Nonmonotonic Diffusion-Localization-Diffusion Transitions of Charged Macromolecules Confined in Hydrogels

Controlling diffusion and retention of biomacromolecules in congested aqueous media is vital for various living processes. When charged macromolecules are trapped inside a hydrogel, the molecules undergo diffusion only if the degree of confinement is either weak or very strong; for intermediate confinements, the center of mass of the macromolecules does not diffuse, but the molecules exhibit glass-like hierarchical dynamics even though the medium is essentially water. Using dynamic light scattering on poly(styrenesulfonate) embedded inside Tetra-PEG hydrogels, we have investigated this enigmatic nonmonotonic diffusion behavior and present a comprehensive dynamical state diagram. After determining the nature of the nondiffusive state at intermediate confinements, we have determined the confinement thresholds for its transition into diffusion when the degree of confinement is either decreased or increased away from the initial confinement. After crossing the higher confinement threshold, the molecules are diffusive, with the coexistence of two populations obeying Rouse dynamics and reptation dynamics. The other threshold condition to fully switch off the nondiffusive state is when the chain size is smaller than the gel mesh size. Our experimental findings are buttressed by the theoretical calculation of the free energy barrier for localizing the nondiffusive state at intermediate confinements. The presented results on the transitions from diffusion to nondiffusion to diffusion again as the degree of confinement progressively increases may reveal the mechanism of how biomacromolecules move toward the targets in the congested aqueous media and provide strategies to design scaffolds for controlled retention and release of macromolecular cargos in crowded aqueous media.