Structure-based screening of binding affinities via small-angle X-ray scattering

Protein-protein and protein-ligand interactions can alter the scattering properties of participating molecules, and thus be quantified by solution small-angle X-ray scattering (SAXS). In such cases, scattering reveals structural details of the bound complex, number of species involved, and in principle strength of the interaction. However, determining binding affinities from SAXS-based titrations is not yet an established procedure with well-defined performance expectations. We thus used periplasmic binding proteins and in particular histidine-binding protein as a standard reference, then examined precision and accuracy of affinity prediction at multiple concentrations and exposure times. By analyzing several structural and comparative scattering metrics, we found that the volatility of ratio between titrated scattering curves and a common reference most reliably quantifies ligand-triggered changes. This ratio permits the determination of affinities at low signal-to-noise ratios and without pre-determining the complex scattering, demonstrating that SAXS-based ligand screening is a promising alternative biophysical method for drug discovery pipelines.