**4.4. Copigmentation**

Copigmentation is almost always a variation toward blueness. This phenomenon induced by the presence of substances [153, 154] that are themselves colorless or only slightly colored in wine has received considerable attention [155, 156]. The basic role of copigments is to protect [125] the colored flavylium cation from the nucleophilic attack of the water molecule. The copigmentation complexes are easily disrupted by dilution returning to the pH-dependent equilibria among the structural forms of anthocyanins. It has been suggested that in acylated anthocyanins, the acyl groups interact with the basic anthocyanin structure, thus avoiding the formation of the hydrated species. This is the basis of characterizing [157] the color due to copigmentation. Copigmentation of flavonoids other than anthocyanins is also possible [158], but it is either a rare or an understudied phenomenon.
