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Notes on phasing with halides

Bromine and iodine display significant anomalous dispersion effects. Bromides can be used through multiwavelength anomalous diffraction or single-wavelength anomalous diffraction (SAD) techniques and iodides through SAD or multiple isomorphous replacement (MIRAS) phasing.

The K absorption edge of bromine (at 0.92 Å or 13,474 eV) is easily accessible for a MAD experiment. 

The absorption edges of iodine are beyond the easily accessible wavelength range (K at 0.37 Å and L1 at 2.39 Å). However, at 1.54 Å X-radiation iodine has significant anomalous signal (f''= 6.8 electron units). Although it is not susceptible to MAD, iodine provides a significant anomalous effect which can be utilized in the SAD or SIRAS.

Dauter et al Novel approach to phasing proteins: derivatization by short cryo-soaking with halides, Acta Crystallogr D (2000) vol. 56 (Pt 2) pp. 232-7
Dauter and Dauter Phase determination using halide ions, Methods Mol Biol (2007) vol. 364 pp. 149