González Ana, von Delft Frank, Liddington Robert C, Bakolitsa Constantina
Stanford Linear Accelerator Center, Menlo Park, CA 94025, USA.
J Synchrotron Radiat. 2005 May;12(Pt 3):285-91. doi: 10.1107/S0909049505003249. Epub 2005 Apr 14.
Radiation damage affects MAD experiments in two ways: (i) increased absorption by the crystal at the wavelengths of interest for the experiment results in faster crystal deterioration; (ii) lack of isomorphism induced by radiation damage causes problems when scaling and merging data at different wavelengths and can prevent accurate measurement of anomalous and dispersive differences. In an attempt to overcome these problems in the case of radiation-sensitive crystals of vinculin, two-wavelength MAD data were collected at the Se absorption-edge inflection and at high-energy remote wavelengths. Although this strategy resulted in a lower total absorbed dose compared with a standard three-wavelength experiment using the peak wavelength, an increase in the unit-cell volume and other effects attributable to radiation damage were still observed. In an effort to extract the maximum information available from the data, different data-processing and scaling procedures were compared. Scaling approaches involving local scaling of unmerged reflections were consistently successful and most ordered Se sites could be located. Subsequent use of these sites for phasing resulted in an interpretable electron density map. This case demonstrates the feasibility of two-wavelength MAD in the presence of moderate radiation damage using conventional data collection strategies and widely available standard software.
辐射损伤对多波长反常散射（MAD）实验有两方面影响：（i）在实验感兴趣的波长下，晶体吸收增加导致晶体更快劣化；（ii）辐射损伤引起的同晶型缺失，在对不同波长数据进行缩放和合并时会产生问题，并且可能妨碍对反常和色散差异的准确测量。为了在纽蛋白辐射敏感晶体的情况下克服这些问题，在硒吸收边拐点和高能远程波长处收集了双波长MAD数据。尽管与使用峰值波长的标准三波长实验相比，该策略导致总吸收剂量更低，但仍观察到晶胞体积增加以及其他归因于辐射损伤的效应。为了从数据中提取最大可用信息，对不同的数据处理和缩放程序进行了比较。涉及未合并反射局部缩放的缩放方法始终成功，并且大多数有序的硒位点可以定位。随后使用这些位点进行相位确定，得到了一个可解释的电子密度图。该案例证明了在存在中等辐射损伤的情况下，使用传统数据收集策略和广泛可用的标准软件进行双波长MAD的可行性。
