Calorimetric investigation of the protein–flexible chain polymer interactions and its relationship with protein partition in aqueous two-phase systems

The binding of polyethyleneglycol of molecular mass 1000, 3300 and 6000 and polyethylene–propylene oxide (molecular mass 8400) to lysozyme and ovoalbumin was measured by isothermal calorimetric titration. A binding process was found to be associated with a saturation effect, which suggests a protein–polymer interaction. The proteins showed an affinity for the polymers in the order of 102 M−1 and it decreased with the increase in the polymer molecular mass. The number of polymer molecules bound per protein molecule varied from 0.01 to 0.2 for polyethyleneglycol 1000, 3300 and polyethylene–polypolypropylene oxide 8400, while for polyethyleneglycol 6000 such number got closer to the unity. The enthalpic change associated with the binding was positive in the order of 1 kcal/mol for lysozyme, while ovoalbumin showed values around 2–3 kcal/mol. Entropic changes were also positive with values around 17–20 e.u. for ovoalbumin and 1–7 e.u. for lysozyme. The heat associated with the protein transfer from a buffer to a medium containing the polymer or the salt (a process similar to protein partitioning in aqueous two-phase systems) was obtained. These results allow the direct calculation of the enthalpic change associated with a protein partition process in aqueous two-phase systems without applying the van’tHoff equation. In this way, it is possible to calculate the associated true heat when the protein is transferred from the bottom to the top phase.