Phase determination using halide ions.

A short soak of protein crystals in cryosolution containing bromides or iodides leads to incorporation of these ions into the ordered solvent shell around the protein surface. The halide ions display significant anomalous signal, bromides in the vicinity of the absorption edge at 0.92 A, and iodides at longer wavelengths, e.g., provided by the copper sources. Bromides can, therefore, be used through multiwavelength anomalous diffraction or single-wavelength anomalous diffraction (SAD) techniques and iodides through SAD or multiple isomorphous replacement (MIRAS) phasing. The halide cryosoaking approach involves very little preparative effort and offers a rapid and simple way of solving novel protein crystal structures.