Porta Jason, Lovelace Jeff, Borgstahl Gloria E O
The Eppley Institute for Research in Cancer and Allied Diseases, University of Nebraska Medical Center, 986805 Nebraska Medical Center, Omaha, NE 68198, USA.
Department of Biochemistry and Molecular Biology, University of Nebraska Medical Center, 985870 Nebraska Medical Center, Omaha, NE 68198, USA.
J Appl Crystallogr. 2017 Jun 30;50(Pt 4):1200-1207. doi: 10.1107/S1600576717007294. eCollection 2017 Aug 1.
Periodic crystal diffraction is described using a three-dimensional (3D) unit cell and 3D space-group symmetry. Incommensurately modulated crystals are a subset of aperiodic crystals that need four to six dimensions to describe the observed diffraction pattern, and they have characteristic satellite reflections that are offset from the main reflections. These satellites have a non-integral relationship to the primary lattice and require vectors for processing. Incommensurately modulated biological macromolecular crystals have been frequently observed but so far have not been solved. The authors of this article have been spearheading an initiative to determine this type of crystal structure. The first step toward structure solution is to collect the diffraction data making sure that the satellite reflections are well separated from the main reflections. Once collected they can be integrated and then scaled with appropriate software. Then the assignment of the superspace group is needed. The most common form of modulation is in only one extra direction and can be described with a (3 + 1)D superspace group. The (3 + 1)D superspace groups for chemical crystallographers are fully described in Volume C of . This text includes all types of crystallographic symmetry elements found in small-molecule crystals and can be difficult for structural biologists to understand and apply to their crystals. This article provides an explanation for structural biologists that includes only the subset of biological symmetry elements and demonstrates the application to a real-life example of an incommensurately modulated protein crystal.
周期性晶体衍射是用三维（3D）晶胞和3D空间群对称性来描述的。非公度调制晶体是非周期性晶体的一个子集，需要四到六个维度来描述所观察到的衍射图案，并且它们具有与主反射偏移的特征卫星反射。这些卫星与主晶格具有非整数关系，并且处理时需要 向量。非公度调制的生物大分子晶体经常被观察到，但到目前为止尚未得到解析。本文的作者一直在率先开展一项确定这种晶体结构的计划。结构解析的第一步是收集衍射数据，确保卫星反射与主反射充分分离。一旦收集到数据，就可以进行积分，然后用适当的软件进行缩放。然后需要确定超空间群。最常见的调制形式仅在一个额外方向上，并且可以用（3 + 1）D超空间群来描述。化学晶体学家的（3 + 1）D超空间群在《 》的C卷中有完整描述。本文包含小分子晶体中发现的所有类型的晶体学对称元素，结构生物学家可能难以理解并应用于他们的晶体。本文为结构生物学家提供了一种解释，仅包括生物对称元素的子集，并展示了其在非公度调制蛋白质晶体实际例子中的应用。
