Essential structure of co-pigment for blue sepal-color development of hydrangea

Blue sepal-color of Hydrangea macrophylla might be due to a supramolecular metal-complex pigment consisting of delphinidin 3-glucoside (1), co-pigments (5-O-caffeoylquinic acid (2), and/or 5-O-p-coumaroylquinic acid (3)) and Al3+ in an aqueous solution around pH 4.0. To clarify the mechanism of blue sepal-color development of hydrangea, we tried to reproduce the blue color in vitro by mixing 1 with designed synthetic co-pigments in the presence of Al3+ at pH 4.0. We at first succeeded in clarifying the essential functional structure in the co-pigment that could form the stable blue solution. Here, we present the structure of the blue pigment caused by an Al-complex coordinating of 1 at ortho-dihydroxyl groups of the B-ring, 1-hydroxy, 1-carboxylic acid, and the carbonyl residue in the ester at 5-position of 2 and/or 3. The hydrophobic interaction between the aromatic acyl residue at 5-position and the nucleus of 1 may also contribute to stabilize the complex.