Conformation-specific self-assembly of thermo-responsive poly(ethylene glycol)-b-polypeptide diblock copolymer

Poly(ethylene glycol)-block-poly(γ-(2-methoxyethoxy)esteryl-L-glutamate) (PEG-b-poly-L-EG2Glu) was synthesized via ring-opening polymerization (ROP) of L-EG2Glu N-carboxyanhydride (NCA) using PEG-NH2 as macroinitiator. This diblock contained a thermo-responsive poly-L-EG2Glu block, which adopted primarily helical conformation in pristine aqueous solution. We found that PEG-b-poly-L-EG2Glu diblock can display two levels of self-assembly behaviors associated with hydrophobic interactions and conformation-specific reassembly, respectively. Upon temperature increase, the PEG-b-poly-L-EG2Glu diblock formed wormlike micelles, in which the poly-L-EG2Glu formed the micelle core and maintained helical conformation. However, extension of thermal annealing time drove the secondary structure transformation of the poly-L-EG2Glu block from helical conformation to β-sheet, which accounted for an assembly structure transition from wormlike micelles to nanoribbons. The critical factor was that poly-L-EG2Glu block can undergo thermo-induced hydrophobicity and conformation transformation, which offered an additional parameter to tune the nature of molecular interactions, i.e., intermolecular versus intramolecular hydrogen bonding interactions. The corresponding conformation and assembly structure changes were characterized using FTIR and electron microscopy, respectively.