Crambin: a direct solution for a 400-atom structure.

The crystal structure of crambin, a 46-residue protein containing the equivalent of approximately 400 fully occupied non-H-atom positions, was originally solved at 1.5 A by exploiting the anomalous scattering of its six S atoms at a single wavelength far removed from the absorption edge of sulfur. The crambin structure has now been resolved without the use of any anomalous-dispersion measurements. The technique employed was an ab initio 'shake-and-bake' method, consisting of a phase-refinement procedure based on the minimal function alternated with Fourier refinement. This method has successfully yielded solutions for a smaller molecule (28 atoms) using 1.2 A data, and a crambin solution was obtained at 1.1 A.