Study On The Dielectric Properties Of Materials In Terahertz Region

Understanding the low-frequency vibrational dynamics of water and biological molecules is crucial for studying their biological functions. Early pioneering work has demonstrated that terahertz spectroscopy of biological molecules is a powerful and a plausible choice for the investigation of the low-frequency, function related dynamics of biomolecules. This thesis research aims to investigate the dynamic properties of water and biomolecules (i.e. DNA and protein) in the terahertz (THz) frequency range (i.e.100GHz-10THz) using terahertz time-domain spectroscopy (THz-TDS) and molecule dynamics simulation techniques.The thesis provides a brief introduction of the THz science and THz-TDS technology and discusses the molecular vibration, dielectric properties and absorption characteristics of substances in the THz region. The thesis also presents the detailed description of experiment data processing using double Debye modelThe dielectric properties of liquid water were studied with THz-TDS. The measured terahertz spectra of water were fitted to equations based on the Debye relaxation theory. Two typical relaxation time,89fs and10.55ps were obtained, which corresponds to the hydrogen bond breaking&re-formation and collective relaxation of water molecular network, respectively. Subsequently, a similar test of different concentrations of DNA is conducted.Following the investigation of water, the terahertz spectroscopy of DNA solution is performed to investigate the interactions between DNA and water molecules. Interestedly, we found that the first relaxation time constant of water decreased with increasing DNA concentration while the second relaxation time gradually increases. Further analysis indicated that a small amount of water molecules transited from the slow relaxation mode into the fast relaxation mode.Finally, with the aim to solve the dependence of spectral features on the convergence of molecular dynamics simulation, we proposed a new statistical method for predicting the spectral features associated with biological molecules. Instead of using long time simulation trajectory, this method calculates the terahertz spectrum of molecules based on many short trajectories. The method was employed to calculate the absorption peak positions of the protein, thioredoxin from Escherichia coli (E. coli), in the frequency range of10-25cm-1. The predicated absorption peak positions of thioredoxin show good correlation with measured results demonstrating that the proposed method can be applied to predict characteristic spectral features of biomolecules in the terahertz region.