Simultaneous refinement of the structure of BPTI against NMR data measured in solution and X-ray diffraction data measured in single crystals.

The structure of the bovine pancreatic trypsin inhibitor (BPTI) has been determined to high resolution by both NMR spectroscopy in solution and X-ray diffraction in crystals. The root-mean-square difference calculated between the two structures for the polypeptide backbone is 0.9 A. Several amino acid side-chains, of which all but one are charged or polar, have different conformations. We find that by refining one structure simultaneously against both the NMR and crystallographic data sets, it can accommodate both. Different starting configurations were used, including the X-ray structure 5pti, an NMR conformer, and the X-ray structure in the full unit cell with extra solvent placed in the bulk solvent region. The X-ray structures quickly converged to accommodate the NMR data in addition to the crystallographic data. Starting from an NMR conformer, however, the convergence to accommodate the more abundant X-ray data in addition to the NMR data is much slower.