MRSAD: using anomalous dispersion from S atoms collected at Cu Kalpha wavelength in molecular-replacement structure determination.

The use of single-wavelength anomalous dispersion (SAD) from S atoms collected in-house to overcome model bias in molecular-replacement (MR) structure determination is demonstrated. The test case considered is a P6(5)22 anti-ssDNA Fab crystal with a theoretical anomalous signal of 0.8% and a diffraction limit of 2.3 A, from which a 360 degrees, 39-fold redundant data set was collected. A nearly complete anomalous scatterer substructure could be quickly built from anomalous difference Fourier analysis based on phases from a full or partial MR solution. The resulting SAD phases were improved with density modification and used to calculate an unbiased electron-density map that could be used for model building. This map displayed clear and continuous density for almost the entire main chain, as well as good density for most side chains. The favorable results obtained from this realistic test case suggest that anomalous differences from S atoms should be routinely collected and used in MR structure determination.