Poly(Acryloyl-L-Serine): A Reactive Polypeptide to Introduce Zwitterion and Amphiphilicity for Stimuli-Responsiveness and Gelability

The rising development in the field of synthetic-functionalized polypeptides aids in the generation of advanced biomaterials comprising versatile functionalities, enabling them to be potential candidates for their progressive applications in different areas. Thus, in this work, the ring-opening polymerization (ROP) of a designed acryloyl-L-serine N-carboxyanhydride (SerA NCA) monomer is conducted to produce side-chain acryloyl-functionalized poly(acryloyl-L-serine) [P(AcSer)] for its facile and selective functionalization to introduce zwitterionic and amphiphilic groups for stimuli-responsiveness as well as gelability via Michael addition and radical polymerization. The postfunctionalization of P(AcSer) with L-cysteine (Cys) leads to the formation of the zwitterionic polypeptide, P(AcSer)-g-Cys, which has an isoelectric point (pI) of 2.7. Near the pI, the aqueous P(AcSer)-g-Cys solution exhibits pH-responsiveness (pH ranges of 2.2-4.1), transforming the transparent solution to a cloudy suspension containing insoluble hierarchical nanoaggregates of various dendritic morphologies. Notably, this cloudy suspension undergoes a upper critical solution temperature (UCST)-type phase transition with tunable cloud points, transforming it into a clear solution upon heating. The addition of triethylene glycol monomethyl ether thiol (MTEG-SH) to P(AcSer) produces amphiphilic P(AcSer)-g-MTEG, which forms vesicular nanoassemblies in water and is capable of encapsulating hydrophobic Nile red (NR) dye present in water. Apart from that, grafting of MTEG moiety introduces lower critical solution temperature (LCST) property to P(AcSer)-g-MTEG in water. Thiol-based cross-linkers and ionic-liquid-based cationic cross-linker are employed for the gelation of acryl-functionalized P(AcSer). The obtained polypeptide gel materials exhibit excellent swelling behaviors in organic/aqueous media. Additionally, the cationic polypeptide gel shows profound dye adsorption capacity under exposure to the aqueous solution of eosin B and sulfasalazine.