Renko Miha, Taler-Verčič Ajda, Mihelič Marko, Zerovnik Eva, Turk Dušan
Department of Biochemistry and Molecular and Structural Biology, Josef Stefan Institute, Jamova 39, SI-1000 Ljubljana, Slovenia.
Acta Crystallogr D Biol Crystallogr. 2014 Apr;70(Pt 4):1015-25. doi: 10.1107/S1399004714000091. Epub 2014 Mar 19.
At present, the determination of crystal structures from data that have been acquired from twinned crystals is routine; however, with the increasing number of crystal structures additional crystal lattice disorders are being discovered. Here, a previously undescribed partial rotational order-disorder that has been observed in crystals of stefin B is described. The diffraction images revealed normal diffraction patterns that result from a regular crystal lattice. The data could be processed in space groups I4 and I422, yet one crystal exhibited a notable rejection rate in the higher symmetry space group. An explanation for this behaviour was found once the crystal structures had been solved and refined and the electron-density maps had been inspected. The lattice of stefin B crystals is composed of five tetramer layers: four well ordered layers which are followed by an additional layer of alternatively placed tetramers. The presence of alternative positions was revealed by the inspection of electron-density score maps. The well ordered layers correspond to the crystal symmetry of space group I422. In addition, the positions of the molecules in the additional layer are related by twofold rotational axes which correspond to space group I422; however, these molecules lie on the twofold axis and can only be related in a statistical manner. When the occupancies of alternate positions and overlapping are equal, the crystal lattice indeed fulfills the criteria of space group I422; when these occupancies are not equal, the lattice only fulfills the criteria of space group I4.
目前，从孪晶获得的数据确定晶体结构已是常规操作；然而，随着晶体结构数量的增加，越来越多的晶格无序现象被发现。在此，描述了一种在stefin B晶体中观察到的此前未被描述的部分旋转有序 - 无序现象。衍射图像显示出由规则晶格产生的正常衍射图案。这些数据可以在空间群I4和I422中进行处理，但有一个晶体在更高对称的空间群中表现出显著的拒斥率。一旦晶体结构得到解析和精修并检查了电子密度图，就找到了这种行为的解释。stefin B晶体的晶格由五个四聚体层组成：四个排列有序的层，后面跟着一层交替排列的四聚体。通过检查电子密度得分图揭示了交替位置的存在。排列有序的层对应于空间群I422的晶体对称性。此外，额外层中分子的位置通过对应于空间群I422的二重旋转轴相关联；然而，这些分子位于二重轴上，只能以统计方式相关联。当交替位置的占有率和重叠相等时，晶格确实满足空间群I422的标准；当这些占有率不相等时，晶格仅满足空间群I4的标准。
