Red cabbage anthocyanins: The influence of D-glucose acylation by hydroxycinnamic acids on their structural transformations in acidic to mildly alkaline conditions and on the resulting color

Anthocyanins acylated by hydroxycinnamic acids (HCAs) are fascinating plant pigments that express a variety of red, purple and blue colors by combining multiple structural transformations and molecular interactions. Acylated anthocyanins are also a promising alternative to artificial food colorants. In this work, the mono- and diacylated anthocyanins of red cabbage have been extensively studied by NMR, UV-visible spectroscopy and circular dichroism. Our results show that HCA residues promote pi-stacking interactions between the phenolic nuclei, thereby efficiently protecting the cyanidin chromophore against water addition leading to colorless forms. For instance, the rate constant of water addition is ca. 0.3 s(-1) for the nonacylated pigment, in the range 0.1-0.2 s(-1) for the three monoacylated pigments and of the order of 0.01 s(-1) for the three diacylated pigments. By contrast, the rate constant of water elimination and the thermodynamic constants of proton transfer between the cationic, neutral and anionic colored forms are only weakly affected by acylation. Thus, through pi-stacking interactions, the diacylated anthocyanins maintain a higher percentage of cationic and neutral colored forms at equilibrium in mildly acidic conditions. In neutral - mildly alkaline conditions, the diacylated anthocyanins adopt persistent anionic forms (very slow water addition), expressing intense blue colors. NMR and CD data suggest that a combination of cyanidin - HCA (intramolecular copigmentation) and cyanidin - cyanidin (self association) interactions operates in the color-stabilizing mechanism, which is also translated in an improved resistance against the long-term color loss in mildly alkaline conditions, signalling the irreversible degradation of the chromophore.