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 th e statistical phasing program SHARP. For the holmium-substituted calcium-bi nding protein psoriasin (22.7 kDa), a set of accurate phases has been obtai ned 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 m ap of a quality comparable to sigma(A)-weighted 2mF(o) - DFc maps derived f rom 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 th e structure of psoriasin can be determined from native data extending to 2. 0 Angstrom alone by exploiting the minute anomalous signal from a bound zin c ion.