Molecular Weight by SAXS

SAXS detects X-rays scattered at small angles by electron density contrasts between solute molecules and solvent. In dilute solutions of globular macromolecules or proteins, the low-q region of the scattering curve contains information about overall particle size and forward scattering intensity, enabling direct estimation of the apparent molecular weight.

Modern laboratory SAXS instruments can reliably determine molecular weights in the 1 to ~300 kDa range.

SAXS molecular weight determination applies to a wide range of globular macromolecules in solution, especially when the biological or physicochemical function depends on the oligomeric state or conformational integrity.

Typical sample types include:

• Globular proteins under native or buffer conditions
• Antibody fragments and protein complexes, including monomer–dimer equilibria
• Biological nano-objects such as lipoproteins or viral components
• Synthetic macromolecules designed to mimic protein-like scattering behavior

Because measurements occur directly in solution, SAXS preserves native folding, hydration, and interaction states, enabling reliable assessment of molecular weight in physiologically relevant environments.

SAXS provides a robust and solution-compatible approach to determining the apparent molecular weight of macromolecules, complementing other structural and biophysical methods. Its main advantages include:

Additionally, SAXS can be coupled to separation methods like SEC (Size-Exclusion Chromatography) or AF4 (Asymmetric Flow Field-Flow Fractionation), allowing molecular weight determination on purified, monodisperse fractions. This combination minimizes aggregation effects and ensures that the low-q scattering reflects the true molecular species in solution.