The Application Research Of Terahertz Spectra In The Molecular Structures Of Amino Acids

Terahertz(Teraherz,THz)science and technology was named as one of the ten key technologies which can change the world in the future.And it has been developed in recent years.THz radiation refers to the electromagnetic frequency between the microwave and infrared radiation.And it is located in the far-infrared region.A lot of information is included in this region,such as hydrogen bonds,van der Waals force and the lattice vibration.THz spectrum is sensitive to collective vibration of molecules.So it can provide an important reference for the biological and medicinal chemistry research.However,there is still room for improvement.Because of the differences of instruments,operational environments and methods,the experimental results which are from different institutions are diverse.The THz spectra of 20 standard a-amino acids were measured in this work.In order to find the relationship between the characteristic peaks and molecular structures,density functional theory(DFT)was carried out to do theoretical simulation.All 20 standard polycrystalline a-amino acids were measured by the use of terahertz time-domain spectroscopy(THz-TDS)at room temperature.Each kind of amnio acid has three forms:L-,D-form and the racemate except for glycine.And Powder X-ray diffraction(PXRD)experiments were carried out to find the relationship between crystal structures and THz spectra.And the results show that the spectral features mainly come from the collective molecular rotation,distortion and molecular translation.The low-frequency vibrational property of L-phenylalanine(L-Phe)and L-phenylalanine monohydrate(L-Phe-H2O)has been investigated by THz-TDS at room and low temperature ranging from 0.5 to 4.5 THz.Distinctive THz absorption spectra of the two compounds were observed.DFT calculations based on the crystal structures have been performed to simulate the vibrational modes of L-Phe and L-Phe H2O and the results agree well with the experimental observations.The study indicates that the characterized features of L-Phe mainly originate from the collective vibration of molecules.And the characterized features of L-Phe H20 mainly come from hydrogen bond interactions between L-Phe and water molecules.L-Phe and L-Phe·H2O were also verified by differential scanning calorimetry and thermogravimetry(DSC-TG)and PXRD examinations.