Abstract
The blue sepal color of hydrangea is due to a metal complex anthocyanin composed of 3-O-glucosyldelphinidin (1) and an aluminum ion with the co-pigments 5-O-caffeoylquinic acid (2) and/or 5-O-p-coumaroylquinic acid (3). The three components, namely anthocyanin, Al3+ and 5-O-acylquinic acids, are essential for blue color development, but the complex is unstable and only exists in an aqueous solution. Furthermore, the complex did not give analyzable NMR spectra or crystals. Therefore, many trials to determine the detailed chemical structure of the hydrangea-blue complex have not been successful to date. Instead, via experiments mixing 1, Al3+ and 2 or 3 in a buffered solution at pH 4.0, we obtained the same blue solution derived from the sepals. However, the ratio was not stoichiometric but fluctuated. To determine the composition of the complex, we tried direct observation of the molecular ion of the complex using electrospray-ionization mass spectrometry. In a very low-concentration buffer solution (2.0 mM) at pH 4.0, we reproduced the hydrangea-blue color by mixing 1, 2 and Al3+ in ratios of 1:1:1, 1:2:1 and 1:3:1. All solution gave the same molecular ion peak at m/z = 843, indicating that the blue solution has a ratio of 1:1:1 for the complex. By using 3, the observed mass number was m/z = 827 and the ratio of 1, 3 and Al3+ was also 1:1:1. A mixture of 1, 3-O-caffeoylquinic acid (4) and Al3+ did not give any blue color but instead was purple, and the intensity of the molecular ion peak at m/z = 843 was very low. These results strongly indicate that the hydrangea blue-complex is composed of a ratio of 1:1:1 for 1, Al3+ and 2 or 3.
Highlights
Hydrangea (Hydrangea macrophylla) originated from Japan, and what we consider to be its flower is not a true flower but a sepal
By addition of 3 eq of co-pigment, 5CQ and 5pCQ gave almost similar color, but the absorbance was much higher with 5pCQ than 5CQ (Table 1, Figure S1). This was because the catechol structure of the caffeoyl residue in 5CQ chelates to Al3+ to form a 5CQ-Al3+
With positive-ion mode detection, the blue solution mixed with 5CQ gave a molecular ion peak at m/z = 843.16 (Figure 3A)
Summary
Hydrangea (Hydrangea macrophylla) originated from Japan, and what we consider to be its flower is not a true flower but a sepal. Its original sepal color is blue and very famous for its changes in hue with soil conditions, from red through purple to blue [1,2]. In the early 20th century, it was already known that hydrangea cultivated in acidic soil is blue [3,4]. In this condition, aluminum ion (Al3+ ) in soil becomes water-soluble and is absorbed from roots, followed by transport to sepal vacuoles to give a blue color [1,5,6]. These compounds can develop all of the reported sepal colors, Molecules 2018, 23, 1424; doi:10.3390/molecules23061424 www.mdpi.com/journal/molecules.
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