Rossmann M G
Department of Biological Sciences, Purdue University, West Lafayette, Indiana 47907.
Acta Crystallogr A. 1990 Feb 1;46 ( Pt 2):73-82. doi: 10.1107/s0108767389009815.
Molecular replacement can be used for obtaining approximate phasing of an unknown structure from a known related molecule and for phase improvement as well as extension in the presence of noncrystallographic symmetry. Emphasis is placed on the latter procedure. It is shown that the real-space method of iterative electron density averaging and Fourier back transformation corresponds to iterative phase substitution in the right-hand side of expressions to give a set of improved phases. Analysis of these expressions (the 'molecular replacement equations') provides insight into the limits of possible phase extension, and the implications for the use of calculated structure factors when there are no observed amplitudes. It is shown that the percentage of observed data and inaccuracy of the observed amplitudes available for phase extension are compensated by the extent of noncrystallographic redundancy and the fraction of crystal cell volume that may be flattened because it is outside the control of noncrystallographic symmetry.
分子置换可用于从已知的相关分子获得未知结构的近似相位，以及在存在非晶体学对称性的情况下进行相位改善和扩展。重点在于后一种方法。结果表明，迭代电子密度平均和傅里叶反变换的实空间方法对应于表达式右侧的迭代相位替换，从而得到一组改进的相位。对这些表达式（“分子置换方程”）的分析有助于深入了解可能的相位扩展极限，以及在没有观测振幅时使用计算结构因子的影响。结果表明，可用于相位扩展的观测数据百分比和观测振幅的不准确性，可由非晶体学冗余程度以及由于不受非晶体学对称性控制而可能被压扁的晶胞体积分数来补偿。
