Probing Biological Water Using Terahertz Absorption Spectroscopy

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.

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“…For our calculations, it is crucial to note that the neurotransmitter molecules exist in an aqueous solution and are thus embedded in a kind of active water matrix, which can be attributed to water not being an inert solvent but, rather, forming hydration layers around solutes [55]. THz spectroscopy of water implies that this frequency band is dominated by highly correlated molecular motion, in particular by collective vibrations of hydrogen bonds, suggesting a significant influence on the dynamics of the solutes that engenders the amplification of vibrational resonances [55,56]. More specifically, studies show that the enhancement of the absorption coefficients of the hydrated biomolecules depends on their hydrophilicity [56].…”

“…THz spectroscopy of water implies that this frequency band is dominated by highly correlated molecular motion, in particular by collective vibrations of hydrogen bonds, suggesting a significant influence on the dynamics of the solutes that engenders the amplification of vibrational resonances [55,56]. More specifically, studies show that the enhancement of the absorption coefficients of the hydrated biomolecules depends on their hydrophilicity [56]. For glutamate, the most lipophobic neurotransmitter, which in aqueous solutions is ionized and found to form NaGlu ion pairs, it turns out that the effective dipole moment of the molecular anions embedded in the water matrix is considerably increased compared to the bare anions [57].…”

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“…Also, BSW water research shows up in studies investigating extreme drought-tolerant plants and in extreme cold-tolerant aquatic animals and fish [87][88][89][90]. Even the latest research with non-invasive monitoring of human hydration levels with terahertz spectroscopy shows that BSW water is present at most cellular interfaces [91][92][93][94]. The literature appears unanimous that in vivo biological water at the interfacial cell surfaces has unique water properties and functions.…”