Optimizing Shake-and-Bake for proteins.
Shake-and-Bake is a direct-methods procedure which has provided ab initio solutions for protein structures containing as many as 1000 independent non-H atoms. This algorithm extends the range of conventional direct methods by repetitively, unconditionally and automatically alternating reciprocal-space phase refinement with filtering in real space to impose constraints. The application of SnB to protein-sized molecules is significantly affected by the choice made for certain critical parameters, including the number of peaks used for density modification, the choice of phase-refinement method and the number of refinement cycles. The effects of parameter variation have been studied for six protein structures, all of which are solvable by Shake-and-Bake using data at 1.1 A or higher resolution. Solvability in the resolution range 1.2-1.4 A appears to be enhanced by the presence of heavier atoms (S, Cl). Furthermore, it appears that in this range the ratio of refinement cycles and triplet phase invariants to atoms in the structure must be increased. Large structures lacking atoms of any element heavier than oxygen also require non-traditional parameter values.