Physics of Protein-Polymer Hybrid Block Copolymers and Complexes
Bradley D. Olsen
Department of Chemical Engineering, Massachusetts Institute of Technology
The self-assembly of proteins into nanostructured materials provides an attractive method for the
fabrication of a wide variety of new functional materials, including tissue engineering gels, biosensors,
biocatalysts, fuel cells, and controlled release materials. Although proteins are amino acid polymers,
their sequence specificity and perfectly monodisperse composition impart them with complex
hierarchical structures and specific interactions that are not observed in synthetic polymers. Using a
diverse set of model materials, we show how these complex shapes and interactions affect selfassembly in protein-polymer hybrids and the dynamics of these molecules.
Rod-coil polymers containing alpha-helical polypetides represent one important category of biohybrid
materials. Despite continued interest in the thermodynamics of rod-coil block copolymers for functional
nanostructured materials, relatively few studies have investigated the dynamics of these systems which
are important for understanding diffusion, mechanics, and self-assembly kinetics. Using molecular
dynamics simulation, we have demonstrated that the mismatch between the curvature of the rod and
coil blocks results in dramatically slower reptation through the entanglement tube. For rod lengths near
the tube diameter, this hindered diffusion is explained by a local curvature-dependent free energy
penalty produced by the curvature mismatch, resulting in a rough energy surface as the rod moves along
the tube contour. Compared to coil homopolymers which reptate freely along the tube, rod-coil block
copolymers undergo an activated diffusion process which is considerably slower as the rod length
increases. For large rods, diffusion of the rod through the tube only occurs when the coil blocks occupy
straight entanglement tubes, which requires ``arm retraction'' as the dominant relaxation mechanism.
In a second example, the self-assembly of globular protein-polymer diblock copolymers into
nanostructured phases is demonstrated as an elegant method for control of nanostructure in protein
materials. The red fluorescent protein mCherry was site-specifically conjugated to a low polydispersity
poly(N-isopropyl acrylamide) (PNIPAM) block using thiol-maleimide coupling was developed to form a
well-defined model globular protein-polymer diblock to explore the fundamental principles guiding selfassembly. Functional protein materials are obtained by solvent evaporation and solvent annealing
above and below the lower critical solution temperature of PNIPAM in order to access different
pathways toward self-assembly. Small angle x-ray scattering and microscopy are used to identify the
type of nanostructures formed as a function of processing conditions and the coil fraction of the diblock
copolymer. UV-vis spectroscopy shows that a large fraction of the protein remains in its folded state
after conjugation, and wide angle x-ray scattering demonstrates that diblock copolymer self-assembly
occurs without protein crystallization.
Templating of proteins into polymers represents an alternative approach to guide self-assembly,
producing encapsulated protein materials that have the potential for significantly enhanced stability.
Using ionic-nonionic block copolymers, we demonstrate for the first time the ability to template
proteins within block copolymers using all-aqueous self-assembly. Ionic interactions between a protein
and a polycationic polymer are used to drive partitioning within a protein matrix, which can be
processed in a manner analogous to traditional block copolymers. These films exhibit both pH and
temperature-dependent release properties. Even in the dehydrated state, approximately 80% of the
protein function is retained.