Structure and Oxidation Effects on Conformation and Thermoresponsiveness of the OEGylated Poly(glutamic acid)-Bearing Side-Chain Thioether Linkers

A series of side-chain thioether-linked OEGylated poly(glutamic acid) (PGAs) have been synthesized by "thiol-ene" synthetic methodology, where both the oligo-ethylene glycol (OEG) length and the hydrophobic linkers at the side chains are varied to learn how these structural features affect the secondary structure and thermoresponsive behaviors in water. Before side-chain oxidation, the structural factors affecting the α-helicity include the backbone length, the OEG length, and the hydrophobic linkers' length at the side chains; however, the OEG length plays the most crucial role among these factors because longer OEG around the peripheral side chains can stop water penetration into the backbone to disturb the intramolecular H bonds, which finally allows stabilizing the α-helix; after the oxidation, the polypeptides show increased α-helicity because of the enhanced hydrophilicity. More interestingly, a rare oxidation-induced conformation transition from the ordered β-sheet to the ordered α-helix can be achieved. In addition, only the OEGylated poly(glutamic acids) (PGAs) with shorter hydrophobic linkers and longer OEG can display the thermoresponsive properties before the oxidation but the subsequent oxidation can cause the polypeptides bearing longer hydrophobic linkers to exhibit the thermosensitivity since sulfone formation at the side chain can lead to final hydrophilicity–hydrophobicity balance. This work is meaningful to understand the secondary structure-associated solution behaviors of the synthetic polypeptides.