Vibrational modes optimization and terahertz time-domain spectroscopy of L-Lysine and L-Lysine hydrate

To date, it is challengeable to clearly identify and accurately characterize molecular vibrational modes of an amino acid and its hydrate. In this work, the terahertz absorption spectra of L -Lysine (Lys) and L -Lysine hydrate (Lys·H 2 O) have been measured by terahertz time-domain spectroscopy (THz-TDS) at the range of 0.3–2.5 THz under room temperature. The structural changes of Lys and Lys·H 2 O are sensitively observed by THz spectroscopy. Based on density functional theory (DFT), the vibrational frequencies of Lys and Lys·H 2 O are analyzed, the Lys single molecule model and the most optimal Lys·H 2 O single molecule model are established. The collective vibrational modes of Lys and Lys·H 2 O have been identified by potential energy distribution (PED) analysis and molecular vibrational animation of GaussView. The results demonstrate that the collective vibrational modes of Lys and Lys·H 2 O at various frequencies are originated from the dihedral torsion or the bond angle bend of different molecular chains in a single molecule, which drives the vibrations of related groups. Therefore, the combination of THz-TDS technology and PED analysis opens up an opportunity for identifying Lys and its hydrate, which provides a theoretical reference for studying vibrational modes of biomolecules.