Description of overall anisotropy in diffraction from macromolecular crystals

Simple formulations are described for evaluating overall anisotropy of the kind sometimes found in diffraction from crystals of macromolecules. The models correspond to whole-body anisotropic vibration of unit cells or of asymmetric macromolecular units that internally also undergo local isotropic atomic motions. These procedures have been implemented in programs that (1) use the structure-factor components from individual molecules to evaluate the anisotropy, (2) use existing F_c data to determine anisotropic parameters for the unit cell, and (3) use expected intensity values from unit-cell contents for unknown structures. The methods have been applied in refinement of the structures of myohemerythrin and other proteins and this led to improved R values and more readily interpreted difference maps.