Topography and high resolution diffraction studies in tetragonal lysozyme

Two complementary approaches are used to enhance the usefulness of X-ray topographies obtained from protein crystals. First, the use of thin plate-like crystals in conjunction with a high intensity, collimated and small source size synchrotron beam produces a large beneficial effect on the level of detail and contrast of topographies for the quantification of local misalignment in the crystal lattice. Second, the recording of topography series along the rocking curve of a diffraction peak is proposed as a technique to combine the benefits of both rocking curves and topographies and produce very detailed data ("rocking maps") on the spatial distribution of lattice misalignments and mosaic spread ("local rocking curves"). The most important crystal features controlling the observed contrast are growth sectors and inter-sector boundaries, clearly observed in the topographies. Systems of parallel fringes are observed in many of the topographies. Two alternative explanations for these fringes are discussed: (a) as moire interference fringes or (b) as Pendellosung fringes in a wedge shaped crystal volume. In both cases, growth sectors play a central role in the physics of fringe generation. Many observations suggest the presence of a relatively large component of dynamical diffraction in these crystals; the consequences of this new scenario are discussed. (C) 1999 Elsevier Science B.V. All rights reserved.