(290b) Invited: Protein Nanomaterials From Bioconjugate Block Copolymer Self-Assembly

Protein-based materials show a great deal of potential as biomaterials, catalysts, sensors, and optoelectronics, where the unique efficiency, selectivity, or activity of enzymes can be captured to improve the performance of these devices.  Control over the structure and orientation of the protein in three dimensions is required to improve transport through the devices, increase the density of active sites, and optimize the stability of the protein.  We demonstrate self-assembly of globular protein-polymer conjugates into nanostructured phases as an elegant and simple method for structural control in biomaterials.  These conjugates may be conceptualized as diblock copolymers, where the first block is the globular protein and the second block is the synthetic polymer.  In order to fundamentally investigate self-assembly in these complex block copolymer systems, proteins are site-specifically conjugated to synthetic polymer blocks to form well-defined model globular protein-polymer diblock copolymers. 

Functional protein materials are obtained by solvent evaporation in order to access different pathways toward self-assembly using polymer-selective, non-selective, and protein-selective solvents.  Similarly, solvent annealing using these different conditions is exploited as a means to both improve ordering and explore the thermodynamic stability of the as-cast nanostructures.  Small angle scattering and transmission electron microscopy are used to explore the dependence of nanostructure formation on processing conditions and the molecular weight of the PNIPAM block.  Wide angle X-ray scattering demonstrates 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 and active state after conjugation. The effect of coil fraction and hydrogen bonding additives on maintaining protein activity within nanostructured phases is also explored, demonstrating methods for fabricating structures with both a high protein density and a high fraction of active protein.  The effect of plasticizing additives on thermal and chemical stability was also explored, illustrating the ability of these materials to dramatically enhance the stability of proteins in polymeric materials.

Phase diagrams for these materials have been prepared as a function of coil fraction and water content in the materials, providing insight into the type of self-assembled nanostructures that may be formed.  Small-angle light scattering allows quantitative measurement of solvent-mediated interactions between the different components of the diblock copolymers, enabling a fundamental understanding of the relationship between molecular interactions and self-assembly.  Comparison of copolymers with variable protein and polymer blocks enables the effects of protein shape and protein-protein interactions in these systems to be understood.  Together, these results begin to lay a foundation for understanding the general principles of self-assembly in block copolymers containing globular proteins.