ABSTRACT
The concept of the hydrodynamic particle is necessary if specific dimensions are to be extracted from physical measurements of the shapes of molecules of protein dissolved in free solution. A molecule of protein dissolved in an aqueous solution of moderate ionic strength is a compact solid of peculiar shape coated with a layer of water that is more or less fixed upon its surface and that has the effect of smoothing its roughness. Molecules of protein are neither prolate nor oblate ellipsoids of revolution, but exact solutions to the hydrodynamic equations are available only for these shapes. The frictional coefficient of a molecule of protein can also be determined by sedimentation velocity. The radius of gyration of the molecule of protein is the molecular parameter that is obtained from the angular dependence of the intensity of the scattered radiation and that provides information about the shape of the molecule of protein.
