A DESCRIPTION OF IMPERFECTIONS IN PROTEIN CRYSTALS

An analysis is given of the contribution of various crystal imperfecti ons to the rocking widths of reflections and the divergence of the dif fracted beams. The crystal imperfections are the angular spread of the mosaic blocks in the crystal. the size of the mosaic blocks and the v ariation in cell dimensions between blocks. The analysis has implicati ons for improving crystal perfection, defining data-collection require ments and for dataprocessing procedures. Measurements on crystals of t etragonal lysozyme at room temperature and 100 K were made in order to illustrate how parameters describing the crystal imperfections can be obtained. At 100 It, the dominant imperfection appeared to be a Varia tion in unit-cell dimensions in the crystal.