Estimating protein partitioning in multicomponent aqueous two-phase extraction based on CG-MALS measurements

Significance of the Topic

The purification of therapeutic proteins demands efficient, scalable and biocompatible extraction methods. Aqueous two-phase systems (ATPS) composed of polymers and salts represent a versatile alternative to traditional organic aqueous extractions due to their high water content, mild conditions and ease of scale-up. However, selecting optimal phase-forming components and displacement agents typically requires extensive experimental screening. Integrating osmotic virial coefficient measurements via composition-gradient multi-angle light scattering (CG-MALS) offers a predictive approach to streamline ATPS design and minimize trial-and-error.

Study Objectives and Overview

This study aims to demonstrate a method for forecasting immunoglobulin G (IgG) partitioning and precipitation affinity in a polyethylene glycol (PEG)–phosphate ATPS. Key goals include:

• Determining protein self-interaction (B22) and protein–solute cross-interaction (B23) parameters in the presence of NaCl and (NH4)2SO4.
• Predicting IgG displacement and precipitation behavior based on osmotic virial coefficients.
• Validating predictions through ATPS partitioning experiments with varied salt additions.

Methodology and Instrumentation

Protein and solute interactions were characterized by static light scattering using a Calypso automated composition-gradient system coupled to an Optilab differential refractometer and a DAWN multi-angle light scattering instrument.

• B22 (second osmotic virial coefficient) quantifies protein–protein interactions and precipitation affinity via measurements of scattered light intensity and refractive index increments.
• B23 (cross-virial coefficient) evaluates interactions between IgG and phase-forming or displacement solutes by fitting light scattering data to established virial equations.
• B33 for small solutes was inferred from literature osmotic coefficient values when direct measurement was impractical.

Main Results and Discussion

B22 measurements of IgG in 0.05 M phosphate buffer (pH 7, 25 °C) revealed increasingly negative values with rising NaCl or (NH4)2SO4 concentrations, indicating stronger attractive protein–protein forces and higher precipitation risk. (NH4)2SO4 had a more pronounced effect than NaCl. Cross-virial B23 values were higher for salts than for PEG or phosphate, suggesting efficient displacement of IgG from the salt-rich phase. Experimental ATPS tests confirmed these predictions: adding NaCl or (NH4)2SO4 to a 7 % PEG 2000–14 % phosphate ATPS increased the IgG partition coefficient from 0.09 (no salt) up to 38.5 (10 % NaCl) or 1.08 (2 % (NH4)2SO4). Salt enrichment in the phosphate-rich phase effectively drove IgG into the PEG-rich phase while avoiding excessive precipitation when guided by B22 and B23 data.

Practical Benefits and Applications of the Method

This CG-MALS based strategy enables rapid ranking of displacement agents and prediction of protein partition behavior without extensive phase screening. It supports early identification of optimal ATPS compositions, reduces development time and resource consumption, and facilitates integration into downstream processing workflows for therapeutic protein production.

Future Trends and Opportunities for Application

Scaling this approach to different proteins and polymer–salt combinations can further accelerate ATPS process design. Integration with automated screening platforms and advanced modeling could enable virtual optimization of phase systems. Emerging light scattering technologies may expand applicability to smaller solutes or complex biomolecule mixtures, broadening the method’s industrial relevance.

Conclusion

By linking osmotic virial coefficients (B22, B23) obtained via CG-MALS to protein partitioning and precipitation tendencies, this work demonstrates a powerful predictive tool for ATPS development. The approach streamlines displacement agent selection, mitigates precipitation risks and ultimately enhances the efficiency of ATPS-based protein purification.

Instrumentation Used

• Calypso Automated Composition-Gradient System
• Optilab Differential Refractometer
• DAWN Multi-Angle Light Scattering Instrument

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