PROTEIN PARTITIONING IN 2-PHASE AQUEOUS POLYMER SYSTEMS .2. ON THE FREE-ENERGY OF MIXING GLOBULAR COLLOIDS AND FLEXIBLE POLYMERS

The free energy of mixing globular colloids (spheroidal and ellipsoidal) and singly-dispersed flexible and linear polymer chains in an incompressible solvent (continuum) is formulated. The deformable and penetrable nature of the polymer coils (apparent in both polymer-polymer and polymer-colloid interactions), as well as the rigid and impenetrable nature of the globular colloids, is incorporated in evaluating the contributions of repulsive steric and short-ranged attractive interactions to the free energy of mixing. The interaction potentials, evaluated by combining a Monte Carlo scheme and the polymer solution theories of Flory and Flory and Krigbaum, are cast in terms of effective hard-sphere potentials (which are nonadditive) in order to evaluate the osmotic pressure and subsequently derive the chemical potentials of the colloid and the polymer at constant temperature and pressure. With an increase in polymer molecular weight, the increased deformability and penetrability of the polymer coils to the colloid can lead to qualitatively different trends in the molecular weight dependence of the chemical potential of the colloid, as compared to the case where the polymer coil is treated as a rigid body characterized by its radius of gyration. The influence of a weak and short-ranged attraction between the polymer-coil segments and the colloids on the predicted thermodynamic properties also reflects the increasing diffuseness of the polymer coils with increasing polymer molecular weight. This theoretical framework is used to predict the partitioning behavior of globular proteins in a two-phase aqueous poly(ethylene oxide) (PEO)-dextran system. To predict experimentally observed trends in protein partitioning behavior with increasing PEO molecular weight, the inclusion of both repulsive steric and weak attractive interactions is necessary. The strength of the attractive interaction between a polymer-coil segment and the protein molecule appears to increase with protein size up to approximately 0. 1 kT, although the overall interaction (which includes the steric interaction) remains repulsive. The effect of shape asymmetry on the predicted partitioning behavior of an ellipsoidal bovine serum albumin protein molecule is evaluated and found to be different from that corresponding to a spherical protein having the same volume. This observation suggests that protein shape (conformation) can be a significant factor in determining the partitioning of proteins in two-phase aqueous polymer systems.