Dorset D L, Kopp S, Fryer J R, Tivol W F
Electron Diffraction Department, Medical Foundation of Buffalo, Inc., NY 14203.
Ultramicroscopy. 1995 Jan;57(1):59-89. doi: 10.1016/0304-3991(94)00161-f.
The Sayre equation was evaluated as a technique for phase refinement in electron crystallography. Atomic-resolution electron diffraction data from copper perchlorophthalocyanine were assigned phase values from the Fourier transforms of various experimental electron micrographs, including one at 2.3 A, containing errors due to lens astigmatism. In each case, an atomic-resolution structure could be found after Fourier refinement. In addition, it was possible to begin with a basis set derived from symbolic addition for phase extension. Such a source of phases was also found to be useful for extending zonal electron diffraction sets from six polymer crystals, even though there was considerable overlap of atomic positions in the projection down the chain axes. Other tests of the Sayre equation were made with zonal protein data sets (bacteriorhodopsin, halorhodopsin) to evaluate what difficulties are to be expected when direct phasing techniques are to be used in macromolecular electron crystallography. Comparison to known values indicated that the low-resolution range (e.g. to 6 A) was reasonably stable for phase extension from a 10-15 A resolution image. Only when a minimum in average intensity was approached (near 5 A) did the direct extension encounter serious difficulties. If this minimum was treated as a "phase node" to generate two possible solutions, a model more similar to the true phase set was found. In general, this rather simple convolutional technique for phase extension seems to be particularly suitable for a variety of electron crystallographic applications.
塞尔方程被评估为电子晶体学中相位细化的一种技术。来自高氯酸铜酞菁的原子分辨率电子衍射数据，通过各种实验电子显微镜图像的傅里叶变换来赋予相位值，其中包括一张2.3埃分辨率的图像，该图像因透镜像散存在误差。在每种情况下，经过傅里叶细化后都能找到原子分辨率结构。此外，还可以从符号相加导出的基组开始进行相位扩展。结果发现，这样的相位来源对于扩展六种聚合物晶体的带状电子衍射数据集也很有用，尽管沿链轴投影的原子位置存在相当大的重叠。还用带状蛋白质数据集（细菌视紫红质、嗜盐菌视紫红质）对塞尔方程进行了其他测试，以评估在大分子电子晶体学中使用直接相位技术时可能会遇到哪些困难。与已知值的比较表明，从10 - 15埃分辨率图像进行相位扩展时，低分辨率范围（例如到6埃）相当稳定。只有当接近平均强度最小值（接近5埃）时，直接扩展才会遇到严重困难。如果将这个最小值视为一个“相位节点”以生成两个可能的解，就会找到一个与真实相位集更相似的模型。总的来说，这种相当简单的用于相位扩展的卷积技术似乎特别适用于各种电子晶体学应用。
