saxs

SAXS stands for Small-Angle X-ray Scattering. It is a technique used to study nanoscale structures in materials and macromolecules in solution or other states. By measuring elastic scattering of X-rays at small angles, SAXS provides information about size, shape, and internal organization of features from roughly 1 to 100 nanometers. The scattering vector q is defined as q = 4π sin(θ)/λ, where 2θ is the scattering angle and λ is the X-ray wavelength.

Samples are exposed to a collimated X-ray beam and the scattered intensity I(q) is recorded. Analysis of I(q) yields low-resolution envelopes of particle shape, size distributions, and, for biological macromolecules, conformational states in solution. SAXS is compatible with native buffers and concentrations that preserve solution structure, making it complementary to crystallography and cryo-electron microscopy.

Key analysis concepts include the Guinier region (very low q) to obtain the radius of gyration Rg; the Porod region for interface and surface information; the pair distance distribution P(r) that provides Dmax; and Kratky plots to assess folding and compactness. Models range from simple form factors (spheres, cylinders) to ab initio shape reconstructions and ensemble modeling.

Experimentally, SAXS requires careful background subtraction, concentration series to check for interparticle interference, and control of radiation damage and temperature. Data can be collected at laboratory X-ray sources or synchrotrons. Limitations include relatively low structural resolution and model dependence; SAXS is commonly used in materials science, soft matter, and structural biology and is often combined with complementary techniques such as SANS.