Triiodide derivatization in protein crystallography

Methods for producing protein derivatives using cryosoak techniques with triiodide solutions are described. The methods have been tested using six different proteins. SAD/SIRAS phasing was attempted for each protein using data measured with conventional Cu Kalpha X-ray equipment and synchrotron radiation. Refinement of all six derivative structures showed that iodine is able to bind as I(-) (as observed with standard halide soaks) and also as the polyiodide anions I(3)(-) and I(5)(-). The various species are able to bind through hydrogen-bond interactions and to more hydrophobic regions of the protein at surface pockets and in intermolecular and intramolecular cavities. On the whole, the derivative agent behaves promiscuously in terms of its binding to proteins and is capable of generating sufficient phasing power from in-house Cu Kalpha data to permit structure solution by SAD. The results of the phasing experiments and structure refinements are presented.