Section 12.1.5.7. Polynuclear reagents

The structure determination of large multicomponent systems such as the 50S ribosomal subunit (Yonath et al., 1986) or the nucleosome core particle (O'Halloran et al., 1987) requires the addition of reagents with a greater number of electrons, preferably in a compact polynuclear structure. Such reagents may be either cluster compounds or multimetal centres having metal–metal bonds.

Polynuclear reagents should preferably be covalently bound to one or a few specific sites, either first in solution or later in the crystals. Spacers of differing length can be inserted into the reagent to increase accessibility. Their low solubility in aqueous solutions can often be overcome by dissolving them in an apolar solvent (e.g. acetonitrile). Tetrakis(acetoxymercurio)methane (TAMM) and di-m-iodobis(ethylenediamine)diplatinum(II) nitrate (PIP) have better solubility in aqueous solutions than other polynuclear heavy-atom compounds.

Polynuclear heavy-atom reagents give an enhanced signal-to-noise ratio in low-resolution MIR studies, but this advantage is offset by the fall-off in scattering amplitude that arises from interference of diffracted waves at higher resolution. In the nucleosome core particle, the scattering reached 50% of its zero-angle value at 7.0 Å, while the relative drop for a single heavy atom was 10% (O'Halloran et al., 1987). Cluster and multimetal reagents that have been successfully employed in protein structure determinations have been reviewed by Thygesen et al. (1996).