Three-dimensional models of protein structures determined by X-ray crystallography are based on the interpretation of experimentally derived electron-density maps. The real-space correlation coefficient (RSCC) provides an easily comprehensible, objective measure of the residue-based fit of atom coordinates to electron density. Among protein structure models, protein-ligand complexes are of special interest, given their contribution to understanding the molecular underpinnings of biological activity and to drug design. For consumers of such models, it is not trivial to determine the degree to which ligand-structure modelling is biased by subjective electron-density interpretation. A standalone script, Twilight, is presented for the analysis, visualization and annotation of a pre-filtered set of 2815 protein-ligand complexes deposited with the PDB as of 15 January 2012 with ligand RSCC values that are below a threshold of 0.6. It also provides simplified access to the visualization of any protein-ligand complex available from the PDB and annotated by the Uppsala Electron Density Server. The script runs on various platforms and is available for download at http://www.ruppweb.org/twilight/.
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机译：通过X射线晶体学确定的蛋白质结构的三维模型是基于对实验得出的电子密度图的解释。实空间相关系数（RSCC）为原子坐标与电子密度的基于残基的拟合提供了易于理解的客观度量。在蛋白质结构模型中，蛋白质-配体复合物特别有意义，因为它们有助于理解生物活性的分子基础和药物设计。对于此类模型的消费者而言，确定主观电子密度解释对配体结构模型的偏倚程度并非易事。介绍了一个独立的脚本Twilight，用于分析，可视化和注释截至2012年1月15日在PDB上存储的2815种蛋白质-配体复合物的预过滤组，其配体RSCC值低于0.6阈值。它还提供了从PDB获得并由Uppsala Electron Density Server注释的任何蛋白质-配体复合物的可视化的简化访问。该脚本可在各种平台上运行，并可从http://www.ruppweb.org/twilight/下载。
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