Metal-complex Pigment Involved in the Blue Sepal Color Development of Hydrangea

The blue sepal color of Hydrangea macrophylla is due to a metal complex named "hydrangea-blue complex" composed of delphinidin 3-O-glucoside, 1, 5-O-caffeoylquinic acid, 2, and/or 5-O-p-coumaroylquinic acid, 3, as co-pigments, and Al3+ in aqueous solution at approximately pH 4.0. However, the ratio of each component in not stoichiometric, but is fluctuating within a certain range. The hydrangea-blue complex exists only in aqueous solution, exhibiting a stable blue color but attempts at crystallization were failure, therefore, the structure still remains obscure. To clarify the basis of the character of the hydrangea blue pigment and to obtain its structural information, we studied the mixing conditions to reconstruct the same blue color as observed in the sepals. In highly concentrated sodium acetate buffer (6 M, pH 4.0) we could measure 1H NMR of both the hydrangea-blue complex composed with 1 (5 mM), 2 (10 mM) and Al3+ (10 mM) and a simple 1-Al3+ complex. We also recorded the spectra of complexes composed with structural different anthocyanins and co-pigments. By comparing those signals, it was indicated that in the hydrangea-blue complex 1 might be under equilibrium between chelating and non-chelating structures having an interaction with 2.

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