Methods to refine macromolecular structures in cases of severe diffraction anisotropy.

Diffraction anisotropy is characterized by variation in diffraction quality with reciprocal lattice direction. In the example presented here, diffraction extended to 2.1 Å resolution along a* and c* directions but only to 3.0 Å along the b* direction. Severe anisotropy such as this is often associated with lack of detail in electron density maps, stalled model improvement, and poor refinement statistics. Published methods for overcoming these difficulties have been combined and implemented in the diffraction anisotropy server. Specifically, the server offers information to diagnose the degree of anisotropy, and then applies ellipsoidal resolution boundaries, anisotropic scaling, and B-factor sharpening to the data set to compensate for the deleterious effects of diffraction anisotropy. Here, I offer advice on implementing these methods to facilitate refinement of macromolecular structures in cases of severely anisotropic data.