The Response Of Silk Fibroin Amino Acids And Their Derivatives In Terahertz Spectroscopy

Low-frequency terahertz(THz)electromagnetic radiation can detect the intermolecular interactions as well as bending or twisting motions of macromolecules,so that THz spectroscopy can accurately reflect the rich and unique information on the vibrational modes of both molecular and hydrogen-bonding networks.Thus it is particularly suitable for evaluating the molecular and functional properties of many materials.Large conformational changes in proteins correspond to transitions among local minima in the protein energy landscape.Based on the fact that the vibration of biological macromolecules is very complicated and difficult to be resolved in the THz band.The intermolecular dynamics of amino acids that make up the silk fibroin are concentrated in the THz frequency range,which can be detected by terahertz time domain spectroscopy(THz-TDS).Therefore,this thesis takes cyclic amino acids of silk fibroin as the research object to study amino acid and its derivatives by THz-TDS.Taking phenylalanine(Phe)and proline(Pro)as the research object and THz-TDS as the main method.Phe,Pro and their hydrates were studied and analyzed combined with thermogravimetry(TG),X-ray diffraction(XRD)and infrared spectroscopy(IR).Anhydrates and hydrates show differences in the mid-infrared and THz bands,but the difference is more significant in the range of 0.25-2.6THz.This is because IR spectroscopy mainly reflects the stretching vibration of functional groups and covalent bonds,and it is difficult to significantly reflect weak interactions such as hydrogen bonds and van der Waals forces.In addition,the change of THz fingerprints of Phe monohydrate is more obvious due to the different forms of water molecules.This study shows that THZ-TDS has unique advantages in reflecting hydrogen bond and structural changes.Therefore,it has certain application value in quality monitoring,polycrystalline state identification.The hydrogen bonding mode of 5-fluorouracil-proline co-crystal(5F-P)was investigated by THz-TDS and density functional theory(DFT)based on the strong response of THz to hydrogen bond vibration.The THz spectrum of 5F-P is significantly different from the monomer of 5-fluorouracil(5-FU)and Pro within the range from 0.25-2.6THz.This result indicates that the two monomers react to form a new phase.Based on unit cell simulation,the theoretical spectrum of configuration 2(N1-H1···O4)is the most consistent with the experimental one.Furthermore,configuration 2 carries the lowest amount of total energy and exhibits the most stable structure.Therefore,the hydrogen bonding mode of configuration 2 is closest to 5F-P.Pro,hydroxyproline(HYP)and pyroglutamic acid(PGA)have similar molecular structures,but their low-frequency vibrations show significant differences in the range of 0.25-2.6THz.For HYP and PGA,the introduction of hydroxyl and carbonyl produces electronic effects and changes the hydrogen bond network structure.Therefore,based on the unit cell simulation,this study investigated the reasons for the differences in the fingerprint spectra of the three amino acids from the perspectives of dielectric properties,hydrogen bond networks and vibration modes.The results show that HYP and PGA have stronger absorption of terahertz waves due to the existence of polar substituents.Furthermore,compared with Pro,the absorption peaks of HYP and PGA are significant red shifted and blue shifted,respectively as the hydrogen bond network changes.Through analysis,it is believed that this is caused by the elongation and shortening effect of intermolecular hydrogen bonds.My findings demonstrate that subtle structural changes can be sensitively detected by terahertz spectroscopy,and the reasons for the changes in the terahertz spectrum caused by the introduction of substituents can be further investigated in combination with density functional theory.