REFINEMENT OF TRICLINIC HEN EGG-WHITE LYSOZYME AT ATOMIC-RESOLUTION

X-ray diffraction data have been collected at both low (120 K) and roo m temperature from triclinic crystals of hen egg-white lysozyme to 0.9 25 and 0.950 Angstrom resolution, respectively, using synchrotron radi ation. Data from one crystal were sufficient for the low-temperature s tudy, whereas three crystals were required at room temperature. Refine ment was carried out using the programs PROLSQ, ARP and SHELXL to give final conventional R factors of 8.98 and 10.48% for data with F > 4 s igma(F) for the low- and room-temperature structures, respectively. Th e estimated r.m.s. coordinate error is 0.032 Angstrom for protein atom s, 0.050 Angstrom for all atoms in the low-temperature study, and 0.03 8 Angstrom for protein atoms and 0.049 Angstrom for all atoms in the r oom-temperature case, as estimated from inversion of the blocked least -squares matrix. The low-temperature study revealed that the side chai ns of 24 amino acids had multiple conformations. A total of 250 waters , six nitrate ions and three acetate ions, two of which were modelled with alternate orientations were located in the electron-density maps. Three sections of the main chain were modelled in alternate conformat ions. The room-temperature study produced a model with multiple confor mations for eight side chains and a total of 139 water molecules, six nitrate but no acetate ions. The occupancies of the water molecules we re refined in both structures and this step was shown to be meaningful when assessed by use of the free R factor. A detailed description and comparison of the structures is made with reference to the previously reported structure refined at 2.0 Angstrom resolution.