X-Ray Topographic Studies of the Effects of Time-Varying Growth Conditions on Protein Crystal Perfection. R.E.Thorne, K.Finkelstein, and I.Dobrianov, Department of Physics, Cornell University High-Energy Synchrotron Source (CHESS)

The lattice constants of protein crystals typically show much larger variations with growth conditions than those of small molecule crystals, reflecting the sensitivity of protein conformation and intermolecular interactions to solution properties. In vapor diffusion and other widely-used growth methods, solution properties can vary substantially during growth of an individual protein crystal. Consequently, the equilibrium crystal lattice constant should vary as a crystal grows. Relaxation of early-growth regions towards the conformation/lattice constant appropriate to late-growth conditions could lead to formation of microdomains, as in martensitic transformations. Overgrowth of layers with later-growth lattice constants onto earlier-growth layers that have not fully relaxed could produce large lattice strains. For many proteins, these effects could be an important source of the molecular disorder that limits the diffraction resolution. Monochromatic X-ray topographs acquired at CHESS have been used to characterize the perfection of lysozyme crystals grown under both uniform and time-varying conditions. Topographs of crystals grown under uniform conditions show relatively little contrast. Crystals grown under time-varying conditions (e.g., time-varying salt concentrations) can show extensive contrast indicating the presence of lattice strains and defects. Crystals for which growth conditions are changed abruptly show a strong difference in diffracted intensity between the pre- and post-change growth regions, indicating a difference in crystal lattice constant and/or mosaic. Measured mosaic widths for these crystals are much larger than those of crystals grown under uniform conditions. These results suggest that variations in equilibrium lattice constant may cause significant degradation of protein crystal quality when growth conditions vary with time.