Broadband terahertz signatures and vibrations of dopamine.

Dopamine (DA) is an essential neurotransmitter and hormone of the nervous system, its structural and conformational properties play critical roles in biological functions and signal transmission processes. Although this neuroactive molecule has been studied extensively, the low-frequency vibration features that are closely related to the conformation and molecular interactions in the terahertz (THz) band still remain unclear. In this study, a broadband THz time-domain spectroscopy (THz-TDS) system in the frequency band of 0.5-18 THz was used to characterize the unique THz fingerprint of DA. In addition, density functional theory (DFT) calculations were performed to analyze the vibrational properties of DA. The results suggest that each THz resonant absorption peak of DA corresponds to specific vibrational modes, and the collective vibration also exists in the broadband THz range. Moreover, the interactions between the DA ligand and the D2 and D3 receptors were investigated by docking, and the simulated THz spectra were obtained. The results indicate the dominant role of hydrogen bonding interactions and the specificity of molecular conformation. This work may help to understand the resonance coupling between THz electromagnetic waves and neurotransmitters.

Probing NaCl hydrate formation from aqueous solutions by terahertz time-domain spectroscopy.

The cooling-induced formation of a hydrate in aqueous NaCl solutions was probed using terahertz time-domain spectroscopy (THz-TDS). It was found that the NaCl hydrate formation is accompanied by the emergence of four new absorption peaks at 1.60, 2.43, 3.34 and 3.78 THz. Combining X-ray diffraction measurements with solid-state based density functional theory (DFT) calculations, we assign the observed terahertz absorption peaks to the vibrational modes of the formed NaCl·2H2O hydrate during cooling. This work shows that THz-TDS based analysis has great potential in studying ionic hydrates and the newly revealed collective vibrational modes could be sensitive indicators to achieve quantitative analysis in phase transitions and lattice dynamics.

Ultra-broadband terahertz fingerprint spectrum of melatonin with vibrational mode analysis.

Melatonin (MLT), as a neurotransmitter and an endogenous neurohormone, plays an important role in physiological functions through interactions with specific receptors. The conformations of MLT are closely related to its biological activities and functions. However, the internal relationship between the structure and interaction of MLT and its allosteric transition remains unclear. In this work, we obtain the broadband fingerprint terahertz (THz) spectrum of MLT in the range of 0.5-18 THz using the air-plasma terahertz time-domain spectroscopy (THz-TDS) system. DFT calculations are employed to analyze the vibration characteristics of MLT. The result shows that the low-frequency vibrations mainly come from the strong coupling between inter- and intramolecular vibrations, and the contribution of intramolecular vibrations gradually dominates with increasing frequency. Meanwhile, the local vibrations of the different functional groups distribute widely in the THz low-frequency band, relating to the diversity of conformational changes in the molecule. The intermolecular hydrogen bonds (HBs) have distinct resonant responses and play critical roles in the THz low-frequency vibrations. The study reveals the complex characteristics of the resonant coupling of MLT with THz electromagnetic waves. The results will help to understand the conformational preferences of MLT in neural signal transmission processes.