Self-assembly And Temperature Sensitive Hydrogel Of Peptide Polymer Based On L-Threonine

Due to the good biocompatibility,biodegradability and unique secondary structures,peptides with stimuli-responsive properties have attracted much more attention in the application of biomedical fields.However,few researches have been reported on the controlling of stimuli-responsive behaviors by the secondary structure,and the responsive properties of hydrogels regulated through the?-sheet structure of peptide have also rarely been reported.In this thesis,homopolymer of L-threonine containing side hydroxyl group and the related derivative copolymers with different modifications(methoxy,benzyl and acetyl)on the side hydroxyl group were synthesized and investigated.¹H-NMR,DMA,DLS,TEM,FTIR,¹³C-NMR,CD and other methods were utilized to characterize the structure,the self-assemble structures and the temperature-sensitive properties of L-threonine-based homopolymers and different derived copolymers.The morphology of assemble structures,such as spherical aggregates,worm-like micelles,nanofibers,nanorods and nanosheets,was regulated by the modification of the side hydroxyl group and the length of the peptide segment,resulting in gel-sol,sol-gel and gel-sol-gel phase transition behaviors with temperature owing to the transition of?-sheet structure.First of all,we synthesized homo-polypeptide of L-threonine(PLT)with controlled segment length,molecular weights and narrow distribution.When the degree of polymerization was below 7,clear aqueous solution was observed and the molecules of PLT self-assembled into spherical aggregates with random coiled secondary structure.Although PLT presented PEG-like thermo-responsiveness,it was not easy to form a temperature-sensitive hydrogel.Second,diblock copolymer methoxy-polyethylene glycols-poly(L-threonine)(mPEG-b-PLT)with narrow molecular weight distribution and tailored molecular weight was synthesized by the ring-opening polymerization of?-amino acid-N-carboxyanhydrides(NCA)with amino-terminated methoxy-polyethylene glycols(mPEG)as initiator.The length of mPEG segment had little effect on the secondary structure of peptide and the self-assemble morphology of the copolymer in aqueous solution.Spherical aggregates with random coil secondary structure were always observed.The increase of PLT length could induce the transformation of secondary structures from random coil to?-sheet,resulting in the morphology changes from nanospheres to nanofibers and the formation of hydrogel.With temperature,the secondary structure of the peptide transformed from?-sheet structure to random coils and mPEG simultaneously dehydrated,resulting in the morphology change from nanofibers to spherical nano-assemblies and the formation of hydrogel with special mechanical properties.Third,the secondary structure,self-assembly and temperature-sensitivity of mPEG-b-PLT and mPEG-b-poly(L-valine)(mPEG-b-PLV)in aqueous solution were investigated.The result showed that mPEG-b-PLV self-assembled more easily into nanorods with?-sheet structure and showed good sol-gel transition behaviors due to the hydrophobic side alkyl groups.Fourth,the influences of L-threonine protection group on the secondary structures,assembly morphologies and thermo-responsive behaviors of mPEG-b-PLT were explored.The secondary structure of?-helix was formed when hydroxyl group was protected by methoxy group,displaying worm-like micelles at the low temperature.The rise of temperature induced the dehydration of PEG and the structural transformation of peptide segment from?-helix to?-sheet so that the sol-gel transition around body temperature occurred.The copolymer with benzyl protection group promoted the formation of regular?-sheet structures due to the?-?stacking,exhibiting the nanofiber morphology and the gelation behaviors at room temperature.The disassembly of?-sheet with temperature caused the morphology change from nanofibers to nanospheres and the gel-to-sol transition.As for the acetyl protection group,the nanosheets structure was observed at proper ratios of PEG/L-threonine and the hydrogel was formed at a certain concentration due to the weakened intermolecular hydrogen bonds by the existence of the rigid ester bond.No gel-sol transition was observed with temperature.Finally,a dual phase transition thermo-hydrogel(gel-sol-gel)was synthesized.The phases transition temperature was easily regulated by the concentration of copolymer and the block length of poly(L-threonine)segment protected by benzyl group.This dual phase transition was ascribed to the crosslinking of nanofibers at the low temperature,the disassembly of?-sheet structures with temperature owing to the polarity increase of benzyl groups,and the dehydration of PEG segments at high temperature,leading to the morphology change from twined nanofibers to nanospheres to large aggregations.In summary,PLT,mPEG-b-PLV,mPEG-b-PLT and series of mPEG-b-PLT derivatives were synthesized.These polymers could self-assemble into spherical,worm-like,lamellar,virgate or fibroid morphologies according to the concentration of polymers,the content of peptide segments and the side group of amino acid.Through controlling side group and peptide length,various of thermo-responsive hydrogels with sol-gel,gel-sol and gel-sol-gel transition behaviors were obtained.These hydrogels showed great potential in the biomedical applications.The temperature responsiveness owing to the transition of peptide secondary structures provided a pathway for the construction of smart peptide-based materials.