Synthesis And Self-assembly Of Light And Metal Ion Responsive Polypeptide Bearing O-nitrobenzyl Group

Polypeptides,possessing the structural similarity to proteins,are known to formα-helices orβ-sheets via intra-and intermolecular interactions.These secondary structures can be further employed as the structural motifs to tune the nanostructures of assemblies.Photo-responsive polymers,which can respond to controllable light in a facile and efficient manner,have received considerable attention as a class of promising smart materials for many potential applications.Metal ions play important roles in many biological processes,so it is essential to investigate the interactions between metal ions and functional biomaterials.In this article,we successfully synthesize some photo-responsive polypeptides and studied the light and ion induced process systematically.The dissertation includes the following three sections:1.The synthesis and characterization of photo-responsive polypeptides bearing o-nitrobenzyl groups:The functional monomer o-nitrobenzyl-_L-glutamate was successfully synthesized.Then,photo-responsive poly（ethylene glycol）-block-poly（o-nitrobenzyl-_L-glutamate）（PEG-b-PNBG）diblock copolymers was synthesized through NCA ring-opening polymerization with polyethylene glycol amine（PEG-NH₂,Mn=2000 g/mol or 5000 g/mol）as macroinitiator.PEG-b-PNBG can self-assemble into spherical micelles in aqueous solution.After irradiation,o-nitrobenzyl groups are cleaved from main chain,which leads to sphere-to-cylinder morphological transition of copolymers.Studies have shown that crucial reason for the morphological transition is pH value.Owing to the cleavage of o-nitrobenzyl groups after irradiation,the generated PGA led to the decrease of pH value to⁴.The stableα-helix structure of polymer and inter-hydrogen bonds resulted in the formation of cylinders.2.Ion-induced the re-assembly of photo-responsive polypeptides bearing o-nitrobenzyl groups:Further addition of different metal ions(Na⁺,Ag⁺,Ca²⁺,Zn²⁺and Fe³⁺)into the PEG-b-PNBG aqueous solutions can induce the pre-formed assemblies re-assemble into the hierarchical structures probably arising from synergistic effects of metal-ion complexation and variations of the secondary structures..As the results,for Ag⁺solutions,during the UV irradiation,the generated PGA chains could be used as the template for the reduction of Ag⁺,and the cluster of Ag could form.For other metal ions,the various valence is the crucial factor on the re-assembly process.The divalent metal ions could increase the length of the cylinder structure of polypeptides and the trivalent ions could induce polypeptides assemble into cluster.3.The synthesis and characterization of photo-responsive polypeptides containing anthracene groups:Photo-responsive poly（ethylene glycol）-block-poly（N^ε-carbobenzyloxy-_L-lysine）（PEG-b-PZLL）diblock copolymers was synthesized through NCA ring-opening polymerization with polyethylene glycol amine as macroinitiator.After deprotection,the anthracene groups were introduced into the end of the polymer side chains by Schiff-base reaction.Finally,we attained poly（ethylene glycol）-block-poly（N^ε-methylene anthracene-_L-lysine）（PEG-b-PLL-An）after reduction reaction.The results showed that the fluorescence intensity of polymer in DMF/H₂O solutions increased with water fraction increasing and the aggregation-annihilated circular dichroism was observed during this process.Besides,after the reduction,diblock copolymers could turn into two kinds of different assemblies in aqueous solutions dependent on the solvents.This variation in morphology also resulted in the different fluorescence properties.