Fanchon E, Hendrickson W A
Howard Hughes Medical Institute, Department of Biochemistry and Molecular Biophysics, Columbia University, New York, New York 10032.
Acta Crystallogr A. 1990 Oct 1;46 ( Pt 10):809-20. doi: 10.1107/s0108767390005967.
The analysis of X-ray diffraction intensities is complicated by the anisotropy of anomalous scattering (AAS) that can occur due to resonance associated with transitions between core electrons and valence molecular orbitals. Substantial AAS has been observed directly in diffraction data near the K edge of selenium in selenolanthionine [Templeton & Templeton (1988). Acta Cryst. A44, 1045-1051] and in pleiochroism of X-ray absorption in selenobiotinyl streptavidin [Hendrickson, Pähler, Smith, Satow, Merritt & Phizackerley (1989). Proc. Natl Acad. Sci. USA, 86, 2190-2194]. The impact of AAS on the multiple-wavelength anomalous diffraction (MAD) method for phase determination is of particular interest in the context of this chemical state of selenium in the light of a general method that has been developed to incorporate selenomethionine into proteins for use in MAD phasing [Hendrickson, Horton & LeMaster (1990). EMBO J. 9, 1665-1672]. The first step of the MAD phasing method necessarily assumes that the anomalous-scattering factors are isotropic and our first aim here is to evaluate the effect of this approximation on initially determined phases. To obtain ultimate phases free from the effects of anisotropy, a least-squares procedure has been written in which global parameters (i.e. pertaining to the whole data set) are refined simultaneously with local parameters (i.e. pertaining to a given node h). The AAS is taken explicityly into account by considering f' and f" as tensors instead of scalars [Templeton & Templeton (1982). Acta Cryst. A38, 62-67], and the components of the f' and f" tensors are among the refinable global parameters. The effectiveness of this procedure is tested with data simulated from the refined atomic model of selenobiotinyl streptavidin. The application of this procedure to actual Photon Factory measurements is also described. The results show that AAS does not cripple the MAD method, and that phases uncorrupted by these effects can be recovered.
X射线衍射强度的分析因反常散射（AAS）的各向异性而变得复杂，这种反常散射可能由于与内层电子和价分子轨道之间跃迁相关的共振而产生。在硒代硫辛酸中硒的K边附近的衍射数据中，已直接观察到显著的反常散射[Templeton & Templeton（1988年）。《晶体学报》A44卷，第1045 - 1051页]，以及在硒代生物素化链霉亲和素的X射线吸收的偏振现象中也观察到了[Hendrickson, Pähler, Smith, Satow, Merritt & Phizackerley（1989年）。《美国国家科学院院刊》86卷，第2190 - 2194页]。鉴于已开发出一种将硒代甲硫氨酸掺入蛋白质以用于多波长反常衍射（MAD）相位测定的通用方法[Hendrickson, Horton & LeMaster（1990年）。《欧洲分子生物学组织杂志》9卷，第1665 - 1672页]，在这种硒的化学状态背景下，反常散射对MAD相位测定方法的影响尤其令人关注。MAD相位测定方法的第一步必然假设反常散射因子是各向同性的，我们这里的首要目标是评估这种近似对最初确定的相位的影响。为了获得不受各向异性影响的最终相位，编写了一种最小二乘法程序，其中全局参数（即与整个数据集相关的参数）与局部参数（即与给定节点h相关的参数）同时进行精修。通过将f'和f"视为张量而非标量，明确考虑了反常散射[Templeton & Templeton（1982年）。《晶体学报》A38卷，第62 - 67页]，并且f'和f"张量的分量是可精修的全局参数之一。用从硒代生物素化链霉亲和素的精修原子模型模拟的数据对该程序的有效性进行了测试。还描述了该程序在实际光子工厂测量中的应用。结果表明，反常散射不会使MAD方法失效，并且可以恢复不受这些影响破坏的相位。
