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Cornflower pigment pictured My love is like a ... blue cornflower? Maybe not, but the same anthocyanin pigment that makes roses red also makes cornflowers blue. It's just packaged differently in the cornflower. A team of Japanese researchers led by Kosaku Takeda of Tokyo Gakugei University has obtained the crystal structure of the blue pigment in cornflowers. They find that the pigment is a complex of six anthocyanin molecules, six flavone molecules, one ferric ion, one magnesium ion, and two calcium ions (Nature 2005, 436, 791). The ions line up along the pseudo-three-fold axis, with each Fe3+ and Mg2+ coordinated to three anthocyanin molecules and each Ca2+ coordinated to three flavone molecules. In the structure, stacking of the anthocyanin and flavone is evident. The C-C and C-O bond lengths indicate that the anthocyanin is in the 4´-keto-quinoidal form. The chelation of the Fe3+ and the Mg2+ with the 4´-keto-quinoidal base is important for the blue color, and the Ca2+ ions help stabilize the complex. The authors think that the tetranuclear metal complex may represent a new type of supramolecular pigment. |
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