ERROR-ESTIMATES IN THE DETERMINATION OF RIGID-BODY DISPLACEMENTS IN PROTEIN CRYSTALS

We report the estimation of random errors in the refinement of the rig id body displacements of the alpha-helices of the enzyme 6-phosphogluc onate dehydrogenase. Least-squares refinement of the TLS parameters of the helices has been carried out using X-ray reflection data of 2.1 a ngstrom resolution, resulting in an R-factor of 19.5%. Standard deviat ions were estimated from the normal matrix. The results show that the translational mean-square displacements of nearly all the helices are significant at this resolution. However the libration parameters are o nly significant when the helices have at least four turns. Screw-rotat ion tensor values cannot be determined at this resolution.