What is diffraction anisotropy?
Diffraction anisotropy is evidenced as a directional dependence in diffraction quality. For example, in the diffraction pattern on the right, the crystal diffracts to higher resolution in the horizontal direction (green pointer) than in the vertical direction (blue pointer). Diffraction anisotropy is commonly observed in protein crystallography, ranging from moderate to severe. It is attributed to whole-body anisotropic vibration of unit cells, for example crystal packing interactions being more uniform in one direction than another. Frequently, the directional dependence coincides with reciprocal cell directions a*, b*, and c*. 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 map
