The influence of phenolic acyl groups on the color of purple sweet potato anthocyanins and their metal complexes

Anthocyanins from purple sweet potato (PSP) are peonidin and cyanidin glycosides acylated by p-hydroxycinnamic and p-hydroxybenzoic acids. For six individual PSP pigments, the thermodynamic constants of proton transfer and water addition were determined, from which the speciation diagrams for the colored and colorless forms and the UV–visible spectra of individual colored forms could be constructed. The data confirm that acylation by phenolic acids protects the chromophore against water addition (a consequence of acyl – anthocyanidin π-stacking interactions) and that this protection depends on the type and number of acyl residues, diacylation being much more efficient than monoacylation, and p-hydroxycinnamoyl more efficient than p-hydroxybenzoyl. Most PSP anthocyanins can bind metal ions (Fe2+, Al3+) through their cyanidin chromophore and/or their caffeoyl residue(s). At pH 7, a cyanidin glycoside bearing a caffeoyl residue can bind a single metal ion by simultaneous involvement of its two binding units. With Fe2+ a strong bluing effect was observed. Although the caffeoyl residues efficiently slow down color loss, their redox activity actually accelerates the thermal degradation of anthocyanins. Consistently, two-electron autoxidation of anthocyanins bearing caffeoyl residues could be evidenced by UPLC-MS/DAD analysis. These new pigments possibly stem from intramolecular coupling between the chromophore and o-quinones derived from the caffeoyl residues.