Crystallographic refinement of the structure of bovine pancreatic trypsin inhibitor at l.5 Å resolution

Abstract

The model of pancreatic trypsin inhibitor determined by Huber, Kukla, Rtihlmann, Epp & Formanek [Naturwissenschaften (1970). 57, 389-392] was fitted to an electron-density map of 1.9 A resolution calculated with isomorphous-replacement phases by application of the real-space refinement procedure of Diamond [Acta Cryst. (1971). A27, 436-452]. Further refinement was done at 1.5 A resolution by cyclic real-space refinement against electron-density maps computed with observed structure-factor amplitudes and phases calculated from the preceding model. Five difference Fourier maps helped to correct severe errors in the model, and to locate solvent molecules. After 15 cycles of constrained crystallographic refinement the R value for all 8079 independent reflexions was 0.225. The R value for 6893 reflexions with intensities above the 2o significance level determined from counting statistics within the range 7.0 A > d> 1-5 A was 0-197. Details of the refined model are presented in terms of hydrogen bonds and conformational angles.