Bovine β‐lactoglobulin was crystallized from 3 M NaCl buffered at pH 3.8 with sodium citrate as thick hexagonal prisms of greater than 1 mm in edge length. Analyses of the X‐ray diffraction intensities using three different current algorithms were unanimous in specifying the space group to be P6₃22, with unit‐cell dimensions a = b = 75.47, c = 140.79 Å. No progress could be made, however, towards an acceptable solution by molecular replacement using this symmetry. In the end, it was found that the true space group was C222₁, a subgroup of P6₃22, with a = 65.89, b = 114.12, c = 140.51 Å, with the apparent 622 symmetry arising from an unusual threefold or tritohedral twinning. An assembly based on a model of the protein in another crystal form (PDB entry 1beb) containing three molecules in the asymmetric unit was refined to 2.3 Å resolution with a final R factor of 0.23 and R_free of 0.26. NCS restraints were maintained throughout. For the most part, the molecules found in this crystal form are virtually the same as in PDB entry 1beb, although there are numerous local variations, particularly in loop elements, rotamer conformation differences and some alterations, including additions, at the termini.

中文翻译：

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pH 3.8 下生长的牛 β-乳球蛋白晶体结构表现出新型三重孪晶。

牛 β-乳球蛋白从 pH 为 3.8 的 3  M氯化钠中用柠檬酸钠缓冲，结晶为边长大于 1 毫米的厚六角棱柱。使用三种不同的当前算法对 X 射线衍射强度进行的分析一致指定空间群为P 6 ₃ 22，晶胞尺寸a = b = 75.47，c = 140.79 Å。然而，在通过利用这种对称性进行分子置换来获得可接受的解决方案方面，并没有取得任何进展。最终，发现真正的空间群是C 222 _{1 ，是}P 6 ₃ 22的一个子群， a = 65.89，b = 114.12，c = 140.51 Å，表观 622 对称性源于不寻常的三重或三面体孪生。基于另一种晶体形式的蛋白质模型（PDB 条目 1beb）的组装，在不对称单元中包含三个分子，该组装被细化至 2.3 Å 分辨率，最终 R 因子为 0.23，R因子为_0.26。NCS 全程保持束缚。在大多数情况下，这种晶体形式中发现的分子实际上与 PDB 条目 1beb 中的分子相同，尽管存在许多局部变化，特别是在环元件、旋转异构体构象差异和末端的一些改变（包括添加）方面。