The same anthocyanin pigment makes roses red but cornflowers blue, a phenomenon that has so far not been entirely explained. Here we describe the X-ray crystal structure of the cornflower pigment, which reveals that its blue colour arises from a complex of six molecules each of anthocyanin and flavone, with one ferric iron, one magnesium and two calcium ions. We believe that this tetrametal complex may represent a previously undiscovered type of supermolecular pigment.

A novel density modification method is applied for the first time to phase reconstruction of x-ray single crystal data of quasicrystals. The structure of icosahedral Zn-Mg-Ho quasicrystals has been determined by means of this ab initio structure determination within a framework of a 6D description. The location, size, and shape of the occupation domains are deduced. The suggested Ho sites in the 3D structure are consistent with the results of magnetic diffuse scattering [T. J. Sato et al., Phys. Rev. Lett. 81, 2364 (1998)].

The X-ray crystal structure of natural commelinin is investigated. The results demonstrate that commelinin is a tetranuclear (4 Mg 2þ ) metal complex, in which two Mg 2þ ions chelate to six anthocyanin molecules, while the other two Mg 2þ ions bind to six flavone molecules, stabilizing the commelinin complex, a new type of supramolecular complex.

A density-modification procedure for phase extension and refinement is described which replaces all density less than one-fifth of the height of a light-atom peak by zero. Its effectiveness is demonstrated by applications to a small and a medium-size protein structure. With high-resolution data, for the small protein, it is possible to extend and refine from 3 to 1 A with a mean phase error less than 30 ° . Successful phase extension from 4/~ is also possible. In general it is shown that phase extension to high resolution gives less error than extension to lower resolution. It has also been shown that for a small protein it is possible to obtain an ab initio solution of the structure by refining from a complete set of random phases for all reflexions. The basis of direct methodsMost direct methods consist of mathematical procedures carried out in reciprocal space which are designed to produce sets of phases satisfying particular constraints. The first powerful and generally applicable direct methods were those based on the

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