50 STRUCTURE OF COMMELININ FROM BLUE PETALS OF COMMELINA COMMUNIS
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Beautiful flower color are mostly composed of anthocyanins. In 1919 Shibata and collaborates proposed the metal-complex theory for blue color development of flower petals. In 1958 Hayashi et al isolated a metallo-anthocyanin, commelinin, from blue petals of Commelina communis (tsuyukusa in Japanese). We determined that commelinin is composed with six molecules each of an anthocyanin, malonylawobanin (M) and a flavone, flavocommelin (F), and two atoms of magnesium, and the gross structure was [M_6F_6Mg_2]^<6-> by electrophoresis, metal analysis by ICP, and molecular weight (10,000) obtained by ultracentrifugation. In commelinin counterclockwise self-association of MM and FF was suggested by negative Cotton effect on CD. By ^1H NMR M are observed to be copigmented with F by cross arrangement. The metal chelated position was determined to be 3' and 4' o-dihydroxy group of B-ring of M by complexation experiment of commelinin-like complex using various anthocyanins. On basis of above-mentioned results we succeeded in X-ray crystallographic analysis of Cd-commelinin (trigonal, P321, a=b=31.256A, c=33.676A, R factor 0.132 at 1.0A resolution), which has the same space group of natural commelinin, and the complete structure of commelinin was determined (Fig. 8). Two cadmium atoms were located on three fold axis at 5.1A distance each other and the two M molecules, which chelates another metals each other, are self-associated. FF are inserted between MMs. Totally all the components are arranged as cyclo-MMFFMMFFMMFF-around the metals. These associations are derived from hydrophobic interaction between aromatic rings and the hydrophilic interaction of glucoses, which cover the outer part of the molecule. Commelinin is a stoichiometorical supra molecule.
