The suitability of multi-metal clusters for phasing in crystallography of large macromolecular assemblies.

The methods commonly used for phase determination in biological crystallography are single and multiple isomorphous replacement (SIR or MIR). Both require the preparation of derivatives, usually by introducing electron-dense compounds into the crystalline lattice at a limited number of distinct locations while keeping the crystal parameters isomorphous with those of the native molecule. As these replacement methods exploit the changes in the structure factor amplitudes resulting from the addition of the heavy atoms, the derivatization reagents are chosen according to their potential ability to induce measurable signals. For proteins of average size, useful heavy-atom derivatives consist of one or a few heavy-metal atoms that usually have an atomic number (Z)>70. However, for producing measurable signals from crystals of very large macromolecules that cannot be subdivided by non-crystallographic symmetry, numerous atoms (approximately 35 per 106 Da) are needed. Such multiple-site derivatives are extremely difficult to locate in the unit cell, and therefore, practically, this approach is not feasible.