X-ray diffraction studies of protein crystal disorder

Protein crystals contain many kinds of disorder, but only a small fraction of these are likely to be important in limiting the diffraction properties of interest to crystallographers. X-ray topography, high-angular-resolution reciprocal space measurements, and standard crystallographic data collecti on have been used to probe three factors that may produce diffraction-limit ing disorder: (1) solution variations during crystal growth, (2) macromolec ular impurities, and (3) post-growth crystal treatments. Variations in solu tion conditions that occur in widely used growth methods may lead to variat ions in equilibrium protein conformation and crystal packing as a crystal g rows, and these may introduce appreciable disorder for sensitive proteins. Tetragonal lysozyme crystals subjected to abrupt changes in temperature, pH , or salt concentration during growth show increased disorder, consistent w ith this mechanism. Macromolecular impurities can have profound effects on protein crystal quality. A combination of diffraction measurements provides insight into the mechanisms by which particular impurities create disorder , and this insight leads to a simple approach for reducing this disorder. S ubstantial degradation of diffraction properties due to conformation and la ttice constant changes can occur during post-growth crystal treatments such as heavy-atom compound and drug binding. Measurements of the time evolutio n of crystal disorder during controlled crystal dehydration - a simple mode l for such treatments - suggest that structural metastability conferred by the constraints of the crystal lattice plays an important role in determini ng the extent to which the diffraction properties degrade. (C) 1999 Elsevie r Science B.V. All rights reserved.