A magic triangle for experimental phasing of macromolecules.

Obtaining phase information for the solution of macromolecular structures is still one of the bottlenecks in X-ray crystallography. 5-Amino-2,4,6-triiodoisophthalic acid (I3C), in which three covalently bound iodines form an equilateral triangle, was incorporated into proteins in order to obtain phases by single-wavelength anomalous dispersion (SAD). An improved binding capability compared with simple heavy-metal ions, ready availability, improved recognition of potential heavy-atom sites and low toxicity make I3C particularly suitable for experimental phasing.