来自区域检测器的单晶X射线衍射数据的处理可以分为两个步骤。首先,原始强度通过衍射图像的积分获得,然后进行数据校正和还原以确定结构因子的振幅及其不确定性。第二步考虑衍射几何形状,样品照明,衰变,吸收和其他影响。虽然吸收在标准大分子晶体学(MX)中只是次要影响,它可能成为长波长实验的最大不确定性来源。当前的MX软件包通常采用经验模型来校正吸收的影响,通过最小化对称等效反射之间的强度差异来确定校正;这些模型非常适合捕获平滑变化的实验效果。然而,对于非常长的波长,经验方法成为一种不可靠的方法来模拟高保真度的强吸收效应。当数据多重性低时,这个问题尤其严重。本文提出了一种基于X射线断层扫描得出的样品体积模型的分析吸收校正策略(在新软件AnACor中实现)。对于所有反射,通过不同样品材料的各个路径长度由光线跟踪方法确定。吸收校正的几种方法(球面谐波校正,分析吸收校正和两者的组合)对两个样品进行比较,膜蛋白OmpK36GD,在λ=3.54的波长下测量,和亚氯酸盐歧化酶,在λ=4.13µ下测量。数据集统计,反常差分傅立叶图中的峰高和实验定相成功用于比较不同吸收校正方法的结果。使用新的分析吸收校正的策略被证明优于标准的球面谐波校正。虽然在3.54的数据中改善不大,在较长波长数据中,分析吸收校正优于球面谐波(λ=4.13µ),这也反映在成功的实验阶段所需的数据量减少。
Processing of single-crystal X-ray diffraction data from area detectors can be separated into two steps. First, raw intensities are obtained by integration of the diffraction images, and then data correction and reduction are performed to determine structure-factor amplitudes and their uncertainties. The second step considers the diffraction geometry, sample illumination, decay, absorption and other effects. While absorption is only a minor effect in standard macromolecular crystallography (MX), it can become the largest source of uncertainty for experiments performed at long wavelengths. Current software packages for MX typically employ empirical models to correct for the effects of absorption, with the corrections determined through the procedure of minimizing the differences in intensities between symmetry-equivalent reflections; these models are well suited to capturing smoothly varying experimental effects. However, for very long wavelengths, empirical methods become an unreliable approach to model strong absorption effects with high fidelity. This problem is particularly acute when data multiplicity is low. This paper presents an analytical absorption correction strategy (implemented in new software AnACor) based on a volumetric model of the sample derived from X-ray tomography. Individual path lengths through the different sample materials for all reflections are determined by a ray-tracing method. Several approaches for absorption corrections (spherical harmonics correction, analytical absorption correction and a combination of the two) are compared for two samples, the membrane protein OmpK36 GD, measured at a wavelength of λ = 3.54 Å, and chlorite dismutase, measured at λ = 4.13 Å. Data set statistics, the peak heights in the anomalous difference Fourier maps and the success of experimental phasing are used to compare the results from the different absorption correction approaches. The strategies using the new analytical absorption correction are shown to be superior to the standard spherical harmonics corrections. While the improvements are modest in the 3.54 Å data, the analytical absorption correction outperforms spherical harmonics in the longer-wavelength data (λ = 4.13 Å), which is also reflected in the reduced amount of data being required for successful experimental phasing.