Abstract
Hydrogen bonding properties of water molecules, which are confined in microcavities of biological interfaces, are significantly different from those of bulk water and drive most of the complex biological processes. While NMR, X-ray and UV–vis-IR spectroscopic techniques have been found inadequate for describing the dynamics of the thick (20–40 Å) sheath of hydration layer around biomolecules, recently developed THz spectroscopy has emerged as a powerful technique to directly probe the collective dynamics of hydrogen bonds in the hydration layer, which control all important functions of the biomolecules in life. Both laser and accelerator-based THz sources are intense enough to penetrate up to about 100 μm thick water samples, which makes THz transmission and/or dielectric relaxation measurements possible in aqueous solutions. These measurements provide valuable information about the rattling and rotational motions of hydrated ions, making, breaking and rearrangement of hydrogen bonds in hydration layer as well as hydrophilic and hydrophobic interactions between biomolecule and water. THz spectroscopy has also been successfully applied to study the effect of modulation of the physical conditions, like temperature, pH, concentration of proteins and chemical additives, on the structure and dynamics of hydration layer. THz spectroscopy has also been applied to study the processes of denaturation, unfolding and aggregation of biomolecules.
Highlights
Water molecules, which reside on the surfaces of proteins or lipid bilayers or in tissues and cells, exhibit properties that are significantly different from those found in pure or bulk water as water molecules in such biological systems face additional interactions [1–7]
Since the development of this understanding that biomolecules are surrounded by a sheath of hydration water, which takes active part in all of their normal activities, extensive efforts have been made to perceive the detailed structure and dynamics of the hydration layer using a variety of spectroscopic methods, such as X-ray and neutron scattering [14, 29, 30], NMR [31–33], second harmonic generation [34, 35] and ultrafast fluorescence and IR spectroscopies [1, 2, 22, 26, 36, 37], assisted by ab initio and molecular dynamics simulations [24, 25, 38, 39]
In a separate study [113], a combined experimental (mid- and far-infrared Fourier transform infrared (FTIR) spectroscopy, THz time-domain spectroscopy (THz-TDS) (0.3–1.6 THz)) and molecular dynamics (MD) simulation technique has been carried out to understand the evolution of the structure and dynamics of water in its binary mixture with 1,2-dimethoxy ethane (DME) over the entire concentration range
Summary
Water molecules, which reside on the surfaces of proteins or lipid bilayers or in tissues and cells, exhibit properties that are significantly different from those found in pure or bulk water as water molecules in such biological systems face additional interactions [1–7]. Since the development of this understanding that biomolecules are surrounded by a sheath of hydration water, which takes active part in all of their normal activities, extensive efforts have been made to perceive the detailed structure and dynamics of the hydration layer using a variety of spectroscopic methods, such as X-ray and neutron scattering [14, 29, 30], NMR [31–33], second harmonic generation [34, 35] and ultrafast fluorescence and IR spectroscopies [1, 2, 22, 26, 36, 37], assisted by ab initio and molecular dynamics simulations [24, 25, 38, 39]. This has offered a unique view of the hydration water in fully solvated biomolecules
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