Environmental and binding dependence of protein terahertz dielectric response
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Terahertz (THz) generation and detection techniques have been developed widely and improved in the last few decades. THz techniques have been applied to many fields, such as medical support (THz imaging for breast cancer and skin cancer), security devices, material studies and astronomy. In our group, we utilize THz time domain spectroscopy (1-67 cm -1 , 30 GHz-2 THz, 0.1-8 meV) to study both intramolecular interaction and intermolecular interaction, such as the flexibility changes due to the electron transfer of cytochrome c and ligand binding of lysozyme and tri-N-acetyl-D-glucosamine. My work is focused on the electron transfer of cytochrome c and the hydration and temperature effects on cytochrome c. In the studies of cytochrome c, I investigated the THz dielectric response of cytochrome c as a function of oxidation, hydration and temperature in order to elucidate the underlying structural mechanisms involved in the electron transfer of cytochrome c. The THz dielectric response of cytochrome c dramatically increases with oxidation, implying either a significant global softening of the potential and/or a significant increase in polarizability with oxidation. Hydration dependence measurements show that a difference in the equilibrium water content with oxidation does not explain the change in THz response. The temperature dependence of both ferri- and ferro-cytochrome c THz dielectric response also cannot elucidate this large increase with oxidation. The results of these different THz dielectric measurements of cytochrome c strongly suggest that the flexibility increases as a function of oxidation state. The experimental results were compared to the normal mode calculations. These calculations were performed to determine what impact electron transfer and hydration have on the harmonic response of cytochrome c. The calculated normal mode density from CHARMM shows similar frequency dependence at low frequencies for all hydrations. The hydration dependence of the calculated absorption intensities are in approximate agreement with the measured THz absorbance, except at low hydrations and at low frequencies. The breakdown in agreement may be due to all waters, including crystal waters, being treated with the simplified TIP3 water force field. In the study of ligand binding, a method for rapid determination of protein ligand binding on solution phase samples using terahertz dielectric spectroscopy is demonstrated. Measurements were performed using terahertz time domain spectroscopy on aqueous solutions below the liquid-solid transition for water. Small ligand binding sensitivity was demonstrated using tri-acetylgluosamine and hen egg white lysozyme with a decrease in dielectric response with binding. The magnitude of the change increases with frequency.