Self-association, anthocyanins

The degradation kinetics of malvidin 3-glucoside in ethanolic solutions under conditions simulating wine accelerated with the increase of ethanol concentration, probably because the extent of anthocyanin self-association decreased with elevated ethanol concentration. ... 

Stability of anthocyanins can be attained by self-association, that is, when two or more anthocyanin molecules are associated. This effect was verihed by increasing the concentration of the cyanidin 3,5-diglucoside solution from 10 M to 10 M with a consequent bathochromic shift in maximum wavelength absorption in the visible region. "... 

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. ... 

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), covalently bound (intramolecular copigmentation) or not (intermolecular copigmentation) to the anthocyanin pigment. 

Miniati, E., Damiani, P, Mazza, G. (1992). Copigmentation and self-association of anthocyanins in food model systems. It. J. Food Sci., 4, 109-116. 

Gonzalez-Manzano, S., Santos-Buelga, C., Duenas, M., Rivas-Gonzalo, J. C., Escribano-Bailon, M. T. (2007b). Colour implications of self-association processes of wine anthocyanins.iiM Food Res. Tech, (in press). DOI 10.1007/s00217-007-0560-9. 

The increase in anthocyanins concentration results in an increase in absorbance at Xmax, greater than expected according to the Beer-Lambert law. It is probably connected with anthocyanins self-association. 

The anthocyanin-ethyl-flavanol adducts thus formed are purple pigments ( nax 5 0 nm), and much more resistant towards acid-catalyzed cleavage than the equivalent colorless ethyl-linked flavanol dimers (29). This presumably results from the displacement of the hydration equilibrium toward the flavylium cation form, due to self association, since similar synthetic pigments have been shown to be greatly stabilized by sandwich-type stacking (28). 

Copigmentation is usually classified in (a) intermolecular anthocyanin-anthocyanin self-association, (b) intermolecular anthocyanin-copigment (i.e., flavonoids, phenolic acids) association, (c) intermolecular anthocyanin-copigment-metal ion association or (d) intramolecular self-association of acylated anthocyanins. 

At the plants pH values (3-7), anthocyanins are mostly present as colorless structures. But a stabilizing mechanism takes place. Such effects are provided by complexation of the color form with other species present in the solution, which can be either an identical anthocyanin molecule (self-association), one of its aromatic acyl groups substituents (intramolecular copigmentation) or another molecule (intermolecular copigmentation). 

The color of anthocyanins is influenced not only by structural features (hydroxylation, methoxylation, glyco-sylation, acylation), but also by the pH of the solution in which they are present, copigmentation, metal complexa-tion and self-association. 

Self-association is the binding of anthocyanin molecules to one another. It has been observed that the complexes absorb more light than the sum of the single molecules. This explains why a 100-fold increase in the concentration of cyanidin 3,5-glucoside results in a 300-fold rise in absorbance. 

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