Ravelli Raimond B G, Nanao Max H, Lovering Andy, White Scott, McSweeney Sean
EMBL, Grenoble, France.
J Synchrotron Radiat. 2005 May;12(Pt 3):276-84. doi: 10.1107/S0909049505003286. Epub 2005 Apr 14.
In the accurate estimation of small signals, redundancy of observations is often seen as an essential tool for the experimenter. This is particularly true during macromolecular structure determination by single-wavelength anomalous dispersion (SAD), where the exploitable signal can be less than a few percent. At the most intense undulator synchrotron beamlines, the effect of radiation damage can be such that all usable signal is obscured. Here the magnitude of this effect in experiments performed at the Se K-edge is quantified. Six successive data sets were collected on the same crystal, interspersed with two exposures to the X-ray beam during which data were not collected. It is shown that the very first data set has excellent phasing statistics, whereas these statistics degrade for the later data sets. Merging several data sets into one, highly redundant, data set only gave moderate improvements as a result of the presence of radiation damage. Part of the damage could be corrected for using a linear interpolation scheme. Interpolation of the data to a low-dose as well as to a high-dose data set allowed us to combine the SAD method with the radiation-damage induced phasing (RIP) technique, which further improved the experimental phases, especially after density modification. Some recommendations are given on how to mitigate the effect of radiation damage during structure determination.
在精确估计小信号时，观测冗余通常被视为实验者的一项重要工具。在通过单波长反常色散（SAD）进行大分子结构测定时尤其如此，此时可利用的信号可能不到百分之几。在最强烈的波荡器同步辐射光束线上，辐射损伤的影响可能会使所有可用信号都被掩盖。本文对在硒K边进行的实验中这种效应的大小进行了量化。在同一晶体上收集了六个连续的数据集，其间穿插了两次对X射线束的曝光，在此期间未收集数据。结果表明，第一个数据集具有出色的相位统计信息，而后续数据集的这些统计信息则会下降。由于存在辐射损伤，将多个数据集合并为一个高度冗余的数据集仅带来了适度的改善。部分损伤可以使用线性插值方案进行校正。将数据插值到低剂量和高剂量数据集，使我们能够将SAD方法与辐射损伤诱导相位法（RIP）技术相结合，这进一步改善了实验相位，尤其是在密度修饰之后。文中给出了一些关于在结构测定过程中如何减轻辐射损伤影响的建议。
