Comparison of radiation-induced decay and structure refinement from X-ray data collected from lysozyme crystals at low and ambient temperatures

Abstract

Two crystals of monoclinic hen egg-white lysozyme were irradiated in a monochromatic synchrotron X-ray beam (λ = 1.488 A), the first as a rapidly frozen crystal mounted at the end of a glass fiber at low temperature (120 K) and the second mounted in a capillary tube at ambient temperature (298 K). Comparison of oscillation photographs, extending in resolution to 1.85 A, and taken from both crystals at zero time and again after a period of exposure in the synchrotron beam (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 sets to 1.9 A resolution at 100 and 298 K were collected from two monoclinic and two tetragonal hen egg-white lysozyme crystals using a rotating-anode source (λ = 1.5418 A). Before inclusion of solvent molecules, the monoclinic and tetragonal structures at low temperature, where data were collected from rapidly frozen crystals, refined to R = 27.5 and 25.2%, respectively. The structures at ambient temperature, however, where crystals were mounted in capillary tubes, refined to significantly lower values of R = 20.9 and 20.6%. After inclusion 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 number of water molecules were included in the low-temperature structures at convergence (406 and 237) than could be assigned in the room-temperature structures (191 and 100). These results suggest that data sets from rapidly frozen crystals might generally be expected to yield higher initial R factors, compared to similar room-temperature structures, but that this difference should diminish appreciably as ordered solvent is included in the model. Apart from the general reduction in atomic temperature factors, the enhancement in resolution observed in diffraction patterns obtained from rapidly frozen crystals is probably due, to some significant degree, to the increase in the ordered-solvent content of the low-temperature structures.