The chemical mechanism for Al3+ complexing with delphinidin: a model for the bluing of hydrangea sepals

The blooms of many hydrangea cultivars can be red or blue, with the color depending on the soil pH. This dependence reflects the availability of Al(3+) to the plant under acidic conditions, as Al(3+) changes the color of the anthocyanin pigment in hydrangea sepals from red to blue. A chemical model, Al(3+) and delphinidin in acidic ethanol, was developed to understand the spectral characteristics and bluing of the hydrangea sepals. Delphinidin as its flavylium cation leads to red solutions in the model system. In the presence of Al(3+), the Al(3+) removes H(+) ions from delphinidin, transforming delphinidin's flavylium cation to its blue quinoidal base anion which complexes with the Al(3+). To further stabilize this complex, a second flavylium cation stacks on top of the complexed quinoidal base anion, creating a bathochromic shift of the cation's spectral signature and accentuating the blue color. This Al(3+)-delphinidin entity forms in adequate concentration for bluing only if there is a sufficient excess of Al(3+), the exact excess being a function of pH and concentration. The role of Al(3+) in bluing is not just to form a primary complex with delphinidin, but also to create a template for the stacking of delphinidin (or possibily co-pigments).