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.