Solid-state NMR Studies On Conformational Transition Of Silk Fibroin Induced By Polylactic Acid

Through the in-depth study of those natural materials to establish the relationship between microstructure and macroscopic properties,it can provide theoretical guidance for the design and preparation of high-performance synthetic polymer materials.Silk fibroin（SF）is a natural polymer with excellent mechanical properties and has wide applications in different fields.However,it is still challenging to tune the microstructures of SF at the molecular level to achieve accurate control of macroscopic properties.Therefore,in this thesis,PLA（polylactic acid）（PLA）was mainly used to control the structure of SF（random coils,α-sheet and β-sheets）,and the mechanism of conformational transitions of SF toβ-sheet structures induced by PLA was elucidated by solid-state nuclear magnetic resonance（NMR）spectroscopy.In this thesis,SF/PLA composite membranes were prepared by solution casting with formic acid as the solvent.By controlling the content,configurations and terminal groups of PLA chains,and comparing effects of PLA and polyacrylic acid（PAA）,the main factors affecting the conformational transition of SF to β-sheet structures were examined in detail.The quantitative carbon spectra of samples were quickly obtained by using a novel multiple cross-polarization method,which enables quantitative determination ofthe relative content of different conformations in SF.Then,¹H-¹³C heteronuclear chemical shift correlation solid-state NMR spectroscopy（HETCOR）reveals the mechanism by which PLA induces the conformational transition of SF.Specific contents include:1)The effect of PLA configurations and contents on the conformational transition of SF was investigated.The results showed that PLA and SF were well mixed at the molecular level.When the racemic PLA content was 60%in the composite membrane,the fraction ofβ-sheet conformation was the highest,which was achieved via the hydrogen bonding interactions between PLA and SF as revealed by 2D¹H-¹³C HETCOR solid-state NMRspectroscopy.2)The end groups of PLA were modified and their effects on the conformational transition of SF toβ-sheet structures were investigated.The results showed that PLA with hydroxyl terminal groups will have stronger hydrogen bonding interactions with SF compared to PLA with carbonxyl or methyl termainal groups,leading to the enhanced conformational transition of SF to β-sheet structures.3)The effect of PAA and PLA on the transformation of SF conformation toβ-sheet structure was compared.The results showed that while PAA has a much larger content of carboxyl than PLA to form hydrogen bonding interactions with SF,PLA can still induced the formation of a higher content of β-sheet structures in the composite membrane.In this thesis,we quantitatively studied the effect of PLA on inducing the conformational transition of SF toβ-sheet structures,and revealed the strong hydrogen bonding interactions between PLA and SF at an atomic level.This study deepens our understanding of the secondary structure of SF,and also offers a new avenue for studying the secondary structure of proteins（especially SF）,which can provide theoretical guidance for the development high performance natural materials.