Application of known triplet phases in the crystallographic study of bovine pancreatic trypsin inhibitor. II: study at 2.0 A resolution.

Direct methods strengthened by the application of about 130 triplet phases, assumed known with a mean error of about +/- 20 degrees, were used to re-determine the structure of BPTI with data at 2.0 A resolution. The triplet phases served to shift the mean direction and enhance the concentration parameter in the corresponding Cochran distributions. These phases were used in combination with the partial structure extracted from successive density maps to control the gradual expansion and refinement of Fourier coefficients. Single phases were developed iteratively from tangent-formula estimation following the path of the convergence map. The a priori triplet phase information was sufficient to initiate solution of the structure at 2.0 A.