Self-Assembly and Biocatalysis with Globular Protein-Polymer Block Copolymers

Brad Olsen from MIT

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.  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 and fusion proteins into nanostructured phases as an elegant and simple method for structural control in biomaterials.  These conjugates or fusions may be conceptualized as diblock copolymers, where the first block is the globular protein and the second block is the synthetic polymer or coil-like protein sequence.

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.  The phase diagram differs significantly from that of traditional block copolymers; the globular protein-polymer conjugates show predominantly hexagonal phases at a coil volume fraction less than 0.5 with lamellar phases above 0.5.  Comparison of copolymers with different polymer blocks shows that new phases are observed when changing polymer chemistry and that the polymer-protein interaction has a strong impact on the order-disorder transition concentration in solution.  Comparisons of structurally similar but chemically different proteins enable the effects of protein shape and protein surface chemistry to be isolated and suggest that monomer-level details of the protein have little impact on self-assembly.  Together, these results begin to lay a foundation for understanding the general principles of self-assembly in block copolymers containing globular proteins.

Self-assembly of the materials in thin films can produce highly functional biocatalysts.  Using a low shear flow-coating process, protein-polymer conjugates may be self-assembled into uniform thin films with thicknesses of 10’s to 100’s of nm.  The type of nanostructure formed and orientation of nanostructures is shown to depend upon the coating condition.  When coatings are prepared from the peroxidase myoglobin, they show a high activity per area, consistent with rapid transport into the films and a high density of active enzyme.  Compared to competing methods for enzyme immobilization or encapsulation in polymer films, block copolymer self-assembly provides a five to ten-fold enhanced activity.