Terahertz Time-Domain Spectroscopy Of Fibroin Oligopeptides And Natural Color Silk

THz wave usually refers to the electromagnetic radiation with the frequency in the range of 0.1 to 10 THz(wavelength of 30-3000μm),which is located between the microwave and the IR radiation in the electromagnetic spectrum.Terahertz time-domain spectroscopy(THz-TDS)is a new spectroscopy technique developed in the 1990 s.After 20 years of development,it has been gradually applied to biological materials,chemical materials,textile materials,security testing,and many other areas due to the characteristics of transient,coherence,broadband,low energy and high spatial resolution.Because the terahertz radiation is very sensitive to the material molecules of their own rotation,the vibration of the molecular skeleton,intermolecular interaction,lattice vibration and so on,it is a new analytical method to study the structural information and vibration information of fibroin oligopeptides and silk by using terahertz time-domain spectroscopy(THz-TDS).In this paper,the four major amino acids(glycine 48%,alanine 27.03%,serine 12.45%,tyrosine 6.44%),which constitute the silk,were measured by using the terahertz time-domain spectroscopy system.The time-domain spectra,frequency spectra,absorption spectra and refractive index spectra of pure amino acids and amino acid mixtures were characterized by the terahertz time-domain spectroscopy system.And the experimental amino acid THz spectra were compared with the previously published amino acids THz spectra to verify the accuracy of the experimental results.Then,the major repeating peptide segments of-(Gly-Ala-Gly-Ala-Gly-Ser)n-and-(Gly-Ala-Gly-Ala-Gly-Tyr)n-were tested by THz spectrum.And the density functional theory of Gaussian 03 software were used to carry out the theoretical simulation of the six kinds of dipeptides in order to discuss the vibrational modes of dipeptides and isomers in the THz band.Then the characteristic absorption peaks in the THz absorption spectra of tetrapeptide GAAS and hexapeptides(GAGAGS and GAGAGY)were compared to further discuss the vibration modes of silk oligopeptides in the THz band.Lastly,using terahertz time-domain spectroscopy,far-infrared spectroscopy and mid-infrared spectroscopy to study the spectral characteristics of four kinds of silk(white silk,naturally colored silk,naturally dyed silk and synthetically dyed silk)in infrared and terahertz regions.By studying the functional groups of silk in the mid-infrared band,the vibration of molecular skeleton and intermolecular interaction,the feasibility of distinguishing the silk with similar structure in infrared and far-infrared bands was discussed.The results show that the small molecules of the amino acids and oligopeptides which constitute the silk,are strikingly sensitive in the terahertz region,and there are distinct characteristic absorption features in the THz absorption spectra.Besides,THz-TDS is efficiently used to distinguish dipeptide from its isomers,which is difficult obtained through mid-infrared spectroscopy.Furthermore,the natural color silk and dyed silk show different absorption coefficient and refractive index in the THz region,as well as distinct absorbance in far-infrared region.Based on the difference in the optical properties of silks,the natural color silk is easily distinguished from the artificial silks dyed with man-made and natural dye.This research can further broaden the application field of far-infrared band,and it has important significance and application prospect for the nondestructive identification of naturally colored silk and dyed silk.