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A series of experiments performed at Cu Kα 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/I2 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.

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