Figure 1.

Visual representation of the radius of gyration and hydrodynamic radius. (Left) The radius of gyration (R_g) of an object can be calculated as the root mean-square distance between each point in the object and its center of mass. Thus, for a protein, it directly reports on the typical distance between an atom and the center of mass of the protein. In the case of a solid sphere, $R_{\text{g}}=r\sqrt{3/5}$. (Right) The Stokes radius or hydrodynamic radius (R_h) of a solute is the corresponding radius of a hard sphere that diffuses at the same rate as that solute. D_t is the translational diffusion coefficient.