Ta6Br14 is a useful cluster compound for isomorphous replacement in protein crystallography.
The metal cluster Ta(6)Br(14) has been used to prepare heavy-metal derivatives of two large proteins, ribulose-1,5-bisphosphate carboxylase/oxygenase and transketolase. In both cases, this cluster compound produced a single-site derivative for which a difference Patterson map, calculated to 5.5 A resolution, could be solved straightforwardly. Ta(6)Br(14) provided enough phase information to unambiguously locate the heavy-atom positions in other multiple-site derivatives. In transketolase, the heavy-metal complex binds at the surface of the protein in a dominantly hydrophobic pocket. In ribulose bisphosphate carboxylase/oxygenase, it binds between two molecules in the crystal lattice. There are negatively charged glutamic and/or aspartic acid residues in the vicinity of the bound clusters. Ta(6)Br(14) is useful over a wide range of pH. For large proteins and/or large unit cells, this compound should be included in the initial screening for heavy-metal derivatives.
Department of Molecular Biology, Swedish University of Agricultural Sciences, Uppsala Biomedical Center, Sweden.
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Schneider
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Lindqvist