Polypeptides are synthetic polymers with a similar main chain structure to proteins, which can form various secondary structures, providing a new dimension for regulating the macroscopic properties of materials. To investigate the effect of secondary structures on the properties of polypeptide materials, we designed polypeptide crosslinking agents with α-helical and random coil structures. We synthesized glutamic acid derivatives with triethylene glycol monomethyl ether as the side chain, and subsequently prepared them as N-carboxy anhydride (NCA) monomers. We then used a tetra-armed initiator to initiate single chiral NCAs and a 1∶1 mixture of enantiomeric NCAs to respectively prepare α-helical and random coil structured polypeptides. Acryloyl chloride was used to modify the end groups of the polypeptides so that they could be introduced as crosslinking agents into the polyacrylamide network of N,N-dimethylacrylamide (DMAM), resulting in the preparation of hydrogels crosslinked with polypeptides of different molecular weights and secondary structures. By comparing the swelling ratio, rheological and tensile properties of the two hydrogels, we studied the effect of α-helical structure of polypeptides on the mechanical properties of hydrogels. The results of swelling tests in aqueous solutions showed that, hydrogels crosslinked with random coil polypeptides exhibited better swelling performance compared to those crosslinked with α-helical structure polypeptides. The smaller mesh size of α-helical peptide networks resulted in fewer water molecules entering during swelling. Rheological characterization revealed that helical peptide hydrogels had lower critical strain and higher storage modulus, and exhibited faster recovery ability due to the high cooperativity of hydrogen bonds. Tensile experiments were conducted, and stress-strain curves were obtained. α-Helical polypeptide crosslinked gels exhibited higher fracture strength but lower fracture strain compared to random coil polypeptide crosslinked gels. The helical structure required a higher force to break the amide bonds due to multiple hydrogen bonds, resulting in lower extensibility. The elastic modulus of helical gels was significantly higher, independent of the crosslinker's molecular weight. Moreover, helical gels had higher fracture energy due to hydrogen bond dissociation and energy dissipation. According to these results, the hydrogel crosslinked with α-helical polypeptides exhibited greater rigidity, higher toughness, and faster recovery compared with the hydrogel crosslinked by random coil polypeptides, demonstrating the characteristics of α-helices as molecular springs and their potential as enhancers for high-performance hydrogels.

Key words: α-helix, polypeptide, secondary structure, hydrogel, material property