Thermodynamic nonideality in macromolecular solutions. Evaluation of parameters for the prediction of covolume effects.

Second virial coefficients and hence covolumes for self-interaction of five proteins, viz. ribonuclease, ovalbumin, bovine serum albumin, catalase and alpha-crystallin, have been determined by analyzing the concentration dependence of the partition coefficient obtained from frontal chromatographic studies on either Fractogel TSK HW55 or porous glass beads. The resulting estimates of the effective radii essentially duplicate their Stokes counterparts and thereby provide further justification for assuming the approximate identity of the thermodynamic and hydrodynamic radii of hydrated globular proteins. Gel chromatographic evaluation of second virial coefficients for protein/dextran systems has led to elimination of the sphere/sphere model as a valid thermodynamic description of the space-filling effects in protein/polymer mixtures, since it does not predict the observed independence of covolume, expressed per unit mass of polymer, upon size of the polymer. This requirement is met by the sphere/rod model [Edmond, E. & Ogston, A. G. (1968) Biochem. J. 109, 569-576] and also by the sphere/flexible-segment model [Hermans, J. (1982) J. Chem. Phys. 77, 2193-2203]. Furthermore, similar studies of the effect of solute radius on covolume for interaction with dextran T70 attest to the adequacy of either model for predicting the thermodynamic nonideality arising from the inclusion of dextrans in protein solutions, and also provide the relevant calibration of the model.