Dynamic response of tetragonal lysozyme crystals to changes in relative humidity: implications for post-growth crystal treatments

The dynamic response of tetragonal lysozyme crystals to dehydration has bee n characterized in situ using a combination of X-ray topography, high-resol ution diffraction lineshape measurements and conventional crystallographic diffraction. For dehydration from 98% relative humidity (r.h.) to above 89% , mosaicity and diffraction resolution show little change and X-ray topogra phs remain featureless. Lattice constants decrease rapidly but the lattice- constant distribution within the crystal remains very narrow, indicating th at water concentration gradients remain very small. Near 88% r.h., the c-ax is lattice parameter decreases abruptly, the steady-state mosaicity and dif fraction resolution degrade sharply and topographs develop extensive contra st. This transformation exhibits metastability and hysteresis. At fixed r.h . < 88% it is irreversible, but the original order can be almost completely restored by rehydration. These results suggest that this transformation is a first-order structural transition involving an abrupt loss of crystal wa ter. The front between transformed and untransformed regions may propagate inward from the crystal surface and the resulting stresses along the front may degrade mosaicity. Differences in crystal size, shape and initial perfe ction may produce the observed variations in degradation timescale. Consequ ently, the success of more general post-growth treatments may often involve identifying procedures that either avoid lattice transitions, minimize dis order created during such transitions or maintain the lattice in an ordered metastable state.