Single-chain self-folding in an amphiphilic copolymer: An integrated experimental and computational study

An amphiphilic random copolymer of hydrophilic poly(ethylene glycol) methyl ether methacrylate with hydrophobic perfluorohexylethyl acrylate, PEGMA77-co-FA23, was synthesized by ATRP and used to investigate self-assembling into nanostructures in water and chloroform solutions, both experimentally and computationally. The dynamic light scattering measurements on water solutions of the copolymer at room temperature evidenced the presence of nanoassemblies with hydrodynamic diameter D-h = 4 +/- 1 nm. The behavior of fluorescence emission intensity of water solutions with added ethidium bromide suggested confinement of the molecular rotor within a hydrophobic environment of the copolymer. Moreover, these nanoassemblies were thermoresponsive and reversibly collapsed into much larger, multi-chain aggregates with D-h = 390 +/- 20 nm at a critical temperature of 55 degrees C (5 mg mL(-1)). Molecular dynamics simulations revealed the formation of single-chain, prolate globular nanoassemblies with a structural variability in water solution at room temperature. The evolutions of the simulated radius of gyration (R-g), asphericity, prolateness and solvent-accessible surface area were analyzed along the folding trajectories. Thus, self-folding appeared to result from the interplay between hydrophobic interactions and structural constraints which leads to rather complex nanostructures (R-g = 20-25 angstrom, 200 ns simulation). By contrast, folding in much more open polymer conformations (R-g= 30-40 angstrom) was predicted for chloroform solutions.