title: Triiodide derivatization and combinatorial counter-ion replacement: two methods for enhancing phasing signal using laboratory Cu Kalpha X-ray equipment.
abstract: A series of experiments performed at Cu Kalpha wavelength on in-house X-ray equipment are presented which investigate two possibilities for enhancing the experimental phasing signal by means of (i) triiodide/iodide soaks using KI/I(2) and (ii) combinations of counter-ions introduced using the short cryosoak method. Triiodide-derivative crystal structures for five test proteins have been refined and reveal that iodine can bind as polyiodide and single iodide ions through hydrophobic and hydrogen-bonding interactions both at the molecular surface and in intramolecular and intermolecular cavities. In three cases, the structures could be automatically determined with autoSHARP using in-house SAD and SIRAS data. The investigation of combinatorial counter-ion replacement using multiple salts with Na(+) and Cs(+) as cations and I(-) and Cl(-) as anions reveals that, for the case of hen egg-white lysozyme, significant improvement in phasing signal is obtained by the combined use of salts compared with SIRAS methods using native and single short-soak derivative data sets.
authors.affiliation: Global Phasing Limited, Cambridge, England.
authors.forename: Gwyndaf
authors.initials: G
authors.lastname: Evans
authors.forename: Gérard
authors.initials: G
authors.lastname: Bricogne
mesh_terms: D000818:Animals; D001052:Apoferritins / Q000737:chemistry; D003460:Crystallization; D018360:Crystallography, X-Ray / Q000379:methods*; D007128:Immunoglobulin Fragments / Q000737:chemistry; D008956:Models, Chemical; D008958:Models, Molecular; D010196:Pancreatic Elastase / Q000737:chemistry
chemical_list: D007128:Immunoglobulin Fragments; D001052:Apoferritins; D010196:Pancreatic Elastase