Self-Assembly of globular protein-block-polymer block copolymers

Group members working on this project: Carla Thomas

Enzymes function as complex natural catalysts whose functions have been fine tuned resulting in high turnover rates, substrate specificities, and efficiencies. These characteristics make enzymes ideal candidates for incorporation into devices such as catalysts and sensors for applications in biofuel cells, carbon sequestration, carbon dioxide reduction, nerve agent decontamination, and pharmaceutical production.

One of the key challenges with harnessing the power of enzymes is incorporating them into functional materials. High enzyme densities are required in order to achieve relevant catalytic rates, and proper protein orientation must be maintained while maximizing enzyme stability. Nanopatterning of enzymes can provide the necessary control over the three dimensional device geometry required to achieve these goals. Our approach is to use block copolymer self-assembly to produce these nanopatterns. Block copolymers are known to self-assemble into a variety of morphologies on the length scale of 10-100 nm depending on the interaction parameter between the two blocks and the relative coil fraction. Protein-polymer block copolymers are synthesized through site-specific bioconjugation. We chose to work with a model system composed of the red fluorescent protein, mCherry, and the thermoresponsive polymer, poly(N-isopropyl acrylamide) (PNIPAM).

We recently demonstrated that bulk samples of these protein-polymer block copolymer materials are capable of self-assembly. Hexagonal cylinders, perforated lamellae, lamellae, or hexagonal and disordered micellar phases are observed depending upon the coil fraction of the block copolymer and the kinetic pathway used for self-assembly.

Proteins have specific sequences, are perfectly monodisperse, and fold into complex shapes.

TEM micrograph showing lamellar self-assembly of the red fluorescent protein mCherry and poly(N-isopropyl acrylamide). The protein domains appear dark due to staining with ruthenium tetroxide. Inset contains overlaid cartoon images depicting the bilayer configuration within the self-assembled nanostructures.

Good solvents for the polymer block produce ordered structures reminiscent of coil-coil diblock copolymers, while an unfavorable solvent results in kinetically trapped micellar structures. Decreasing solvent quality for the protein improves long-range ordering, suggesting that the strength of protein interactions influences nanostructure formation.

Proteins have specific sequences, are perfectly monodisperse, and fold into complex shapes.

Schematic depicting different pathways towards self-assembly which become accessible by using different types of solvents.

Exploring the Interactions Governing Globular Protein-Polymer Block Copolymer Self-Assembly

Group members working on this project: Chris Lam

Block copolymers comprising two synthetic Gaussian coil blocks are known to self-assemble into different nanostructures, and they have been well studied, both experimentally and theoretically. Replacing one of the coil blocks with a globular protein results in a protein-polymer diblock, the self-assembly of which has not been explored in concentrated solutions or solid-state materials. To complement Carla Thomas' studies of the self-assembly of mCherryS131C-b-PNIPAM bioconjugates, my project aims to explore the nature of the interactions that lead to self-assembly—is it protein-protein, polymer-polymer, protein-polymer interactions, or some combination—and to quantify them. One powerful technique of understanding these interactions involves the use of small-angle X-ray scattering (SAXS) to measure the order-disorder transition concentration behavior. For mCherryS131C-b-PNIPAM, it is observed that the ODTC at temperatures below the lower critical solution temperature (LCST) of the bioconjugate reaches a minimum near fPNIPAM = 0.50. Because water is a good solvent at low temperatures, the ODTC minimum near the symmetric composition signifies that repulsive mCherry-PNIPAM interactions account for the largest contribution to the net protein-polymer repulsion leading to self-assembly. To quantify each pairwise-interaction, static light scattering (SLS) is used to measure the self-virial coefficients of mCherry and PNIPAM and their cross-virial coefficient.

Order-disorder transition concentration SAXS patterns.

SAXS scattering patterns for temperature sweeps of mCherryS131C-b-PNIPAM17k at (a) 20 wt %, (b) 40 wt %, and (c) 50 wt % showing ODTC behavior.