Qualitative and Quantitative Study on DL-Glutamic Acid and Its Monohydrate using Terahertz Spectra

Abstract

The rotation, translation, and vibration of many amino acid molecules fall in the terahertz (THz) region, thus qualitative and quantitative researches on amino acids can be carried out through their THz absorption characteristic fingerprint peaks. In this paper, the room temperature THz absorption spectra of DL-glutamic acid and its monohydrate at 0.5-3.0THz were measured by utilizing terahertz time-domain spectroscopy (THz-TDS). It is found that the THz characteristic peaks of these two amino acids are obviously different. Moreover, according to the change of the absorption amplitude of the characteristic absorption peak (1.24THz) of DL-glutamate monohydrate at different sample concentrations, a quantitative analysis was carried out on the mixed samples of DL-glutamate and its monohydrate, and then the quantitative analysis formula was also verified. In addition, the optical mode of DL-glutamic acid and its monohydrate in THz region was predicted by using density functional theory (DFT). Under the condition that the theoretical data matched with the experimental peaks, the origins of THz absorption peaks obtained in the experiment were discussed and summarized. The results show that the THz characteristic peaks (<2.80THz) of DL-glutamic acid and its monohydrate come from the intermolecular interaction modes, and the other absorption peaks are resulted from the combination of intermolecular and intramolecular interaction modes.