Sjölin L, Prince E, Svensson L A, Gilliland G L
Institute for Inorganic Chemistry, Chalmers University of Technology, Göteborg, Sweden.
Acta Crystallogr A. 1991 May 1;47 ( Pt 3):216-23. doi: 10.1107/s0108767390012077.
An efficient algorithm for the determination of an everywhere positive electron-density distribution that agrees with observed structure amplitudes has been used to determine the phases of X-ray diffraction data from recombinant bovine chymosin, a protein with 323 amino-acid residues in the molecular chain whose structure was recently determined using molecular replacement methods. A systematic procedure for testing the signs of centric reflections, using the total entropy of the map as a figure of merit, was used to produce a low-resolution map. The phases of acentric and additional centric reflections were then chosen by adding them to the map with various possible phases and computing the total entropy of the resulting map. Of 159 centric reflections whose phases were chosen by this procedure, 141 had the same phase as in the refined structure. The median absolute phase difference for 1811 acentric reflections was 32 degrees. A map produced from these 1970 reflections, out of 12,346 reflections in the data set, showed a remarkable agreement with the refined structure. This molecule is many times larger than any whose structures have previously been determined without the use of isomorphous replacement, molecular replacement or anomalous dispersion, and the map demonstrates the potential of maximum-entropy methods in macromolecular structure determination.
一种用于确定与观测结构振幅相符的处处为正的电子密度分布的有效算法，已被用于确定重组牛凝乳酶的X射线衍射数据的相位。重组牛凝乳酶是一种分子链中有323个氨基酸残基的蛋白质，其结构最近用分子置换法确定。一种使用图谱的总熵作为品质因数来测试中心反射符号的系统程序，被用于生成低分辨率图谱。然后通过将非中心反射和额外的中心反射以各种可能的相位添加到图谱中，并计算所得图谱的总熵，来选择它们的相位。通过该程序选择相位的159个中心反射中，有141个与精修结构中的相位相同。1811个非中心反射的中位数绝对相位差为32度。从数据集中的12346个反射中的这1970个反射生成的图谱与精修结构显示出显著的一致性。该分子比以前在不使用同晶置换、分子置换或反常色散的情况下确定结构的任何分子大许多倍，并且该图谱证明了最大熵方法在大分子结构测定中的潜力。
