Polymer Nanoparticle Hydrogels with Autonomous Affinity Switching for the Protection of Proteins from Thermal Stress

Non-Thermoresponsive Decanano-sized Domains in Thermoresponsive Hydrogel Microspheres Revealed by Temperature-Controlled High-Speed Atomic Force Microscopy

Despite the tremendous efforts devoted to the structural analysis of hydrogel microspheres (microgels), many details of their structures remain unclear. Reported in this study is that thermoresponsive poly(N-isopropyl acrylamide) (pNIPAm)-based microgels exhibit not only the widely accepted core-shell structures, but also inhomogeneous decanano-sized non-thermoresponsive spherical domains within their dense cores, which was revealed by temperature-controlled high-speed atomic force microscopy (TC-HS-AFM). Based on a series of experiments, it is concluded that the non-thermoresponsive domains are characteristic for pNIPAm microgels synthesized by precipitation polymerization, and plausible structures for microgels prepared by other polymerization techniques are proposed.

Selective Uptake and Refolding of Globular Proteins in Coacervate Microdroplets

Intrinsic differences in the molecular sequestration of folded and unfolded proteins within poly(diallyldimethylammonium) (PDDA)/poly(acrylate) (PAA) coacervate microdroplets are exploited to establish membrane-free microcompartments that support protein refolding, facilitate the recovery of secondary structure and enzyme activity, and enable the selective uptake and exclusion of folded and unfolded biomolecules, respectively. Native bovine serum albumin, carbonic anhydrase, and α-chymotrypsin are preferentially sequestered within positively charged coacervate microdroplets, and the unfolding of these proteins in the presence of increasing amounts of urea results in an exponential decrease in the equilibrium partition constants as well as the kinetic release of unfolded molecules from the droplets into the surrounding continuous phase. Slow refolding in the presence of positively charged microdroplets leads to the resequestration of functional proteins and the restoration of enzymatic activity; however, fast refolding results in protein aggregation at the droplet surface. In contrast, slow and fast refolding in the presence of negatively charged PDDA/PAA droplets gives rise to reduced protein aggregation and misfolding by interactions at the droplet surface to give increased levels of protein renaturation. Together, our observations provide new insights into the bottom-up design and construction of self-assembling microcompartments capable of supporting the selective uptake and refolding of globular proteins.