Protein Encapsulation in Nanometric Domains: Insights from the Continuous Phase.

Inside cells, biomolecular function depends on both intrinsic and environmental properties. Oppositely charged macro-ions can induce liquid-liquid phase separation, mimicking intracellular organization mechanism. When one macro-ion is a charged-neutral diblock copolymer, polyelectrolyte complex micelles form, crucial for biomolecule encapsulation as they stabilize nanostructures under varying conditions. However, the molecular understanding becomes more challenging when complex phases are confined within core-shell aggregates at colloidal scales. Here, we investigate protein encapsulation in both bulk-phase and micellar polyelectrolyte complexes, with micelles having core compositions similar to the bulk phase. Our results show that critical salt concentrations for bulk and core systems are similar, indicating comparable interaction strengths and phase behaviors. Using fluorescently labeled polyelectrolytes and proteins, we observed that high protein concentration in the bulk phase leads to multiphase complexes: one formed by oppositely charged polyelectrolytes and another by protein-polyelectrolyte complexes. Light scattering analysis shows that micelle size and stability are tied to core composition, mirroring bulk behavior. These findings suggest that bulk complexes offer valuable insights into the phase behavior and immiscibility in polyelectrolyte complex cores, enhancing our understanding of cellular organization at nanoscale dimensions.