Dorset D L
Electron Diffraction Department, Hauptman-Woodward Medical Research Institute, Inc., Buffalo, NY 14203-1196, USA.
Acta Crystallogr A. 1996 May 1;52 ( Pt 3):480-9. doi: 10.1107/s0108767396001420.
Zonal diffraction amplitudes and crystallographic phases, derived from an averaged electron micrograph of two-dimensionally crystalline E. coli Omp F outer membrane porin (plane group p31m, a = 72 A), embedded in glucose, were used as a model data set to test the feasibility of direct phase extension and ab initio direct phase determination. If 17 phase terms derived from e.g. a 10 A (diffraction) resolution image are expanded to 6 A by the Sayre-Hughes equation, the unknown phases are found with reasonable accuracy (mean error 43 degrees for 25 reflections). This, however, is not the most optimal starting point. As a function of initial image resolution, the accuracy of the phase extension to 6 A is approximately a parabolic function. That is, an optimal basis resolution, found at 11 A (i.e. 14 defined reflections), produces a least mean error of 18 degrees for 28 new reflections. In addition, ab initio phase determination is possible via a multisolution technique, using a test for density flatness as a figure of merit. The success of the determination, again is sensitive to the size of the starting basis set generated from the permuted unknown reflections. If an annealing step is used to improve the basis set, the test for flatness will identify which reflections should be changed in phase. However, this figure of merit is not absolutely reliable for finding the exact value of the unknown phases.
从嵌入葡萄糖中的二维结晶大肠杆菌OmpF外膜孔蛋白（平面群p31m，a = 72 Å）的平均电子显微照片中获得的区域衍射振幅和晶体学相位，被用作模型数据集来测试直接相位扩展和从头算直接相位确定的可行性。如果通过塞尔-休斯方程将例如从10 Å（衍射）分辨率图像导出的17个相位项扩展到6 Å，则未知相位能以合理的精度被找到（25个反射的平均误差为43度）。然而，这并不是最理想的起始点。作为初始图像分辨率的函数，将相位扩展到6 Å的精度近似为抛物线函数。也就是说，在11 Å（即14个定义的反射）处找到的最佳基础分辨率，对于28个新反射产生的最小平均误差为18度。此外，通过多解技术，使用密度平整度测试作为品质因数，可以进行从头算相位确定。确定的成功同样对从置换后的未知反射生成的起始基集的大小敏感。如果使用退火步骤来改进基集，平整度测试将识别哪些反射的相位应该改变。然而，这个品质因数对于找到未知相位的精确值并非绝对可靠。
