Affinity Partitioning of Proteins

Biopolymers can be separated by partitioning between two aqueous phases generated by two polymers dissolved together in water (1 ,2 ). The partitioning of proteins and nucleic acids between the two phases may be affected by changing the concentration of polymers, usually dextran and polyethylene glycol (PEG), and by including various salts and adjusting the pH value of the system (1 ,3 ). A more effective way to adjust the partitioning and also to strongly increase the selectivity in the partitioning of biopolymers has been to bind charged groups, hydrophobic groups, or affinity ligands to one of the polymers that localizes the attached groups to the phase enriched in this polymer (4 –6 ). Mainly by using a variety of methods, affinity ligands have been bound to PEG, concentrated in the top phase (7 ). However, dextran has been used also as ligand carrier for affinity partitioning (8 ). Affinity partitioning can be used both for single-step extractions (9 ) and for countercurrent distribution (10 ). The two-phase systems can be applied in chromatographic processes by adsorbing one of the phases to a matrix and using the other one as the mobile phase (11 ). Besides the extraction and separation of proteins (6 ,12 ) and nucleic acids (13 ), affinity partitioning also has been used for fractionation of particulate biomaterials such as membranes (14 –16 ).