1.7 Å structure of the stabilized REI_v mutant T39K. Application of local NCS restraints

The X-ray structure of the T39K mutant of the variable domain of a human immunoglobulin κ light chain has been determined at room temperature to 1.7 Å resolution with a conventional R factor of 0.182. T39K crystallizes in the triclinic space group P1 [a = 35.4 (1), b = 40.1 (1), c = 43.1 (1) Å, α = 66.9 (1), β = 85.4 (1), γ = 73.8 (1)°]. The unit-cell contains two monomers, related by a non-crystallographic twofold axis. The use of a novel type of local non-crystallographic symmetry restraints on related isotropic displacement parameters and 1–4 distances as incorporated in the refinement program SHELXL improves the model and quality of the maps, but local differences between both monomers in areas subject to different packing contacts can still be observed. 12 overall anisotropic scaling parameters were refined. These may have compensated for the difficulties in accurately scaling single rotation axis image-plate data from a triclinic crystal, because of the scarcity of common equivalent reflections. The final model has been used to perform a number of tests on anisotropic scaling, non-crystallographic symmetry, anisotropic refinement, determination of standard uncertainties and bulk solvent correction. It is remarkable that removal of the NCS restraints from the final model caused R_free to increase. These tests clarify the strategies for optimum use of SHELXL for refinement at medium as opposed to atomic resolution.