We explore using two-polymer coacervation to encapsulate model proteins and present design rules to guide encapsulation of novel targets.

Protein encapsulation is a growing area of interest, particularly in the fields of food science and medicine. The sequestration of protein cargoes has been achieved using a variety of methods, each with benefits and drawbacks. One of the most significant challenges associated with protein encapsulation has been achieving high loading while maintaining protein viability. This difficulty has been exacerbated because many encapsulant systems require the use of organic solvents. In contrast, nature has optimized strategies to compartmentalize and protect proteins inside the cell-a purely aqueous environment. Although the mechanisms whereby aspects of the cytosol is able to stabilize proteins are unknown, the crowded nature of many newly discovered, liquid phase separated 'membraneless organelles' that achieve protein compartmentalization suggests that the material environment surrounding the protein may be critical in determining stability. Here, we focus on encapsulation strategies based on liquid-liquid phase separation, and complex coacervation in particular, which has many of the key features of the cytoplasm as a material. We review the literature on protein encapsulation via coacervation and discuss the parameters relevant to creating protein-containing coacervate formulations. Additionally, we highlight potential opportunities associated with the creation of tailored materials to better facilitate protein encapsulation and stabilization.

Oppositely charged polyelectrolytes can undergo an associative phase separation, in a process known as polymeric complex coacervation. This phenomenon is driven by the electrostatic attraction between polyanion and polycation species, leading to the formation of a polymer-dense coacervate phase and a coexisting polymer-dilute supernatant phase. There has been significant recent interest in the physical origin and features of coacervation; yet notably, experiments often use weak polyelectrolytes the charge state of which depends on solution pH, while theoretical or computational efforts typically assume strong polyelectrolytes that remain fully charged. There have been only a few efforts to address this limitation, and thus there has been little exploration of how pH can affect complex coacervation. In this paper, we modify a transfer matrix theory of coacervation to account for acid–base equilibria, taking advantage of its ability to directly account for some local ion correlations that will affect monomer charging. We show that coacervation can stabilize the charged state of a weak polyelectrolyte via the proximity of oppositely charged monomers, and can lead to asymmetric phase diagrams where the positively and negatively charged polyelectrolytes exhibit different behaviors near the pK a of either chain. Specifically, there is a partitioning of one of the salt species to a coacervate to maintain electroneutrality when one of the polyelectrolytes is only partially charged. This results in the depletion of the same salt species in the supernatant, and overall can suppress phase separation. We also demonstrate that, when one of the species is only partially charged, mixtures that are off-stoichiometric in volume fraction but stoichiometric in charge exhibit the greatest propensity to form coacervate phases.

Widespread vaccine coverage for viral diseases could save the lives of millions of people each year. For viral vaccines to be effective, they must be transported and stored in a...