Anisotropy in the dielectric spectrum of hydration water and its relation to water dynamics

Abstract

Proteins, molecules, and macromolecular assemblies in water are surrounded by a nanometer-sized hydration layer with properties very different from bulk water. Here, we use classical molecular dynamics simulations to study the dielectric response of hydration water next to hydrophobic and hydrophilic planar surfaces. We find the interfacial dielectric absorption of water to be strongly anisotropic: compared to bulk water, which shows a broad dielectric absorption maximum around 15 GHz in the imaginary part of the dielectric function, the absorption for electric fields parallel to the surface is of similar strength and shows a slight redshift, while for perpendicular electric fields it is strongly attenuated and blueshifted. This anisotropy is generic for hydrophobic and hydrophilic surfaces. From our spatially resolved dielectric functions and a modified Maxwell-Garnett theory that accounts for anisotropic hydration layers around spherical particles, the dielectric absorption of solutions of organic molecules and micelles is derived to exhibit the experimentally known attenuation in combination with a redshift. These two features are traced back to the subtle interplay of interfacial depolarization effects and the dielectric anisotropy in the hydration layer. By a detailed analysis of the individual water molecule dynamics the perpendicular blueshift is shown not to be linked to accelerated water reorientation, but rather to dielectric boundary effects. Carefully conducted angularly resolved experiments at planar aqueous interfaces will be able to resolve this dielectric anisotropy and thus to confirm the subtle connection between spectral absorption features and the molecular water dynamics in hydration layers.

Sub-terahertz spectroscopy reveals that proteins influence the properties of water at greater distances than previously detected

“…Increasing numbers of research groups around the world have studied bio-molecules in a solvated state by means of THz radiation. Significant progress has been made in interpretation of complex dielectric spectra of solvated bio-molecules partially facilitated by molecular dynamics (MD) simulations 23 and technological advances in THz spectroscopy. 24 Studies by the Group of Havenith are focused mainly about 2.1-2.7 THz, with the probing radiation produced by a pgermanium laser, and applied to the study of a wide range of bio-molecules, e.g., DNA, proteins, peptides, amino acid, and carbohydrates.…”

Electrostatics of liquid interfaces

“…Both equations (19) and (20) show that the only nonlocal part of the microscopic field E m caused by the polarized dielectric comes from the field of the surface charges. According to Eq.…”

“…Qualitatively similar results have been reported for aqueous interfaces. 19,25,26 On the other hand, the longitudinal symmetry of the polarization field, which makes direct calculations according to Eq. (25) largely impractical, can be used to obtain an equation more suitable for numerical applications.…”

“…According to Eq. (19), it is the only field of the polarized dielectric that a measuring device (e.g., a spectroscopic probe) placed either outside of the dielectric or inside a void can directly detect. It is, therefore, this field, and the corresponding surface charge density σ, that is the main focus of our formalism.…”

“…14 Effective screening of the perturbation produced by creating the interface occurs as the result of this structural adjustment, and both the density and orientational perturbations of the liquid propagate only a few molecular layers into the bulk. [15][16][17][18][19][20] The polar response is then dominated by the interface, and the language of interfacial polarization is the most relevant for describing polarized polar liquids. 17 If, next, the microscopic orientational structure of the interface is incorporated into the definition of the surface charge density, one can arrive at a physically motivated formulation of the electrostatics of liquid dielectrics.…”