Solute-induced retardation of water dynamics probed directly by terahertz spectroscopy

Abstract

Hydrogen bond rearrangement in water occurs on the picosecond time scale, so relevant experiments must access these times. Here, we show that terahertz spectroscopy can directly investigate hydration layers. By a precise measurement of absorption coefficients between 2.3 THz and 2.9 THz we could determine the size and the characteristics of the hydration shell. The hydration layer around a carbohydrate (lactose) is determined to extend to 5.13 ؎ 0.24 Å from the surface corresponding to Ϸ123 water molecules beyond the first solvation shell. Accompanying molecular modeling calculations support this result and provide a microscopic visualization. Terahertz spectroscopy is shown to probe the collective modes in the water network. The observed increase of the terahertz absorption of the water in the hydration layer is explained in terms of coherent oscillations of the hydration water and solute. Simulations also reveal a slowing down of the hydrogen bond rearrangement dynamics for water molecules near lactose, which occur on the picosecond time scale. The present study demonstrates that terahertz spectroscopy is a sensitive tool to detect solute-induced changes in the water network.hydration water dynamics ͉ molecular dynamics simulations of biomolecules ͉ solvated lactose T he many unusual properties of water combined with its importance as the solvent of life account for its continued study by numerous researchers. Indeed, the properties of water are essential in the behavior of all biological systems. For example, water plays a central role in the folding and function of proteins, and the function of carbohydrates. The dynamics of water surrounding a solute is of fundamental importance in a wide range of processes in solution. In particular, the properties of water molecules near the surface of a biomolecule have been the subject of numerous and sometimes controversial experimental and theoretical studies (1-5). The characteristics and role of this ''biological'' water, with properties that differ considerably from those of bulk water, are still not fully understood (6).The importance of the hydration layer or the biological water is obvious in specific protein processes, such as opening and closing of channels, the kinetics of which has been found to be coupled with the solvent fluctuations (1). Important questions that remain so far unanswered include the following: How does solvation water differ from bulk water, and how large is the solvation layer that can be attributed to solvent water? Simulations suggest the existence of rather rigid water structures around proteins (5) and carbohydrates (7), but experimental studies that characterize the hydration layer are limited. Hydrogen bond rearrangements in water occur on the picosecond time scale (8), so that a detailed understanding of the relevant processes at a molecular level requires experimental techniques that are able to probe the hydration layers on this time scale. However, experimental investigations of fast dynamics of hydration water still remain a ch...

Dissecting the THz spectrum of liquid water from first principles via correlations in time and space

“…One can even detect the onset of another modulation of α P ðν ref ¼ 80 cm −1 ;R 0 Þ around R 0 ≈ 6.7 Å which coincides with the third solvation shell according to the third maximum of the g com ðrÞ. The intensity ratio between the maximum value found in the second shell and the bulk value, i.e., been (18,20) and ascribed to an "extended dynamical solvation shell" (20). Overall, the long-ranged modulation of the distance-dependence in the THz frequency domain, stemming from intermolecular modes, is distinctly different from the essentially monotonous behavior as observed for intramolecular modes in the IR regime, as exemplified for the bending mode at ≈1;630 cm −1 .…”

“…The second frequency window,ν ref ≈ 80 AE 10 cm −1 , is selected based on the results of previous studies in which the absorption at 2.4 THz ≈ 80 cm −1 revealed long-range effects in solvation dynamics of various molecules using THz laser spectroscopy (18,20). Most interestingly, the distance-resolved IR spectra, α P ðν ref ;R 0 Þ, uncover a strong and long-ranged spatial modulation atν ref ≈ 80 cm −1 where no peak can be discerned in the bulk absorption spectrum, αðν ref Þ, see Fig.…”

“…Using THz laser spectroscopy, water dynamics has been shown to be influenced within an "extended dynamical hydration shell" (20) of about 6-7 Å for small solutes like carbohydrates (18) by changes in the absolute absorption intensity as probed at 2.4 THz (i.e., ≈80 cm −1 ). For proteins (20), this shell extends even up to ≈20 Å and is affected by e.g., folding state (21).…”

“…The intermolecular vibrational modes in the frequency window from 1 THz up to about 10 THz (16,17), corresponding to ≈30-300 cm −1 , have been proven recently to be a unique tool to study the hydration dynamics around solutes (18)(19)(20)(21). Using THz laser spectroscopy, water dynamics has been shown to be influenced within an "extended dynamical hydration shell" (20) of about 6-7 Å for small solutes like carbohydrates (18) by changes in the absolute absorption intensity as probed at 2.4 THz (i.e., ≈80 cm −1 ).…”

The effect of the flexibility of hydrogen bonding network on low-frequency motions of amino acids. Evidence from Terahertz spectroscopy and DFT calculations

“…The photon energy of THz radiation can give rise to collective atomic motions of compound. These low-frequency vibrational modes are usually associated with motions of hydrogen-bonding networks which play essential roles on stabilization of biomolecules in solid state [1,2], folding of proteins [3] and solvation of proteins [4] and some other biomolecules [5,6,7,8] in aqueous phase.…”