Distribution of proteins in aqueous two-phase systems formed by dextran and polyethylene glycol. Influence of protein hydrophobicity.

Aqueous two-phase systems formed by mixing polyethylene glycol (PEG) and dextran (DXT) were studied. The composition of DXT and PEG phase was established by the size exclusion HPLC method. The concentrations of DXT and PEG in both phases as a function of total concentration of both polymers was fitted by nonlinear regression. This procedure yielded an empirical numerical model (with two independent variables: bulk concentration of DXT and PEG) characterizing the composition of either phase at any applied concentration of the polymers. The distributions of bovine serum albumin (BSA) and immunoglobulin G (IgG) were studied in this system in relation to the composition of both phases. Both proteins were predominantly distributed in the dextran phase. The protein affinity to the respective phase was found to increase with: i) the increasing phase dextran concentration; and ii) the decreasing phase PEG concentration. Chemical modification of primary amino groups of BSA by 2,4,6-trinitrobenzene sulfonic acid (TNBS) raised the affinity of the conjugate to the less polar PEG phase.