Characteristic fingerprint spectrum of neurotransmitter norepinephrine with broadband terahertz time-domain spectroscopy.

The neurotransmitter norepinephrine (NE) was investigated by broadband terahertz time-domain spectroscopy (THz-TDS) in the air-plasma system range from 0.5 to 18.2 THz. NE has a unique absorption spectrum in the THz band, which can be used as a characteristic fingerprint for molecular detection and identification. The temperature-dependent THz spectra of NE were recorded in the range from 83 to 293 K, and a blue-shift of the absorption peaks was observed as the temperature decreased. A solid-state density functional theory (DFT) calculation was implemented to better understand the low-frequency vibrational property of NE, and the calculated results agree well with the THz experimental observations. This result suggests that the broadband THz system can obtain more abundant spectral signals of NE and each THz resonance peak has its own specific vibrational mode, which corresponds to a specific structure and interaction. Even with the adjacent absorption peaks, the vibrational behaviors are different. The deformation of the aromatic ring and the flexibility of the side chain directly affect the NE molecular conformation, which may be closely associated with the receptor binding preference for the neurotransmitter. The conformational diversity of NE may help to understand the biological function of the neurotransmitter in the nervous system.