Phasing methods for protein crystallography

The phase problem has been formulated as one of constrained global minimization and it leads to the minimal principle, which is the theoretical basis of 'Shake-and-Bake'--an algorithm for the automatic solution of the phase problem, ab initio. A related approach (termed 'Half-Baked') that, as in Shake-and-Bake, alternates phase refinement in reciprocal space with density modification in real space, has recently been formulated. The traditional techniques of direct methods have been integrated with isomorphous replacement and anomalous scattering and the first applications have been made. The accurate measurement of a limited number of structure invariants by means of multiple beam X-ray diffraction has become a reality. A feasibility study shows that, in combination with the direct method, the ability to measure the values of the structure invariants experimentally will strengthen existing techniques.