Metal Complex Pigment Involved in the Blue Sepal Color Development of Hydrangea

Abstract

Anthocyanins exhibit various vivid colors from red through purple to blue and are potential sources of food colorants. However, their usage is restricted because of their instability, especially as a blue colorant. 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 copigments, and Al(3+) in aqueous solution at approximately pH 4.0. However, the ratio of each component ins not stoichiometric, but is fluctuates within a certain range. The hydrangea-blue complex exists only in aqueous solution, exhibiting a stable blue color, but attempts at crystallization have failed; therefore, the structure 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 (1)H NMR of both the hydrangea-blue complex composed of 1 (5 mM), 2 (10 mM), and Al(3+) (10 mM) and a simple 1-Al(3+) complex. We also recorded the spectra of complexes composed with structurally different anthocyanins and copigments. Comparison of those signals indicated that in the hydrangea-blue complex 1 might be under equilibrium between chelating and nonchelating structures having an interaction with 2.

Direct Observation of Hydrangea Blue-Complex Composed of 3-O-Glucosyldelphinidin, Al3+ and 5-O-Acylquinic Acid by ESI-Mass Spectrometry

“…As previously reported [13,14,17], we performed reproduction experiments by mixing Dp3G ( 1 ), co-pigment and Al 3+ in various buffer solutions at a concentration of 100 mM. For NMR measurements, we used an unusually concentrated buffer such as 6 M [17].…”

“…Recently, we measured the 1 H-NMR spectrum of the hydrangea blue-complex by mixing 1 , 2 and Al 3+ in a ratio of 1:2:1 in 6 M acetate buffer at pD 4.0 [17]. Analysis of the spectrum gave a partial structural information, indicating that in the hydrangea-blue complex, Dp3G ( 1 ) might exist in an equilibrium between chelating and non-chelating structures that have an interaction with 5CQ ( 2 ).…”

“…Analysis of the spectrum gave a partial structural information, indicating that in the hydrangea-blue complex, Dp3G ( 1 ) might exist in an equilibrium between chelating and non-chelating structures that have an interaction with 5CQ ( 2 ). Therefore, we proposed a schematic structure of the hydrangea blue-complex as a 1:1:1 complex of 1 , 2 and Al 3+ [17]. However, the composition was not determined unambiguously.…”

From vine to wine: photophysics of a pyranoflavylium analog of red wine pyranoanthocyanins

“…inter-and intramolecular complex formation between the anthocyanin and colorless electron-rich molecules (copigments) also present in the vacuoles or covalently attached to the sugar residues [4,5]. Anthocyanins with vicinal OH groups in the B-ring can chelate metal ions like Al(III), resulting in a change in the color from red-purple for the free anthocyanin to blue in the metal complex [6] (the chemical basis of the red/blue color change in Hydrangea flowers [7]). The blue pigments of some flowers are even more complex, the colored species being a self-organizing metal-ion-stabilized supramolecular complex of defined stoichiometry formed between anthocyanins and copigment molecules (e.g.…”

Chemical analysis of color change and transcriptional profile in flavonoids biosynthesis during flower development of Tibouchina pulchra (Cham.) Cong.

“…Ser., Hydrangeaceae, Cornales) are the best-known example of flavonoid-metal complexation influencing the corolla color. Under high aluminum content and acidity of the soil, the flowers are blue due to a nonstoichiometric complex with Al 3+ , delphinidin 3-glucosides, quinic acid derivatives, while upon low aluminum concentrations, the same metal-pigment-copigment complex became light pink, as the result of soil pH change (Yoshida et al 2003, Kondo et al 2005, Oyama et al 2015.…”