Planar Anthocyanin Conformer Leads To Color Change

A class of anthocyanins—the chemicals behind the coloring of many flowers and fruit—exhibits an unusual color change from red to blue when a solution of the chemicals is frozen. Now, a group of Portuguese researchers believe they have found the mechanism behind the transformation. Led by André Melo of the University of Porto, the group used computational methods to investigate vinylpyranoanthocyanin pigments, or portisins, that incorporate a pyranoanthocyanin moiety linked to a phenolic group by a vinyl bridge. Melo and colleagues found that freezing an aqueous solution of the pigments stabilizes a planar conformer of the chemical structure, probably due to interactions between hydroxyl groups and water (J. Ag. Food Chem., DOI: 10.1021/jf904246g). The planar conformation improves electron delocalization across the molecule, leading to a shift in the absorption spectrum.