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首页> 外文期刊>Acta crystallographica.Section D. Biological crystallography >ACORN: a review
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ACORN: a review

机译：小青:一个回顾

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The ACORN system was originally developed as a means of ab initio solution of protein structures when atomic resolution data were available. The first step is to obtain a starting set of phases, which must be at least slightly better than random,. These may be calculated from a fragment of the structure, which can be anything from a single metal atom to a complete molecular-replacement model A number of standard procedures are available in ACORN to orientate and position such a fragment. The fragment provides initial phases that give the first of a series of maps that are iteratively refined by a dynamic density-modification (DDM) process. Another FFT-based procedure is Sayre-equation refinement (SER), which modifies phases better to satisfy the Sayre equation. With good-quality atomic resolution data, the final outcome of applying DDM and SER is a map similar in appearance to that found from a refined structure, which is readily interpreted by automated procedures. Further development of ACORN now enables structures to be solved with less than atomic resolution data. A critical part of this development is the artificial extension of the data from the observed limit to 1 A resolution. These extended reflections are allocated unit normalized structure amplitudes and then treated in a similar way to observed reflections except that they are down-weighted in the calculation of maps. ACORN maps, especially at low resolution, tend to show C atoms less well, in particular C~alpha atoms which fall within the first diffraction minimum of their three neighbours. Two new density-modification procedures (DDM1 and DDM2) and a density-enhancement procedure (ENH) have been devised to counter this problem. It is demonstrated that high-quality maps showing individual atoms can be produced with the new ACORN ACORN has also been demonstrated to be very effective in refining phase sets derived from physical processes such as those using anomalous scattering or isomorphous derivative data. Future work will be directed towards applying A CORN to resolutions down to 2 A.
机译：橡子系统最初是作为开发从头开始解决蛋白质结构的方法当原子分辨率的数据可用为止。第一步是获取一组开始的阶段,必须至少略优于随机的,。的结构,可以从任何东西单一的金属原子一个完整molecular-replacement模型的标准程序可以在ACORN进行定位和位置这样的片段。提供了初始阶段,给的第一篇一系列迭代精制的地图动态density-modification (DDM)的过程。另一个是Sayre-equation FFT-based过程细化(SER),最好修改阶段满足塞尔方程。原子分辨率数据,最终的结果应用DDM和爵士是一个相似的地图发现从一个精致的外观结构,很容易解释自动化的过程。小青现在使结构能够得到解决小于原子分辨率数据。这种发展是人工扩展从观察到的数据限制为1决议。分配单位振幅归一化结构然后以类似的方式处理反射只是他们down-weighted地图的计算。在低分辨率,倾向于显示C原子更少特别是C ~α原子而下降在第一衍射最低的三个邻居。程序(DDM1和DDM2)和一个density-enhancement过程(掺)设计来应对这个问题。表明,高质量的地图单个原子可以产生新的橡子橡子也被证明是非常集来自有效的精炼阶段那些使用异常等物理过程散射或同形导数数据。工作将针对应用玉米决议2。

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