The effective configuration of a molecule in ultrafiltration: Its definition and determination

Summary The time taken to descend through a measured depth of glycerol was recorded for some metal boxes, cylinders, ellipsoids and spheres having axial ratios from 0.2 to 2.4. The results were used to determine the relation between each nonspherical form, its axial ratio, and the radius of its dynamically equivalent sphere with respect to glycerol. Expressed in dimensionless terms, these relations closely predict thereal gross configuration of four regularly shaped molecules (inulin, ribonuclease, hemoglobin and hen egg-white lysozyme), and sensibly predict theeffective gross configuration of one irregularly shaped molecule (Gl immunoglobulin) from their known axial ratios and Stokes-Einstein radii. Similarly, they predict theeffective gross configuration of a molecule whose real shape is not known (serum albumin). Since the relations established for the models apply to all the molecules considered, it seems likely that they can be used in conjunction with membrane-diffusion studies (E. Middleton, 1975,J. Membrane Biol. 20:347) and free-diffusion studies to determine the effective configuration in ultrafiltration of any molecule.