Investigation of the Correlations between Amino Acids, Amino Acid Mixtures and Dipeptides by Terahertz Spectroscopy

Terahertz (THz) spectroscopy is regarded as a professional technology to probe collective molecular vibrational modes and intensity of bio–molecules, which is potential to analyze amino acids and short peptides. In this study, THz spectroscopy is utilized to investigate the correlations of glycine (Gly), Serine (Ser), amino acid mixtures of Gly/Ser, and dipeptides (GS and SG) in the frequency range of 0.3–2.5 THz. The results manifest that the collective molecular vibrational intensity of Ser is larger than Gly, implying that the hydroxyl methyl group (–CH2–OH) on the side chain of Ser exhibits larger motions than that of –H in Gly. Additionally, the THz spectra of amino acid mixtures of Gly/Ser fit the linear addition of the individual amino acid absorption spectrum. Herein, the THz spectra of dipeptide GS and its isomer SG are distinguished according to the different collective molecular vibrations produced by the terminal groups on the carbon skeleton. Compared to physical amino acid mixtures of Gly/Ser, the dipeptides exhibit diverse THz spectra, indicating that the different collective vibrational modes are caused by peptide bonds and terminal groups. These solid evidences confirm that THz spectroscopy can be availably used to analyze the correlations of amino acids, amino acid mixtures, and dipeptides, as well as to identify dipeptide and its isomer from molecular level.