Picosecond dynamics of hydrated water in biomolecular solution revealed by terahertz time-domain attenuated total reflection spectroscopy

We propose a novel method to characterize hydration state in solution from dielectric responses in terahertz (THz) frequency region. Slower motion of hydrated water molecules than that of bulk water molecules results in the decrement in dielectric loss, which is detected using THz time-domain attenuated total reflection (TD-ATR) spectroscopy. By fitting experimental data with a dielectric function derived based on local field theory, we can determine several physical constants characterizing hydration state e.g. hydration number. Our method provides new tool to investigate picosecond dynamics of hydration directly which plays important roles in various phenomena in solution.