 Analytical ultracentrifugation

Analytical ultracentrifugation observes concentration profiles during rotation, rather than only recovering a separated pellet. Sector-shaped sample cells rotate past optical detectors. Absorbance scans measure attenuation at selected radii. A Rayleigh interferometer compares a sample sector with a solvent reference, converting concentration-dependent refractive-index changes into fringe shifts. Repeating scans follows a sedimentation boundary or the approach to a stationary distribution.^[ref]

For a dilute solute of buoyant mass $m_b$ and translational friction $f_h$, radial force balance gives $s=m_b/f_h$, where $s$ is the sedimentation coefficient. Diffusion supplies $D=k_{B}T/f_h$. A velocity experiment resolves boundary motion and broadening; a sedimentation equilibrium experiment instead measures $d\ln c/d(r^2)=m_b{\omega }^2/(2k_{B}T)$. This follows from the Boltzmann distribution in centrifugal potential $-m_b{\omega }^2{r}^2/2$. The two approaches separate frictional size information from buoyant mass information; shape, hydration and solvent properties must be specified to infer a unique geometric dimension.

Analytical ultracentrifugation (AUC) is a powerful biophysical technique used to study the properties of macromolecules, such as proteins, nucleic acids, and complex mixtures, in solution. It exploits the principles of sedimentation under a high centrifugal field to provide insights into various aspects of these molecules, including their size, shape, mass, and interactions.