Phasing at high resolution using Ta₆Br₁₂ cluster

The Ta₆Br cluster compound is known to be a powerful reagent for derivatization of crystals of large macromolecules at low resolution. The cluster is a regular octahedron of six Ta atoms with 12 bridging Br atoms at the edges of the octahedron. The cluster is compact, of approximately spherical shape, with a radius of about 6 Å. Both tantalum and bromine display a significant anomalous diffraction signal at their absorption edges at 1.25 and 0.92 Å, respectively. At resolutions lower than 5 Å the tantalum cluster behaves as a super-atom and provides very large isomorphous and anomalous signals, which significantly diminish at about 4 Å. However, beyond 3 Å the individual Ta atoms can be resolved and the phasing power of the cluster increases again. The Ta₆Br cluster has been used for phasing four different proteins at high resolution. Ta₆Br appeared to be a mild derivatization reagent and, despite partial incorporation, led to a successful solution of crystal structures by the single-wavelength anomalous diffraction (SAD) approach.