Self-Assembly of Globular Protein-Polymer Diblock Copolymers

Protein-based materials show a great deal of potential as catalysts and optoelectronics, where the unique efficiency, selectivity, or activity of enzymes can be captured to improve material performance.  The self-assembly of globular protein-polymer diblock copolymers into nanostructured phases is demonstrated as an elegant and simple method for structural control in protein-based biocatalysts or bioelectronics.  In order to fundamentally investigate self-assembly in these complex block copolymer systems, a mutant of the red fluorescent protein mCherry is expressed in E. coli and site-specifically conjugated to a low polydispersity poly(N-isopropyl acrylamide) (PNIPAM) block.  Using thiol-maleimide coupling, a well-defined model globular protein-polymer diblock copolymer is formed.  Nanostructured protein materials are self-assembled by the evaporation of water from concentrated solutions of the diblock copolymer.  Through changes in the temperature of the solution or use of volatile acids and bases, different pathways toward self-assembly may be accessed.  Small angle X-ray scattering and transmission electron microscopy are used to explore the dependence of nanostructure formation on the processing pathway, and solvent annealing was used to achieve more fully equilibrated structures.  The effect of the molecular weight of the PNIPAM block was also investigated, demonstrating the ability to self-assemble a variety of different nanostructured morphologies from globular proteins.  Wide angle X-ray scattering illustrates that diblock copolymer self-assembly results in a noncrystalline structure within the protein nanodomains.  Circular dichroism , UV/Vis spectroscopy, and Fourier transform infra-red (FTIR) spectroscopy show that a large fraction of the protein remains in its folded state after conjugation, and the activity of the protein is explored as a function of processing conditions to identify conditions that maximize the preservation of protein function.