Particle or Macromolecular Shape by SAXS

SAXS encodes structural information through the scattering intensity profile, which reflects the internal pairwise distances within the particle. Fourier transformation converts this information into the pair distance distribution function (PDDF), which represents all distances between points inside the particle.

From the PDDF, SAXS provides:

Overall particle shape, distinguishing globular, rod-like, disk-like, or more complex geometries
Characteristic dimensions, such as maximum particle dimension (Dmax), radii, or aspect ratios
Internal symmetry and compactness, inferred from the PDDF profile shape
Presence of extended or flexible domains, visible through long tails or broadened distributions

Figure 1. PDDF obtained from lysozyme solution SAXS data using XSACT Pro, showing a compact globular profile. PDDF is obtained through a Bayesian approach, with no information loss. The user input of auxiliary values such as the maximum distance within the particle (D_max) of the smoothness parameter are not required.

Figure 2. Comparison between the PDDF of a spherical particle (40 nm in diameter) and a cylindrical particle (40 nm length and 4 nm diameter.), illustrating how shape information is encoded in real-space distance distributions.

SAXS shape determination applies to a wide range of nanoscale systems, particularly those that are monodisperse or moderately polydisperse.

Typical samples include:

Protein solutions, including monomeric and oligomeric species
Dilute nanoparticles in aqueous or organic media
Surfactant assemblies and micelles, including block-copolymer micelles
Lipid-based colloidal systems such as liposomes or vesicles
Polymeric or inorganic nano-objects stable in suspension

Because researchers perform SAXS measurements in solution or in soft-matter environments, they preserve the native, hydrated structure of the particle.

Ensemble-averaged statistical relevance

SAXS probes millions of particles within the illuminated volume, ensuring ensemble-averaged statistical relevance.

Non-destructive and label-free analysis

This preserves the sample’s native state.

Measurements under physiological or formulation conditions

Enabling shape determination in buffers, solvents, surfactant environments, or other realistic media.

Compatibility with flexible or partially disordered systems

Where high-resolution crystallography or microscopy may be limited.

Straightforward workflow

Allowing shape determination from a single scattering curve without the need for crystallization or staining.

Particle or Macromolecular Shape by SAXS

What is measured?

Samples