Controlling nanostructures of globular protein-polymer block copolymers in bulk solutions and in thin films

Abstract

The self-assembly of globular protein-polymer diblock copolymers represents a promising technology for protein nanopatterning. The self-assembled materials have a high density of proteins and internal nanostructures that serve as continuous transport pathways for substrates, products, cofactors, and/or charges. The polymer block can act as a protective matrix for the protein, improving its stability and longevity in materials. The self-assembly of protein-polymer diblock copolymers is substantially different from that of traditional synthetic diblock copolymers due to the globular and rigid shape, heterogeneous composition, and anisotropic interactions of proteins. This thesis focuses on the control of nanostructures in self-assembled materials with a goal to gain a better understanding of the governing principles in self-assembly. This thesis presents experimental studies on the effect of modulated interactions between protein and polymers on the self-assembly of globular protein-polymer block copolymers. Bioconjugates composed of a model red fluorescent protein, mCherry, and a synthetic homopolymer with different chemical moieties are synthesized. Modulated interactions between protein and polymer by introducing polymer blocks with different hydrogen bonding capabilities change order-disorder transition concentrations in solution and the type of nanostructures formed. Bioconjugates with a weakly segregating polymer block are found to form a double gyroid structure with Ia3d symmetry, as opposed to perforated lamellae of bioconjugates with a strongly segregating polymer block. Common phase behaviors are also revealed, including the order of lyotropic order-order transitions and a re-entrant disordering behavior at high concentrations. Birefringence of the disordered solutions with increasing protein fraction suggests the formation of a nematic liquid crystalline phase arising from protein interactions. Self-assembly of proteinzwitterionic polymer bioconjugates shows that electrostatic segregation of mCherry constitutes one of the major driving forces for microphase separation. Nanostructures of the conjugates are further controlled by changing solvent selectivity. Important considerations in preparing bioconjugate thin films are also presented and discussed. Surface effects as well as kinetics such as solvent evaporation rate and film coating speed are shown to have a large impact on the long-range order of self-assembled nanostructures.