Self-assembly of protein-zwitterionic polymer bioconjugates into nanostructured materials

Abstract

The microphase separation of a bioconjugate made of a globular protein and a zwitterionic polymer is studied in order to elucidate the role of charge in the polymer block on the self-assembly of protein–polymer bioconjugates. Zwitterionic polymer surfaces are resistant to nonspecific protein adsoprtion due to strong hydration; however, bioconjugates constructed from a red fluorescent protein, mCherry, and a zwitterionic polymer, PDMAPS, show a relatively narrow range of conditions for self-assembly in concentrated systems. The bioconjugates demonstrate weaker segregation strengths compared to previously studied mCherry–polymer conjugates with non-ionic polymers, as demonstrated by higher order-disorder transition concentrations (C_ODT) and a narrower range of ordered concentrations in the phase diagram. The results suggest that electrostatic segregation of mCherry is one of the main parameters governing the self-assembly of protein–nonionic polymer bioconjugates, and this driving force is perturbed by the zwitterionic polymer. Disruption of ordering upon addition of NaCl confirms that electrostatics play a critical role in the bioconjugate self-assembly. Order–disorder–order transitions are observed with increasing concentration of a kosmotropic salt, ammonium sulphate, due to the initial salt-in followed by salt-out effect, suggesting that stabilization of protein domains by enhancing attractive interactions between proteins can significantly improve long range ordering.