The interdependence of wavelength, redundancy and dose in sulfur SAD experiments
The interdependence of wavelength, redundancy and dose in sulfur SAD experiments
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DOI：
10.1107/s0907444908030503
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发表时间：
2008-12-01
影响因子：
2.2
通讯作者：
Suzuki, Atsuo
中科院分区：
文献类型：
--
作者：
Cianci, Michele;Helliwell, John R.;Suzuki, Atsuo
近十年来，作为解决大分子晶体学中相位问题的一种快速、简便的方法，硫SAD异常色散实验在同步加速器用户中迅速普及。在SRS（英国达斯伯里实验室）的光束线10上，一个通用的设计允许在0.979和2.290 a之间的六个波长收集测试数据集，以便评估实验变量的重要性和相互依赖性，如Bijvoet比率，波长，分辨率限制，数据冗余和每个数据集样品中的吸收x射线剂量。实验所用样品均为优质蛋清溶菌酶晶体。在给予晶体0.20 x 10(7) Gy的总剂量后，发现x射线损伤会影响二硫化物桥。然而，在这样的总剂量下，仍然有可能在所有情况下找到一种策略来收集数据集，以确定硫的亚结构，并通过选择波长、曝光时间和冗余度的最佳组合来产生高质量的相。所有分辨率的壳大于1.5是成功定位硫SAD子结构的必要条件。如果实现了这一点，似乎有可能在波长、冗余和剂量之间找到最佳折衷方案，以提供相位信息。波长的选择应遵循样品的组成和晶体的衍射特性。对于强衍射晶体，可以选择等于或小于1.540埃的波长来捕获可用数据（前提是Bijvoet比合理），而对于较弱衍射晶体，必须使用较长的波长，以获得尽可能高的Bijvoet比。这些结果表明，基于完整的晶体系统描述和数据采集仪器的硫SAD实验方法是有用的。
In the last decade, the popularity of sulfur SAD anomalous dispersion experiments has spread rapidly among synchrotron users as a quick and streamlined way of solving the phase problem in macromolecular crystallography. On beamline 10 at SRS (Daresbury Laboratory, UK), a versatile design has allowed test data sets to be collected at six wavelengths between 0.979 and 2.290 A in order to evaluate the importance and the interdependence of experimental variables such as the Bijvoet ratio, wavelength, resolution limit, data redundancy and absorbed X-ray dose in the sample per data set. All the samples used in the experiments were high-quality hen egg-white lysozyme crystals. X-radiation damage was found to affect disulfide bridges after the crystals had been given a total dose of 0.20 x 10(7) Gy. However, with such a total dose, it was still possible in all cases to find a strategy to collect data sets to determine the sulfur substructure and produce good-quality phases by choosing an optimum combination of wavelength, exposure time and redundancy. A greater than 1.5 for all resolution shells was a necessary requirement for successful sulfur SAD substructure location. Provided this is achieved, it seems possible to find an optimum compromise between wavelength, redundancy and dose to provide phasing information. The choice of the wavelength should then follow the sample composition and the diffracting properties of the crystal. For strongly diffracting crystals, wavelengths equal or shorter than 1.540 angstrom can be selected to capture the available data (provided the Bijvoet ratio is reasonable), while a longer wavelength, to gain as high a Bijvoet ratio as possible, must be used for more weakly diffracting crystals. These results suggest that an approach to a sulfur SAD experiment based on a complete description of the crystal system and the instrument for data collection is useful.