Redinbo M R, Yeates T O
UCLA Department of Chemistry and Biochemistry and Molecular Biology Institute, Los Angeles, CA 90024, USA.
Acta Crystallogr D Biol Crystallogr. 1993 Jul 1;49(Pt 4):375-80. doi: 10.1107/S090744499300294X.
The structure determination of a macromolecule from a hemihedrally twinned crystal specimen with a twinning fraction of one-half is described. Twinning was detected by analysis of crystal-packing density and intensity statistics. The structure was solved using molecular replacement, and the positioned search model was used to overcome the twinning by a novel method of 'detwinning' the observed data. Estimates of the unobservable crystallographic intensities from each of the twin domains were obtained and used to refine the model. The structure of a new algal plastocyanin from Chlamydomonas reinhardtii was determined by this method to 1.6 A resolution with a 'twinned' R factor of 15.6%. Additional data from a crystal specimen with a low twinning fraction were used to establish the accuracy of the structure solution from the perfectly twinned data, and to finalize the refinement to 1.5 A resolution and a true R factor of 16.8%. Methods for detecting twinning and obtaining a molecular-replacement solution in the presence of twinning are discussed.
描述了从孪晶分数为二分之一的半面孪晶晶体标本中确定大分子结构的方法。通过晶体堆积密度分析和强度统计检测到孪晶。使用分子置换法解析结构，并通过一种对观测数据进行“解孪晶”的新方法，利用定位搜索模型克服孪晶现象。获得了来自每个孪晶域的不可观测晶体学强度的估计值，并用于完善模型。用这种方法确定了莱茵衣藻一种新的藻蓝蛋白的结构，分辨率达到1.6 Å，“孪晶”R因子为15.6%。利用来自低孪晶分数晶体标本的额外数据，从完全孪晶数据确定结构解析的准确性，并最终将精修分辨率提高到1.5 Å，真实R因子为16.8%。讨论了在存在孪晶的情况下检测孪晶和获得分子置换解的方法。
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