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The major macromolecular crystallographic refinement packages restrain models to ideal geometry targets defined as single values that are independent of molecular conformation. However, ultrahigh-resolution X-ray models of proteins are not consistent with this concept of ideality and have been used to develop a library of ideal main-chain bond lengths and angles that are parameterized by the φ/ψ angle of the residue [Berkholz et al. (2009), Structure, 17, 1316–1325]. Here, it is first shown that the new conformation-dependent library does not suffer from poor agreement with ultrahigh-resolution structures, whereas current libraries have this problem. Using the TNT refinement package, it is then shown that protein structure refinement using this conformation-dependent library results in models that have much better agreement with library values of bond angles with little change in the R values. These tests support the value of revising refinement software to account for this new paradigm.

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