Applications of single-wavelength anomalous dispersion at high and atomic resolution

Two examples of the application of single-wavelength anomalous dispersion (SAD) in macromolecular structure determination are described, both using the statistical phasing program SHARP. For the holmium-substituted calcium-binding protein psoriasin (22.7 kDa), a set of accurate phases has been obtained to a resolution of 1.05 Angstrom without recourse to an atomic model of the molecule. The accuracy of the phases resulted in an electron-density map of a quality comparable to sigma(A)-weighted 2mF(o) - DFc maps derived from the final model refined with SHELX97. Comparison of the refined and SAD electron-density maps showed significant discrepancies resulting from the iterative refinement in reciprocal space. Additionally, it is shown that the structure of psoriasin can be determined from native data extending to 2.0 Angstrom alone by exploiting the minute anomalous signal from a bound zinc ion.