Charge-Transfer Complexation as a General Phenomenon in the Copigmentation of Anthocyanins

Abstract

Color intensification of anthocyanin solutions in the presence of natural polyphenols (copigmentation) is re-interpreted in terms of charge transfer from the copigment to the anthocyanin. Flavylium cations are shown to be excellent electron acceptors (E_red ≈ −0.3 V vs SCE). It is also demonstrated, for a large series of anthocyanin−copigment pairs, that the standard Gibbs free energy of complex formation decreases linearly with EA_Anthoc − IP_Cop, the difference between the electron affinity of the anthocyanin, EA_Anthoc, and the ionization potential of the copigment, IP_Cop. Based on this correlation, copigmentation strengths of potential candidates for copigments can be predicted.