Can diffraction anisotropy impede crystallographic refinement?
Moderate cases of diffraction anisotropy are adequately handled by automatically applied anisotropic scaling algorithms such as those applied in REFMAC or CNS. But, when the directional dependence in resolution becomes strong or severe, refinement can stall at high R-factors and electron density maps can appear featureless, thus inhibiting model building efforts. In the case cited above (Strong et al., 2006), the difference between the best and worst diffracting directions of the crystal were 2.3 and 3.3 Angstroms, respectively. R-factors were stalled at Rwork=38.5%, Rfree=43.4%. One of the frames from the data set is pictured on the right. How does diffraction anisotropy stall improvement of the R-factor? There are two means by which anisotropy impacts the R-factor. The first has to do with the inclusion of numerous poorly measured reflections in anisotropic data sets (i.e. F/sigma less than 3.0). Because they make up a considerable fraction of the data set, the R-factor in anisotropi
