ABSTRACT De novo single-wavelength anomalous dispersion (SAD) determination of macromolecular structures requires accurate measurement of anomalous signal from naturally occurring (S, P, Ca, etc.) or incorporated (Se, Hg, etc.) anomalous scatterers. The quality and the level of the anomalous signal in SAD datasets can be assessed using, as an indicator, the average anomalous signal-to-noise level ⟨|ΔIanom|/σ(ΔI)⟩ based on intensities or ⟨|ΔFanom|/σ(ΔF)⟩ based on amplitudes. The value for the average anomalous signal-to-noise for a successful SAD experiment is a matter of debate given its high variability from one SAD experiment to another. We present a mathematical model relating the ⟨|ΔIanom|/σ(ΔI)⟩ to the overall final ⟨I/σ(I)⟩, necessary for successful SAD phasing. By statistical analysis of the data from 115 successful native SAD experiments, as available in the PDB, we show that the experimental values of ⟨|ΔIanom|/σ(ΔI)⟩ follow a Gamma distribution with an average value of 1.2 ± 0.3. This mathematical model allows a-priori prediction of the overall ⟨I/σ(I)⟩, necessary for successful SAD phasing given the description of the sample expressed. The results reported here are of general applicability to any phasing experiment involving the measurement of anomalous signal from any anomalous scatterer or X-ray source, including X-FEL data.

中文翻译：

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SAD 实验中的信噪比

摘要 大分子结构的从头单波长反常色散 (SAD) 测定需要准确测量来自天然（S、P、Ca 等）或掺入（Se、Hg 等）反常散射体的反常信号。SAD 数据集中异常信号的质量和水平可以使用基于强度的平均异常信噪比水平 ⟨|ΔIanom|/σ(ΔI)⟩ 或 ⟨|ΔFanom|/σ 作为指标进行评估(ΔF)⟩ 基于振幅。一个成功的 SAD 实验的平均异常信噪比值是一个有争议的问题，因为它从一个 SAD 实验到另一个的高度可变性。我们提出了一个数学模型，将 ⟨|ΔIanom|/σ(ΔI)⟩ 与整体最终 ⟨I/σ(I)⟩ 相关联，这是成功 SAD 定相所必需的。通过对 115 个成功的原生 SAD 实验数据的统计分析，正如 PDB 中可用的那样，我们表明 ⟨|ΔIanom|/σ(ΔI)⟩ 的实验值遵循伽玛分布，平均值为 1.2 ± 0.3。这个数学模型允许对整体 ⟨I/σ(I)⟩ 进行先验预测，这对于成功的 SAD 定相是必要的，给出了所表达的样本的描述。此处报告的结果普遍适用于任何涉及测量来自任何异常散射体或 X 射线源的异常信号的定相实验，包括 X-FEL 数据。