the intact forms, J. Agric. Food Chem., 49, 1546, 2001. 

Four anthocyanin species exist in equilibrium under acidic conditions at 25°C/ according to the scheme in Figure 4.3.3. The equilibrium constant values determine the major species and therefore the color of the solution. If the deprotonation equilibrium constant, K, is higher than the hydration constant, Kj, the equilibrium is displaced toward the colored quinonoidal base (A), and if Kj, > the equilibrium shifts toward the hemiacetalic or pseudobase form (B) that is in equilibrium with the chalcone species (C), both colorless." - Therefore, the structure of an anthocyanin is strongly dependent on the solution pH, and as a consequence so is its color stability, which is highly related to the deprotonation and hydration equilibrium reaction constant values (K and Kj,). 

FIGURE 4.3.3 Interconversion pathways of various forms of anthocyanins in acidic aqueous medium. 

Attempts to stabilize anthocyanins by complex inclusion with a- and P-cyclo-dextrins failed on the contrary, a discoloration of anthocyanin solutions was observed.Thermodynamic and kinetic investigations demonstrated that inclusion and copigmentation had opposite effects. In the anthocyanins, the cw-chalcone colorless structure is the best species adapted to inclusion into the P-dextrin cavity, shifting the equilibrium toward colorless forms. "... 

Studies of the stability and stabilization of anthocyanins are still required, based on the extreme importance of those pigments for food colors. Modem HPLC-MS equipment also allows us to easily follow the copigmentation reactions in detail, calculate their kinetic and thermodynamic parameters, identify the products formed during the reactions, and thus shed new light on the stability and stabilization of these pigments. Since anthocyanins play important roles as natural colorants for... 

Francia-Aricha, E.M. et al.. New anthocyanin pigments formed after condensation with flavanols, J. Agric. Food Chem., 45, 2262, 1997. 

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