småvinkelrøntgenspredning

Småvinkelrøntgenspredning (small-angle X-ray scattering, SAXS) is a non-crystalline structure-determination technique that probes nanometer-scale features in materials by measuring elastically scattered X-rays at small angles. The scattered intensity I(q) is recorded as a function of the scattering vector q = (4π/λ) sin(θ/2), where λ is the X-ray wavelength and θ the scattering angle. SAXS is commonly applied to samples in solution, gels, or other disordered media, and provides ensemble-averaged information about size, shape and internal structure in the approximate 1–100 nm range.

Key outputs include the radius of gyration (Rg), the maximum particle dimension (Dmax) from the pair-distance distribution p(r), and the overall low-q behavior. The Guinier region (very small q) yields I(q) ≈ I(0) exp(-q^2 Rg^2/3). The high-q region follows the Porod law for sharp interfaces, with I(q) ∼ q^-4. Kratky plots help assess compactness and flexibility. Data can be analyzed in a model-free way or used to fit atomic or coarse-grained models. Ab initio shape reconstructions (for example, through programs in the ATSAS suite) provide low-resolution envelopes, while tools like CRYSOL or FoXS enable fitting to atomic structures.

Experimental considerations include sample monodispersity, concentration effects, and aggregation, which can distort I(q). Buffer scattering must be accurately subtracted, and radiation damage minimized. SAXS measurements can be conducted at synchrotron facilities or laboratory instruments, usually with capillary or flow-cell geometries and stringent temperature control.

Applications span structural biology (proteins, nucleic acids and complexes in solution), polymers, and nanomaterials. SAXS complements neutron scattering and cryo-electron microscopy by delivering information about shape and size in native-like environments, though it provides lower spatial resolution and may involve model ambiguities.