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 singlewavelength 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.