For a normal periodic crystal, the X‐ray diffraction pattern can be described by an orientation matrix and a set of three integers that indicate the reciprocal lattice points. Those integers determine the spacing along the reciprocal lattice directions. In aperiodic crystals, the diffraction pattern is modulated and the standard periodic main reflections are surrounded by satellite reflections. The successful indexing and refinement of the main unit cell and q vector using TWINSOLVE, developed by Svensson [(2003). Lund University, Sweden], are reported here for an incommensurately modulated, aperiodic crystal of a profilin:actin complex. The indexing showed that the modulation is along the b direction in the crystal, which corresponds to an `actin ribbon' formed by the crystal lattice. Interestingly, the transition to the aperiodic state was shown to be reversible and the diffraction pattern returned to the periodic state during data collection. It is likely that the protein underwent a conformational change that affected the neighbouring profilin:actin molecules in such a way as to produce the observed modulation in the diffraction pattern. Future work will aim to trap the incommensurately modulated crystal state, for example using cryocooling or chemical crosslinking, thus allowing complete X‐ray data to be collected.

The mosaic structure of a single protein crystal was analyzed by reflection profiling and topography using highly parallel and monochromatic synchrotron radiation. Fine-'-sliced diffraction images (0.002 stills) were collected using a conventional large-area CCD detector in order to calculate reflection profiles. Fine-'-sliced topographic data (0.002 ) stills were collected with a digital topography system for three reflections in a region where the Lorentz effect was minimized. At room temperature, several different mosaic domains were clearly visible within the crystal. Without altering the crystal orientation, the crystal was cryogenically frozen (cryocooled) and the experiment was repeated for the same three reflections. Topographs at cryogenic temperatures reveal a significantly increased mosaicity, while the original domain structure is maintained. A model for the observed changes during cryocooling is presented.research papers

A system which joins digital topography with fine '-sliced reflection profiling has been developed and applied to cryocrystallography. In this demonstration, fifteen fine '-sliced reflection profiles with corresponding topographic sequences are evaluated: twelve reflections from a crystal at cryogenic temperatures and three reflections from a room-temperature crystal. The digitally collected data show results comparable with film, albeit at a lower resolution, but are acquired at a substantially higher rate. Additionally, antiblooming circuitry in the CCD was tested and shown to provide useful data even when pixels were overloaded. research papers J. Appl. Cryst. (2005). 38, 512-519 Jeffrey J. Lovelace et al. Digital topography 513