Activation of Upper Critical Solution Temperature Behaviors of Zwitterionic Poly(l-methionine-g-poly(sulfobetaine methacrylate) _m ) with a Bottlebrush Structure

Aqueous poly(sulfobetaine methacrylate) (PSBMA) solutions exhibit upper critical solution temperature (UCST) behaviors, which are influenced by the topological structure and relative molar mass of the polymer chain. In this study, a series of well-defined PSBMA-based zwitterionic bottlebrush poly(l-methionine) derivatives, namely, poly(l-methionine-g-(PSBMA)(m)) (MS-m), were synthesized via ring-opening polymerization of l-methionine N-carboxyanhydride, followed by bromo-alkylation and atom transfer radical polymerization. The aqueous MS-70 (m = 70) solution showed UCST-type thermoresponsive properties with a cloud point (T-cp) at around 38.6 degrees C. The variable temperature nuclear magnetic resonance results of H-1 and 2D H-1-H-1 correlated spectroscopy and nuclear Overhauser effect spectroscopy (NOESY) demonstrated correlated peaks and coupling interactions between protons in the sulfobetaine units. When the temperature increased up to or beyond T-cp, the NOESY spectra showed that the zwitterionic bottlebrush MS-70 polypeptides transformed from the aggregated state to the monodisperse state owing to the weakened inter/intramolecular interactions between the positively and negatively charged groups. In contrast, the observed UCST transition near the physiological temperatures could be negligible in linear PSBMA or the linear-like SBMA polymer of MS-70 degraded by papain. Results indicated that the bottlebrush structure activated the solution properties of zwitterions. In brief, this study presents a precise strategy for synthesizing zwitterionic bottlebrush polypeptides with an elucidated mechanism of thermal responsiveness in aqueous solution. It provides valuable insights into expanding applications of zwitterions in biology, such as protein stabilization or zwitterionic hydrogels.