SAXS
Also known as: SAXS, small-angle scattering
Small-Angle X-ray Scattering (SAXS) is a solution-phase X-ray scattering technique that measures the overall shape and size of macromolecules and nanoparticles by analyzing scattering intensity at low angles (0.1-10 degrees). Developed by Kratky and colleagues in the 1950s, SAXS provides information about molecular radius, aggregation state, and overall shape without requiring crystallization or fixing, making it ideal for studying native protein conformations and dynamics.
Key highlights
- Native conditions: measurements in physiological buffers and temperatures preserve native structures
- Label-free: no isotopic labeling or dyes needed
- Conformational flexibility: can detect mixtures of conformations and estimate exchange rates
- Non-destructive: samples can be recovered and used for other measurements
Intuition
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How it works
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When to use it
Apply SAXS to determine the shape and size of molecules in solution: proteins, protein complexes, RNA, DNA, nanoparticles, and colloids. SAXS is ideal for flexible molecules and complexes where crystallography fails. SAXS can detect conformational changes upon ligand binding or pH change. Requires milligram quantities of pure sample.
Strengths & limitations
- Native conditions: measurements in physiological buffers and temperatures preserve native structures
- Label-free: no isotopic labeling or dyes needed
- Conformational flexibility: can detect mixtures of conformations and estimate exchange rates
- Non-destructive: samples can be recovered and used for other measurements
- Low resolution: SAXS provides overall shape, not atomic detail
- Shape ambiguity: multiple different molecular shapes can fit the same SAXS curve
- Polydispersity: heterogeneous samples (aggregates, dissociation) confound interpretation
- Radiation damage: X-rays can cause oxidative damage to proteins during measurement
Common pitfalls
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Applications
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Frequently asked
What is the radius of gyration (Rg) measured by SAXS?
Rg is the root-mean-square distance of all atoms in a molecule from its center of mass. Compact, folded proteins have low Rg; extended, unfolded proteins have high Rg. From SAXS, Rg is extracted using Guinier analysis at the smallest angles where the data are linear in q².
Can SAXS determine absolute molecular weight?
Yes, by comparing the scattering intensity at zero angle (the forward scattering intensity I(0)) to that of a standard. Alternatively, if the sample concentration is known, molecular weight can be calculated from I(0) using protein-specific parameters.
How does SAXS compare to dynamic light scattering (DLS)?
DLS measures diffusion coefficients and reports hydrodynamic radius; SAXS measures scattering and reports radius of gyration. Both are complementary; together they constrain molecular shape. SAXS provides more detailed shape information; DLS is faster and requires less sample.
Sources
- 1.Glatter, O., & Kratky, O. (1982). Small Angle X-ray Scattering. Academic Press.
- 2.Koch, M. H., Vachette, P., & Svergun, D. I. (2003). Small-angle scattering: a view on the properties, structures and structural changes of biological macromolecules in solution. Quarterly Reviews of Biophysics, 36(2), 147-227.
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Cite this page
ScholarGate. (2026, June 3). SAXS. ScholarGate. https://scholargate.app/spectroscopy/saxs