Vibrational spectra of serotonin by terahertz time domain spectroscopy and DFT simulations

Serotonin is an important biogenic monoamine neurotransmitter that has major influences on mental health disorders; its structural and conformational changes have important roles in the biological functions of the human body. The decreased serotonin levels in the human body are majorly attributed to the causes of anxiety, depressive disorders, mood disorders, etc. Therefore, the quantification of serotonin in our bodies is of utmost importance in unearthing the origin of such physiological disorders. In this study, Terahertz-Time Domain Spectroscopy (THz-TDS) is employed to characterize the unique THz fingerprint of serotonin in the frequency range 0-3 THz. The characteristic THz absorption peaks of serotonin are observed at 0.54, 0.84, and 1.10 THz. In addition, Density Functional Theory (DFT) calculations are performed to investigate the vibrational properties of serotonin. For the vibrational assignment of modes, we have used Potential Energy Distribution (PED) analysis. Furthermore, studies have been conducted on the variation of serotonin concentration in a polyethylene (PE) host medium. The effect of the serotonin concentration in the PE host is studied using the complex refractive index (CRI) model. The sensitivity of detection of serotonin concentration is 0.015 for an increment of 2% concentration in PE medium. This work maps the spectral features of serotonin in the THz range, suggesting that THz-TDS can be used to understand and treat the physiological disorders related to serotonergic systems.