Conformation-specific Self-assembly of Thermo-responsive Poly(ethylene glycol)-b-polypeptide Diblock Copolymer

Junyang Shen, Chongyi Chen, Wenxin Fu, Linqi Shi *, and Zhibo Li *

Langmuir, 2013, 29 (21), 6271–6278

Poly(ethylene glycol)-block-poly(gamma-(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 beta-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.