Quantitative characterization of temperature-independent polymer-polymer interaction and temperature-dependent protein-protein and protein-polymer interactions in concentrated polymer solutions.

To study the effect of non-specific interactions arising from proteins being in a crowded environment on physiological processes, the self-interaction of concentrated Dextran T70 and Ficoll 70 and the interactions between a dilute protein and these polymeric macromolecules were quantified using non-ideal tracer sedimentation equilibrium. Sedimentation equilibria of each polymer were measured between 5 and 37 °C, and sedimentation equilibria of 2 mg cm-3 superoxide dismutase (SOD) in 0-0.1 g cm-3 of each polymer was also measured. Results were analyzed using a model-free thermodynamic virial expression of activity coefficients in terms of the concentration of polymer and a structural model using a statistical thermodynamics approximation. The equilibrium gradients of each of the polymers suggest repulsive interaction, which is independent of temperature. However, the net repulsive interaction between superoxide dismutase (SOD) species and the polymers is dependent on temperature. The ratio of the solvation energy of SOD in Dextran T70 to that in Ficoll 70, lnγSOD (Dex)/lnγSOD (Fic) at the same w/v concentration was about 1.8 at 37 °C, 1.6 at the intermediate temperature, and ranges from 1.2 to 1.6 at 5 °C over the entire concentration range. The enthalpy and entropy of interaction of SOD with dilute Dextran T70 are - 14 kJ mol-1 and - 5.6 J K-1 mol-1 , respectively. For SOD in dilute Ficoll 70, the enthalpy and entropy are - 8.1 kJ mol-1 and 12.9 J K-1  mol-1 , respectively. Thus, Dextran T70 contributes more to the attractive protein-polymer interaction and to self-association of protein than Ficoll 70 and reasons for this are discussed.