Plevka Pavel, Kaufmann Bärbel, Rossmann Michael G
Biological Sciences, Purdue University, 240 South Martin Jischke Drive, West Lafayette, IN 47907-2032, USA.
Acta Crystallogr D Biol Crystallogr. 2011 Jun;67(Pt 6):568-77. doi: 10.1107/S090744491101448X. Epub 2011 May 17.
The constraints imposed on structure-factor phases by noncrystallographic symmetry (NCS) allow phase improvement, phase extension to higher resolution and hence ab initio phase determination. The more numerous the NCS redundancy and the greater the volume used for solvent flattening, the greater the power for phase determination. In a case analyzed here the icosahedral NCS phasing appeared to have broken down, although later successful phase extension was possible when the envelope around the NCS region was tightened. The phases from the failed phase-determination attempt fell into four classes, all of which satisfied the NCS constraints. These four classes corresponded to the correct solution, opposite enantiomorph, Babinet inversion and opposite enantiomorph with Babinet inversion. These incorrect solutions can be seeded from structure factors belonging to reciprocal-space volumes that lie close to icosahedral NCS axes where the structure amplitudes tend to be large and the phases tend to be 0 or π. Furthermore, the false solutions can spread more easily if there are large errors in defining the envelope designating the region in which NCS averaging is performed.
非晶体学对称性（NCS）对结构因子相位施加的限制允许相位改善、将相位扩展到更高分辨率，从而实现从头算相位确定。NCS冗余越多，用于溶剂扁平化的体积越大，相位确定的能力就越强。在此分析的一个案例中，二十面体NCS相位似乎失效了，尽管后来当NCS区域周围的包络收紧时，成功的相位扩展是可能的。失败的相位确定尝试得到的相位分为四类，所有这些都满足NCS约束。这四类分别对应于正确解、相反对映体、巴比涅反转以及具有巴比涅反转的相反对映体。这些错误解可能源自属于倒易空间体积的结构因子，这些体积靠近二十面体NCS轴，在那里结构振幅往往较大且相位往往为0或π。此外，如果在定义指定进行NCS平均的区域的包络时存在较大误差，错误解可能更容易传播。
