Sedimentation

Abstract Sedimentation is a classic method of biochemistry that provides first‐principle hydrodynamic and thermodynamic information about the purity, size, shape, molar mass, association energy, association stoichiometry and thermodynamic nonideality of molecules in solution. The fundamental measurement in sedimentation is the concentration as a function of radial position. Any of the three optical systems provides the necessary concentration profiles, making sedimentation a versatile tool for analysing biological solutions. There are two distinct sedimentation methods: sedimentation velocity and sedimentation equilibrium. Data analysis from either method uses computer programs developed around fundamental equations. Velocity sedimentation is used to check for impurities and also to characterise molecular interactions. Equilibrium sedimentation is not used as widely, but it is used to provide first‐principle insights into intermolecular interactions such as macromolecular binding and thermodynamic nonideality. Key concepts: Sedimentation velocity provides information about the size and shape of molecules in solution. Sedimentation equilibrium provides information about the interactions of macromolecules in solution. The quaternary structure of a molecule may be determined under different solvent conditions. Mass‐action equilibria may be characterised with respect to the stoichiometry and association energies. Sedimentation velocity analysis is used to detect aggregates and fragments of highly purified molecules. Thermodynamic nonideality may be characterised from sedimentation equilibrium analysis. Analysis programs for sedimentation velocity data often use solutions of the Lamm equation. Analysis programs for sedimentation equilibrium data are based on thermodynamic first principles.