Synthetic Strategy to Build High-Molecular-Weight Poly(L‑tyrosine) and Its Unexplored β‑Sheet Block Copolymer Nanoarchitectures

Synthesis of high-molecular-weight polypeptides and their block copolymer macromolecular architectures from β-sheet-promoting L-amino acids is still an unresolved problem. Here, an elegant steric hindrance-assisted ring-opening polymerization (SHAROP) strategy is introduced to access β-sheet poly(L-tyrosine) having more than 250 units. The scope of the synthetic methodology is expanded to access unexplored poly(L-tyrosine)-based higher-order β-sheet block copolymer nanoassemblies. In this strategy, a tert-butyl benzyl unit is employed as a steric handle that imbibes the solubility by promoting the α-helical conformation in the propagating polypeptide chains. The living ROP process enables the synthesis of well-defined block copolymers initiated by poly(L-tyrosine) living-chain ends or growing the poly(L-tyrosine) chains from the pre-existing macroinitiators of poly(L-glutamate) or poly(L-lysine). Acid-catalyzed postpolymerization deprotection restores the poly(L-tyrosine) blocks in their nascent β-sheet conformations. Thioflavin-T fluorescence assay establishes the β-sheet core–shell structures of these nanoassemblies, which are found to be nontoxic to mammalian cell lines.