Effects of Secondary Structures and pH on the Self-Assembly of Poly(ethylene glycol)-b-polytyrosine

Abstract

Different from conventional synthetic polymers, polypeptides exhibit a distinguishing characteristic of adopting specific secondary structures, including random coils, α-helixes, and β-sheets. The conformation determines the rigidity and solubility of polypeptide chains, which further direct the self-assembly and morphology of the nanostructures. We studied the effect of distinct secondary structures on the self-assembly behavior of polytyrosine (PTyr)-derived amphiphilic copolymers. Two block copolymers of enantiopure poly(ethylene glycol)-b-poly(L-tyrosine) (PEG-b-P(L-Tyr)) and racemic poly(ethylene glycol)-bpoly(DL-tyrosine) (PEG-b-P(DL-Tyr)) were synthesized through the ring-opening polymerization of L-tyrosine N-thiocarboxyanhydride (L-Tyr-NTA) and DL-tyrosine N-thiocarboxyanhydride (DL-Tyr-NTA), respectively, by using poly(ethylene glycol) amine as the initiator. PEG 44 -b-P(L-Tyr) 10 adopts a β-sheet conformation and self-assembles into rectangular nanosheets in aqueous solutions, while PEG 44 -b-P(DL-Tyr) 9 is primarily in a random coil conformation with a tiny content of β-sheet structures, which self-assembles into sheaf-like nanofibrils. A pH increase results in the ionization of phenolic hydroxyl groups, which decreases the β-sheet content and increases the random coil content of the PTyr segments. Accordingly, PEG 44 -b-P(L-Tyr) 10 and PEG 44 -b-P(DL-Tyr) 9 selfassemble to form slender nanobelts and twisted nanoribbons, respectively, in alkaline aqueous solutions. The secondary structuredriven self-assembly of PTyr-derived copolymers is promising to construct filamentous nanostructures, which have potential for applications in controlled drug release.