Well-characterized and safe vaccine adjuvants and delivery vehicles are recognized as a key strategic factor in the development of new and improved vaccines. Polyelectrolytes are a class of macromolecules that may associate with antigens by physico-chemical means, such as charge complexation, rather than covalent binding. This offers an elegant route to alter and modify vaccine formulations through aqueous solution mixing. However, such approaches may also lead to unexpected and undesirable solution stability and associative behavior. We will report on an automated titration dynamic light scattering (DLS) method that provides an insightful and rapid characterization of the hydrodynamic radius size distribution of soluble polymer-protein complexes as a function of the stoichiometry of mixing. Poly[di(carboxylatophenoxy) phosphazene] (PCPP) provides a well-defined model immunoadjuvant that forms complexes with antigens. The automated titration DLS method was applied to follow the formation of PCPP complexes with lysozyme across 2 decades in stoichiometry of mixing. An interplay of stable, soluble PCPP complexes with large molar excess of lysozyme was observed and transitioned to a saturated PCPP complex as the stoichiometry of polyelectrolyte to protein increased eventually leading to a transition to multi-chain associates. This phase behavior illustrates a relationship to the more general problem of charge-driven associative phase separation, or polyelectrolyte complex coacervation, that observes liquid-liquid phase separation upon heating. The experimental accessibility of the critical temperature outlines a temperature, salt concentration and polymer concentration coexistence surface. We show how associative behavior within the homogeneous phase enhances the concentration fluctuations upon approach to the phase boundary.

Structure formation in polyelectrolyte mixtures from phase behavior to applications

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ACS Fall 2022

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