Figure 1.

Schematic of a typical small-angle X-ray scattering (SAXS) experiment. An X-ray beam from either a synchrotron or home/lab source is incident on a capillary containing a solution of macromolecules. The primary sample is typically a buffered solution containing protein, nucleic acid, or protein-nucleic acid complex at dilute concentrations. To avoid radiation damage from the X-ray beam, the sample is typically oscillated or flowed continuously. The scattered X-rays are imaged onto an area detector while the primary beam is either blocked or greatly attenuated by a beamstop. These images are then pooled, averaged, and azimuthally integrated to obtain SAXS profiles of intensity as a function of scattering vector, $I(0)$. For each biomolecule containing sample, a corresponding measurement of the buffer is made, and the resulting buffer profile is subtracted from the biomolecule sample profile to obtain the SAXS scattering profile from the macromolecule. SAXS intensities can be calibrated on an absolute scale.