Phase transitions in concentrated solution self-assembly of globular protein–polymer block copolymers

Abstract

The phase behaviour of mCherry-b-PNIPAM (mChP) block copolymers with four different PNIPAM coil fractions is investigated in concentrated aqueous solution as a function of both concentration and temperature, demonstrating both order–order transitions (OOTs) and order–disorder transitions (ODTs) in globular protein–polymer block copolymers. Independent of coil volume fraction from 0.25 to 0.70, the temperature–concentration phase diagrams share several common features. At low concentrations, mCherry-b-PNIPAM forms a homogeneous disordered phase, and macrophase separation into an ordered conjugate-rich phase and a solvent-rich phase is observed at temperatures above the PNIPAM thermoresponsive transition temperature. mChP solutions are also observed to undergo a low-temperature ODT driven by increasing concentration. The order–disorder transition concentration (ODTC) behaviour of mChP is minimized for symmetric conjugates, suggesting that repulsive solvent-mediated protein–polymer interactions provide a driving force for self-assembly. Both coil fraction and solvent selectivity have large effects on the morphologies formed—disordered micelles, hexagonally packed cylinders, lamellae, and perforated lamellae are identified with the combination of small-angle X-ray scattering (SAXS), depolarized light scattering (DPLS), turbidimetry, and differential scanning calorimetry (DSC). An OOT is observed upon increasing temperature for three of the studied coil fractions at concentrations of 40–50 wt% due to changing solvent selectivity. SANS contrast-matching experiments show that water is weakly selective for PNIPAM at low temperatures and strongly selective for mCherry at high temperatures.