The analytical ultracentrifuge remains one of the most reliable tools for studying macromolecular complexes in solution. In sedimentation equilibrium, the sample is spun in the ultracentrifuge until the distribution of macromolecules becomes constant. In the case of a single species, this equilibrium distribution is related to the molecular weight of the molecule under study. Interpretation of the distribution is more involved if there is self-association or more than one macromolecular species present. In either case the measurement is thermodynamically rigorous and requires no “calibration standards”. Sedimentation velocity uses higher speeds than equilibrium and measures the rate at which macromolecules move. These data can help enumerate the number of species present and are a primary source of data on a molecule’s hydrodynamic radius, the Stokes radius. The Stokes radius is dependent on the molecule’s mass, shape, and solvent components that move with the molecule.
The instrument can measure molecular weights from several hundred to millions, and is ideal for the study of weak, noncovalent associations, including both self-association and interactions between different macromolecules.