Copigment complex

There are two basic problems with all of the previous spectral methods. The first is that pH is uncontrolled and since the copigmented form and free anthocyanin display quite different pH response (72), the estimate of the anthocyanin content and degree of ionization are unreliable. The second problem is that color of the anthocyanins in the copigment complex is included... 

The method used to measure the amount of the copigment complex in a red wine, and the other measures, is referred to as the copigmentation assay. This method measures the absorbance of a sample altered by dilution together with those resulting from additions of acetaldehyde or sulfur dioxide (7, 8). The procedure differs from previous methods in that all readings are made at a standardized pH (3.6) and at 12% ethanol and makes no assumptions about the extinction coefficients, ionization or the color shifts. It also subtracts the color due to copigmentation from wine color in order to make the anthocyanin estimate. 

The correlation of wine color with the anthocyanin color, the color due to copigmentation and the color of the polymeric pigments, individually, are straight-line relationships. More important is the fact that a higher correlation exists for the color contribution of the copigment complex, (R = 0.77), Figure 3, than for the color due to anthocyanins (R = 0.57) or polymeric pigments (R = 0.66), data not shown. 

The percentage of the total color at pH 3.6 due to the anthocyanins in the copigment complex ranges from 25 to 50% with an average of 40%. This... 

Jurd, L., Asen, S. Formation of metal and copigment complexes of cyanidin 3-d-glucoside. Phytochemistry 5, 1263 (1966)... 

Studies on the antioxidant properties of anthocyanins on human low-density lipoprotein (LDL) and lecithin liposome systems in vitro showed that the inhibition of oxidation increased dose-dependently with antioxidant concentration. The oxidation was catalyzed by copper in the LDL system and the effects of the anthocyanins were explained by several antioxidant mechanisms including hydrogen donation, metal chelation and protein binding [33]. Anthocyanins also prevented the oxidation of ascorbic acid (vitamin C), through chelate formation with the metal ions, and finally by the formation of an ascorbic (copigment)-metal-anthocyanin complex [49]. 

Copigmentation is driven by hydrophobic vertical stacking between the anthocyanin and the copigment to form tt-tt complexes from which water is excluded. The flavylium cation as well as the quinonoidal base are planar hydrophobic structures and can be involved in such complexes whereas the hemiketal form cannot. The association thus results in displacement of the anthocyanin hydration equilibrium from the colorless hemiketal to the red flavylium form that can be easily measured by spectrophotometry. 

Hydrophobic effects are thought to position the anthocyanin chromophore and the copigment to form a ti-ti complex [244], and by this way, the most efficient overlap would occur with planar flavonols when compared with hydroxyciimamoyl, galloyl esters, or with the nonplanar flavan-3-ols [245]. Usually, it is thought that the flavylium ion is the major colored species that contributes to the copigmentatirm phenomenon [244, 246]. However, some authors have suggested that the neutral quinoidal base is the main species involved [247, 248]. 

In addition to UV-visible absorption spectroscopy, copigmentation can also be followed by H NMR techniques, which provide further evidence of the formation of a 1 1 vertical stacking complex between the pigment and copigment molecules (Wigand et al, 1992). 

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. 

---