What is hydrodynamic radius?
Hydrodynamic radius (Rh), is a parameter used to describe the size of a particle or molecule in a fluid, taking into account its shape, mass, and interactions with the surrounding fluid molecules. The hydrodynamic radius is defined as the radius of a hypothetical sphere that diffuses at the same rate as the particle or molecule in question, under the same conditions (Einstein, 1905).
The hydrodynamic radius is used to describe the behaviour of biomolecules, such as proteins, in solution. Techniques such as flow-induced dispersion analysis (FIDA) are used to determine the hydrodynamic radius experimentally. The hydrodynamic radius is a crucial parameter in drug discovery, biotechnology, and environmental science, where the behaviour of molecules in solution is essential to their function and properties
In practice, the hydrodynamic radius is calculated from the diffusion coefficient of the molecule using the Stokes–Einstein equation:
where D is the diffusion coefficient, kB is the Boltzmann constant, T is the absolute temperature, η is the solvent viscosity, and Rh is the hydrodynamic radius. This relation links molecular motion in a liquid to its effective size.
Importantly, the hydrodynamic radius is not always the same as the geometric radius. It reflects how a molecule behaves in solution, which can be influenced by hydration layers, conformational flexibility, oligomerization, or interactions with other molecules. As a result, Rh is often considered a more physiologically relevant parameter than molecular weight alone.
Because of these features, hydrodynamic radius is widely applied in the study of protein folding, aggregation, stability, molecular interactions, and formulation science, where solution behavior must be understood in detail.
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Einstein, A. (1905). Über die von dermolekularkinetischen Theorie der Wärme geforderte Bewegung von in ruhendenFlüssigkeiten suspendierten Teilchen. Annalen Der Physik, 322(8),549–560. https://doi.org/10.1002/andp.1905322080679
