Effect of excluded-volume and hydrophobic interactions on the partition of proteins in aqueous micellar two-phase systems composed of polymer and nonionic surfactant

The partitioning behaviors of several model proteins were investigated in aqueous micellar two-phase systems (AMTPS) composed of polyethylene glycol (PEG) with different molecular weight (4,000, 8000 and 20,000) and nonionic surfactant (TX-100), respectively. The influences of proteins molecular size or hydrophobicity, tie line length (TLL), compositions concentrations and PEG molecular weight on the partition coefficients of proteins were extensively studied. In order to evaluate the contributions of excluded-volume interaction and hydrophobic interaction to proteins partitioning in PEG/TX-100 AMTPS in detail, firstly, the various proteins partition coefficients in PEG/Na2SO4 aqueous two-phase systems (ATPS) and TX-100/Na2SO4 AMTPS were investigated for the cross-sectional radius fitting in PEG-rich phase and TX-100-rich phase, respectively. The cross-sectional radius fitting results embodied that the order of cross-sectional radius in various phases are PEG4,000-rich phase, PEG8,000-rich phase, TX-100-rich phase and PEG20,000-rich phase. Secondly, the quantitative assessment of excluded-volume and hydrophobic contributions to proteins partitioning in PEG/TX-100 AMTPS were further investigated using the cross-sectional radius in PEG-rich phase and TX-100 phase, respectively. It was indicated that the excluded-volume interaction was stronger than hydrophobic interaction for proteins partitioning in PEG/TX-100 AMTPS. Finally, the effect of hydrophobic interaction on proteins partitioning was also analyzed in detail. The intrinsic hydrophobicity (P0) and the hydrophobic resolution (R) of PEG/TX-100 AMTPS with various TLL were assessed according to the model proteins partitioning and proteins' surface hydrophobicity. For proteins partitioning in PEG/TX-100 AMTPS, the molecular weight of PEG played an important role in excluded-volume interaction, not in hydrophobic interaction.