| Chiral substances widely exist in various fields,such as agriculture,food,materials and biomedicine.Different chiral isomers may show different physiological and pharmacological effects.Among all chiral compounds,amino acids are a significant class.They are closely related to human life and health,and their applications in agriculture are also widely used.In addition,amino acids are also the basic components of proteins.Among the 20 amino acids that make up proteins,there are two enantiomers(L-and Damino acid)except glycine.A large number of studies have shown that life almost only uses L-amino acids in its biological activities,while the corresponding D-amino acid and racemic compound DL-amino acid may have different biological or physiological characteristics.In this paper,the enantiomers and racemates of solid-phase amino acids and liquid-phase amino acids were studied based on infrared spectroscopy and Raman spectroscopy.The enantiomers and racemic compounds of solid-phase amino acids were detected based on transmission terahertz time domain spectroscopy(THz-TDS).The polarization direction of linearly polarized terahertz light can be changed by using attenuated total reflection THz-TDS system and the optical rotation of amino acids.The enantiomers and racemates of amino acids in liquid state were studied,and the enantiomers were identified by using the opposite optical rotation of L-and D-amino acid.Based on the density functional simulation,the theoretical and experimental values were verified each other,and the vibration mode of Terahertz absorption characteristic peak of solid amino acids was explored.The main research contents and conclusions of this paper were as follows:(1)The enantiomers and racemates of solid-phase and liquid-phase amino acids were studied based on infrared spectroscopy.The infrared absorption spectra of enantiomers(L-and D-)and racemic compounds(DL-)of alanine,tyrosine and proline in solid phase are different,but the spectra between L-and D-amino acids are similar.Due to the different arrangement of molecules in the lattice of amino acid enantiomers and racemic compounds,their solid-phase spectra are different from each other.The infrared absorption spectra of the enantiomers(L-and D-)and racemates(DL-and D+L-)of alanine,tyrosine and proline aqueous solutions are not significantly different.(2)The enantiomers and racemates of solid-phase and liquid-phase amino acids were studied based on Raman spectroscopy.The Raman absorption spectra of enantiomers(Land D-)and racemic compounds(DL-)of alanine,tyrosine and proline in solid phase are different,but the spectra between L-and D-amino acids are similar.The Raman absorption spectra of the enantiomers(L-and D-)and racemates(DL-and D+L-)of alanine,tyrosine and proline aqueous solutions are not significantly different.(3)The enantiomers and racemic compounds of solid-phase amino acids were studied based on transmission THz-TDS technique.The absorption spectra of L-and Dalanine are almost the same,with obvious absorption peaks near 2.20 and 2.51 THz,while DL-alanine has an absorption peak near 1.25 THz.The difference of peak frequency shows that these bands come from intermolecular vibration modes;L-tyrosine and Dtyrosine have similar characteristic peaks,showing four peaks at 0.95,1.92,2.06 and 2.60 THz,while DL-tyrosine has three peaks at 1.5,2.15 and 2.40 THz,which is different from L-and D-tyrosine.It can be seen that the difference of stereoisomer structure and crystal arrangement between tyrosine enantiomer and racemic compound leads to the difference of THz spectral characteristics;The spectra of L-and D-proline are similar.There is a characteristic absorption peak at 1.78 THz,while DL-proline has two characteristic absorption peaks at 1.3 THz and 1.86 THz respectively.The absorption peak at 1.3 THz is weak,while the vibration absorption peak at 1.86 THz is strong.Therefore,it can be considered that the L-,D-and DL-forms of amino acids have different properties,and it also provides a method to distinguish enantiomers from racemic compounds.(4)Based on the attenuated total reflection THz-TDS system and the characteristic that the optical rotation of amino acids(D,L-amino acid)can change the polarization direction of linearly polarized terahertz light,the enantiomers and racemates of amino acids in liquid state were studied.The chiral effect was studied by using different sample concentrations and different solvents.The results showed that the absorption coefficient of amino acid enantiomer was higher than that of racemate.The optical rotation of optically active substances is directly proportional to the concentration.At high concentration,the recognition effect between enantiomers and racemic compounds and enantiomers is better than that at low concentration.The optical activity is not only related to the molecules of substances,but also related to the interaction between molecules(including the interaction between solute and solute,and between solute and solvent).Compared with inorganic solvent environment(absolute ethanol),it is more advantageous to recognize chiral enantiomers in organic solvent environment(pure water).In alcohol solvent environment,the conclusion is similar to that in pure water and absolute ethanol environment,that is,the absorption coefficient of amino acid enantiomer is higher than that of racemate.(5)The single molecule and cell structures of L-type amino acids of three amino acids were calculated by Gaussian 09 software and Materials Studio software.The results show that the calculated absorption peak position is close to the experimental absorption peak position.It is also found that the intermolecular force of amino acids and the intramolecular benzene ring structure have the greatest influence on the THz spectral absorption.Finally,the three spectral techniques are compared,and it is concluded that THz-TDS technology has more advantages in distinguishing the enantiomers and racemates of amino acids in this experiment. |
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