Self-assembly of globular protein-polymer diblock copolymers

Abstract

Self-assembly of protein-polymer block copolymers provides a simple bottom-up approach towards protein nanopatteming for the fabrication of more effective and efficient bioelectronic and biocatalytic devices. Changes in shape and surface chemistry between proteinpolymer conjugates and classical coil-coil block copolymers result in significant differences between the self-assembly of these two classes of molecules. A model material is used to explore the self-assembly behavior of globular protein-polymer block copolymers as well as investigate protein functionality, stability, and secondary structure in the resulting nanostructured materials. Across a wide range of polymer coil fractions from 0.21 to 0.82, a variety of morphologies including hexagonally packed cylinders, lamellae, perforated lamellae, weakly ordered nanostructures and a disordered phase are observed. Surprisingly, a lyotropic re-entrant order-disorder transition is observed in all materials between 30 and 70 wt% indicating the solvent-mediated effective interaction potential is non-monotonic with concentration. Solid state materials are prepared through evaporation of aqueous solvent, which leads to the formation of kinetically determined nanostructured morphologies. The type of nanostructure is strongly determined by the solvent quality for the polymer block. Good solvents produce well-ordered nanostructures similar to those observed in coil-coil block copolymers, while poor solvents produced an aggregated micellar structure. Importantly, protein secondary structure remains largely unaltered, even in a completely dehydrated environment. As much as 80% of the protein solution functionality is retained in these solid state materials. This quantity depends primarily on the processing conditions, but also the polymer fraction, with ambient temperatures and materials composed of 45-60% polymer retaining the highest levels of protein functionality. Interestingly, there exists some fraction of protein functionality which is reversibly lost in the solid state and regained upon rehydration. The addition of small molecule osmolytes is demonstrated to eliminate this reversible loss and improve protein functionality retention up to 100% in the solid state. Osmolytes with a high glass transition temperature are capable of increasing the thermal stability of dehydrated films by 15 °C, while those with a low glass transition temperature decrease it.