TA6BR12(2-RAY CRYSTALLOGRAPHY(), A TOOL FOR PHASE DETERMINATION OF LARGE BIOLOGICAL ASSEMBLIES BY X)

The title compound Ta6Br122+ is of interest for the analysis of biolog ical structures as a heavy-metal derivative with great potential for t he structure determination of large protein systems. Ln macromolecular crystallography the phases of the measured structure factor amplitude s have to be determined. The most widely used method for novel structu res is isomorphous replacement by introducing electron-rich compounds into the protein crystals. These compounds produce measurable changes of the diffraction intensities, which allow phase determination. We sy nthetized the Ta6Br122+ cluster in high yields, crystallized it, and d etermined its crystal structure by X-ray diffraction analysis at atomi c resolution. The cluster is a regular octahedron consisting of six me tal atoms with 12 bridging bromine atoms along the 12 edges of the oct ahedron. The cluster is compact, of approximately spherical shape with about 4.3 Angstrom radius and highly symmetrical. One Ta6Br122+ ion a dds 856 electrons to a protein, a considerable contribution to the sca ttering power even of large proteins or multimeric systems. At low res olution all atoms of the cluster scatter in phase and act as a super h eavy-atom, which is easy to locate in the difference Patterson map. We investigated its binding sites in the biologically significant high-r esolution structures of an antibody V-L domain, dimethyl sulfoxide red uctase, GTP-cyclohydrolase I, and the proteasome. With the randomly or iented cluster, treated as a single site scatterer, phases could be us ed only up to 6 Angstrom resolution. In contrast, when the cluster is correctly oriented, phases calculated from its 18 atom sites can be us ed to high resolution. We present the atomic structure of the Ta6Br122 +, describe a method to determine its localization and orientation in the unit cell of protein crystals of two different proteins, and analy se its phasing power. We show that phases can be calculated to high re solution. The phase error is lower by more than 30 degrees compared to the single site approximation, using a resolution of 2.2 Angstrom Fur thermore, Ta6Br122+ has two different strong anomalous scatterers tant alum and bromine to be used for phase determination. (C) 1997 Academic Press Limited.