Polymer-induced phase separation and crystallization in immunoglobulin G solutions

Area of Science:

• Biophysics
• Physical Chemistry
• Materials Science

Background:

• Understanding protein phase behavior is crucial for crystallization and therapeutic formulation.
• Nonglobular proteins like immunoglobulin G (IgG) exhibit complex phase diagrams.
• Polymer additives and ionic strength are key factors influencing protein solution properties.

Purpose of the Study:

• To investigate the influence of polymer additive size and ionic strength on the phase behavior of immunoglobulin G (IgG).
• To develop predictive models for polymer-induced protein phase separation.
• To provide practical guidelines for optimizing protein crystallization conditions.

Main Methods:

• Utilized a four-site model to represent the shape of IgG.
• Employed Derjaguin-Landau-Verwey-Overbeek (DLVO) and Asakura-Oosawa depletion potentials to model protein interactions.
• Calculated liquid-liquid and fluid-solid equilibria using Gibbs ensemble Monte Carlo and Gibbs-Duhem integration (GDI).
• Determined absolute Helmholtz energy for initial coexisting point calculations.

Main Results:

• Discovered a nonmonotonic relationship between critical polymer concentration and polymer-to-protein size ratio (q).
• Developed a predictive equation for the minimum polymer concentration required for liquid-liquid phase separation: rho(PEG)(*) approximately [q(1+q)(3)].
• Observed that liquid-liquid phase separation transitions from metastable to stable with increasing polymer molecular weight (q).
• Identified small polymer sizes as essential for inducing protein crystallization.

Conclusions:

• Polymer size is a critical parameter controlling protein phase separation and crystallization.
• The study provides a theoretical framework and practical insights for manipulating protein phase behavior.
• Findings offer guidance for selecting appropriate polymer additives and ionic strengths in protein crystallization processes.