COMPARISON OF RADIATION-INDUCED DECAY AND STRUCTURE REFINEMENT FROM X-RAY DATA COLLECTED FROM LYSOZYME CRYSTALS AT LOW AND AMBIENT-TEMPERATURES

Two crystals of monoclinic hen egg-white lysozyme were irradiated in a monochromatic synchrotron X-ray beam (lambda = 1.488 angstrom), the f irst as a rapidly frozen crystal mounted at the end of a glass fiber a t low temperature (120 K) and the second mounted in a capillary tube a t ambient temperature (298 K). Comparison of oscillation photographs, extending in resolution to 1.85 angstrom and taken from both crystals at zero time and again after a period of exposure in the synchrotron b eam (60 min exposure at 120 K; 8 min at 298 K), reveals that radiation -induced decay is not observed at 120 K but is observed, particularly at high resolution, at 298 K. In a separate set of experiments, data s ets to 1.9 angstrom resolution at 100 and 298 K were collected from tw o monoclinic and two tetragonal hen egg-white lysozyme crystals using a rotating-anode source (lambda = 1.5418 angstrom). Before inclusion o f solvent molecules, the monoclinic and tetragonal structures at low t emperature, where data were collected from rapidly frozen crystals, re fined to R = 27.5 and 25.2%, respectively. The structures at ambient t emperature, however, where crystals were mounted in capillary tubes, r efined to significantly lower values of R = 20.9 and 20.6%. After incl usion of solvent, the R values at convergence were 20.3 and 17.6% for the monoclinic and tetragonal low-temperature structures and 17.9 and 16.2% for the room-temperature structures. Approximately twice the num ber of water molecules were included in the low-temperature structures at convergence (406 and 237) than could be assigned in the room-tempe rature structures (191 and 100). These results suggest that data sets from rapidly frozen crystals might generally be expected to yield high er initial R factors, compared to similar room-temperature structures, but that this difference should diminish appreciably as ordered solve nt is included in the model. Apart from the general reduction in atomi c temperature factors, the enhancement in resolution observed in diffr action patterns obtained from rapidly frozen crystals is probably due, to some significant degree, to the increase in the ordered-solvent co ntent of the low-temperature structures.