Abstract: Protein-based materials show a great deal of potential as catalysts, sensors, and optoelectronics, where the unique efficiency, selectivity, or activity of enzymes can be captured to improve the performance of these devices.  However, careful 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 bioelectronics.  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, a mutant of the red fluorescent protein mCherry was expressed in E. coli and site-specifically conjugated to a low polydispersity poly(N-isopropyl acrylamide) (PNIPAM) block using thiol-maleimide coupling to form a well-defined model globular protein-polymer diblock copolymer.

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 X-ray 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.  In addition, comparison of mCherry-b-PNIPAM diblocks with diblocks that incorporate green fluorescent protein (GFP-b-PNIPAM) and block copolymers containing polyester-based synthetic polymers 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.

Bio:  Bradley Olsen is the Raymond A. and Helen E. St. Laurent Career Development Assistant Professor in the Department of Chemical Engineering at MIT.  He earned his S.B. in Chemical Engineering at MIT, his Ph.D. in Chemical Engineering at the University of California – Berkeley, and was a postdoctoral scholar at the California Institute of Technology.  He started as an assistant professor at MIT in December 2009.  Olsen’s research expertise is in materials chemistry and polymer physics, with a particular emphasis on molecular self-assembly, block copolymers, polymer networks and gels, and protein biomaterials.  In addition to his academic appointments, Olsen has worked at Dow Chemical and W.R. Grace Construction products on the development of extruded polymer foams and pressure-sensitive adhesives, respectively.   He has been recognized with a Hertz Fellowship, Tau Beta Pi Fellowship, Beckman Postdoctoral Fellowship, NIH Postdoctoral Fellowship, the American Physical Society Division of Polymer Physics/UK Polymer Physics Group Exchange Lectureship for Young Investigators, and the Air Force Young Investigator Award.

     Olsen’s research interests focus on engineering new biofunctional and bioinspired materials and understanding the novel polymer physics required to control the nanoscale structure and properties of these complex systems.  To do this, his group applies cutting-edge polymer chemistry and protein engineering to synthesize new materials at the interface of biology and the physical sciences.  To intelligently design such systems, they investigate the relationships between molecular structure and self-assembly, applying concepts from block copolymer assembly and polymer gels to understand complex biohybrid materials.  Efforts are aimed at enabling the application of highly functional biological components or biological design principles to dramatically extend the capability of soft materials such as solar energy converters, catalysts, and biomedical hydrogels.  Through the study of protein-based systems, his lab also hopes to produce a new sustainable source of functional polymers.