Terahertz Detection and Density Functional Theory Identify Analysis of Typical Feed Amino Acid
LIANG Shuang1, 2, WANG Ying3, ZHAO Wen-wen2, ZHANG Zhi-yong1, YUE Jian-min2, LI Bin1, 2*
1. College of Agricultural Engineering, Shanxi Agricultural University, Taigu 030801, China
2. Intelligent Equipment Research Center, Beijing Academy of Agriculture and Forestry Sciences, Beijing 100097, China
3. Lenovo(Beijing) Information Technology Corporation Ltd., Beijing 100085, China
Abstract：To explore the terahertz band fingerprint properties of typical amino acids used for feeding low-protein diets to livestock and poultry and their correlation with molecular structure, this paper presents experimental analysis and theoretical vibrational calculations of six feed amino acids, including methionine, based on terahertz time-domain spectroscopy technique and Density Functional Theory (DFT). First, experimental samples of amino acids were prepared. Their terahertz absorption spectra were measured from 0.5 to 2.0 THz. It was found that methionine at 1.48 and 1.54 THz, lysine at 0.95 and 1.67 THz, tryptophan at 1.2 and 1.4 THz, threonine at 1.42 THz, histidine at 1.4 THz, isoleucine at 1.41 and 1.51 THz showed significant absorption characteristics; Then, GaussView software was used to construct the single-molecule structures of the six amino acids, and the structure was optimized by applying the B3LYP/6-311G (d, p) level of theory of DFT theory. The spectral properties of their terahertz bands were obtained through theoretical calculations. It was found that methionine at 1.51 THz, lysine at 0.89 and 1.68 THz, tryptophan at 0.67 and 1.39 THz, threonine at 1.4 THz, histidine at 1.4 THz, and isoleucine at 1.56 and 1.75 THz certain absorptionwere present. Which is consistent withthe actual experimental measurement of the absorption peaks. Finally, the vibrational modes of the six amino acids at the absorption peaks in the 0.5～2.0 THz band were analyzed by Gaussian software, and it was found that the absorption peak at 1.51 THz for methionine originated from the in-plane rocking vibration of itsmethyl group; the absorption peaks at 0.89 and 1.68 THz for lysine originated from the rotation of carboxyl group and the in-plane bending vibration of itsamino group; the characteristic peak at 1.39 THz for tryptophan originated from the out-of-plane bending of itscarboxyl group; the absorption peak at 1.4 THz for threonine originated from the out-of-plane bending of itscarboxyl group and the in-plane rocking vibration of itsamino group; the absorption peak at 1.4 THz for histidine originated from the in-plane bending of itsamino group and the overall rocking vibration; the absorption peaks at 1.56 THz for isoleucine originated from the overall rocking of its methyl groupsand amino groups and the out-of-plane rocking vibration of its carboxyl group, and the characteristic peaks at 1.75 THz originated from the overall rocking of its methyl group and out-of-plane rocking vibration of its carboxyl group. This study is a theoretical reference for the rapid qualitative and quantitative studies of typical feedamino acids using terahertz spectroscopy.
Key words：Low-protein diets; Amino acid; Terahertz; Density functional
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