The signal-to-noise ratio in SAD experiments
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.