FLOWER colours, from red through purple to blue, are mostly from anthocyanins, a type of flavonoid1–5. Although there are many colours, only a few anthocyanidins, chromophores of the pigments, have been found. The colour of the pigments is stable in plants for a few days to one month but the extracted anthocyanins are, nevertheless, unstable and quickly lose colour by hydration in a neutral aqueous solution1–5. In 1915 Willstätter proposed that flower-colours vary because anthocyanins change their colour with the pH6. Shibata and Shibata questioned the theory because most plant cell sap was weakly acidic or neutral. Their alternative, based on metal-complex theory7 was refuted by Everest8. In 1958 Hayashi isolated in crystal form a blue pigment9, commelinin, a metal-complex anthocyanin (named metalloanthocyanin)10,11, from the blue petals of Commelina communis. But the existence of a blue-coloured magnesium complex was denied by Bayer et al.12. We obtained the same blue pigment as intact commelinin by reconstruction from its components13. We also prepared Cd-commelinin in which the complexation metal Mg2+ was replaced with Cd2+. We report here the X-ray crystal structure of a real anthocyanin and a sugar-containing flavonoid, using Cd–commelinin. The blue flower-colour development and the stability of the colour can be explained by metal complexation of antho-cyanin and intermolecular hydrophobic association.
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