9B Flavanols, Flavonols and Dihydroflavonols 487
- hydrogen bonding, which is a non covalent bond arising mainly from per-
manent dipole – permanent dipole interactions (a particular type of Keesom
interactions).- london interactions, also called dispersion forces, occurring between two induced
dipoles. These interactions are further strengthened by the partial desolvation
of the hydrophobic surfaces coming into contact and release of some
water molecules from the solvation shell into the bulk phase (hydrophobic
effect).These forces are rather weak when compared to covalent binding, but their range
of action is larger.
In addition to these non-covalent and reversible bonds, covalent binding through
nucleophilic addition or radical coupling can occur, in particular as a result of
oxidation reactions. Copigmentation has thus been proposed to be the first step
leading to formation of anthocyanin-flavanol adducts in red wine (Brouillard and
Dangles 1994). Besides, quinones arising from oxidation of phenolic compounds
can suffer nucleophilic addition from theSH or NH2 groups of proteins (Pierpoint
1969).
9B.4.2 Flavonoid Interactions
9B.4.2.1 Copigmentation
Among flavonoid interaction processes, copigmentation has been extensively stud-
ied; for a review see Brouillard and Dangles (1993).
Actors
It involves, on one hand an anthocyanin under its flavylium or quinonoidal base
form, and on the other hand another planar hydrophobic structure that can be another
anthocyanin unit (self-association), or another colorless species (copigment), cova-
lently bound (intramolecular copigmentation) or not (intermolecular copigmenta-
tion) to the anthocyanin pigment.
Interaction Mechanisms
Copigmentation is driven by hydrophobic vertical stacking between the anthocyanin
and the copigment to form - 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 redflavylium form that can be easily measured
by spectrophotometry.