Abstract
Two aspects of low-resolution macromolecular crystal structure analysis are considered: (i) the use of reference structures and structural units for provision of structural prior information and (ii) map sharpening in the presence of noise and theeffects of Fourier series termination. The generation of interatomic distance restraints by ProSMART and their subsequent application in REFMAC5 is described. It is shown that the use of such external structural information can enhance the reliability of derived atomic models and stabilize refinement. The problem of map sharpening is considered as an inverse deblurring problem and is solved using Tikhonov regularizers. It is demonstrated that this type of map sharpening can automatically produce a map with more structural features whilst maintaining connectivity. Tests show that both of these directions are promising, although more work needs to be performed in order to further exploit structural information and to address the problem of reliable electron-density calculation.
Highlights
Heterogeneous organization of molecules in the crystal lattice can lead to the diffraction data being of poor quality
We have presented two tools to aid in lowresolution refinement, namely external structural restraints and regularized map sharpening
The use of external restraints based on homologous reference structures and/or structural fragments gives promising results
Summary
Heterogeneous organization of molecules in the crystal lattice can lead to the diffraction data being of poor quality. Such heterogeneities may arise from effects such as crystal mosaicity, molecule/chain flexibility and localized disorder This results in weak diffraction intensities, causing the data to be collected using a lower resolution threshold and with low information content. There are other factors that can reduce the information content of macromolecular crystallographic (MX) data, reducing effective resolution These include crystal-growth peculiarities such as twinning and OD order–disorder. Data from low-resolution crystals usually exhibit high isotropic and anisotropic B values This contributes to the observation of smeared regions of electron density, with vanishing side chains, secondary-structural elements and even domains. When data are collected to the crystal diffraction limit and no map sharpening is used, such noise usually dies out approaching the high-resolution limit.
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