Structural basis of blue-colour development in flower petals from Commelina communis

Abstract

FLOWER colours, from red through purple to blue, are mostly from anthocyanins, a type of flavonoid^1–5. 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 solution^1–5. 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.