how accurate are refinement targets and how much should protein structures be allowed to deviate from them?
The Protein Data Bank and Cambridge Structural Database were analyzed with the aim of verifying whether the restraints that are most commonly used for protein structure refinement are still appropriate 15 years after their introduction. From an analysis of selected main-chain parameters in well ordered fragments of ten highest resolution protein structures, it was concluded that some of the currently used geometrical target values should be adjusted somewhat (the C-N bond and the N-C(alpha)-C angle) or applied with less emphasis (peptide planarity). It was also found that the weighting of stereochemical information in medium-resolution refinements is often overemphasized at the cost of the experimental information in the diffraction data. A correctly set balance will be reflected in root-mean-square deviations from ideal bond lengths in the range 0.015-0.020 A for structures refined to R factors of 0.15-0.20. At ultrahigh resolution, however, the diffraction terms should be allowed to
