4. Structure of Ionic Hydration Shells 1351 4.1. Results from Diffraction Experiments 1351 4.1.1. X-ray Diffraction 1351 4.1.2. Neutron Diffraction 1351 4.2. Results from Computer Simulations 1352 4.3. Results from Spectroscopic Measurements 1354 4.3.1. Vibrational Spectroscopic Measurements 1354 4.3.2. EXAFS Spectroscopy 1354 4.3.3. NMR Relaxation Studies 1355 4.3.4. Dielectric Relaxation Studies 1355 4.4. Summary of the Structure of Ionic Hydration Shells 1355

Effect of ions on the structure of water: structure making and breaking.

The dynamics of water molecules next to hydrophobic solutes is investigated, specifically addressing the recent controversy raised by the first time-resolved observations, which concluded that some water molecules are immobilized by hydrophobic groups, in strong contrast to previous NMR conclusions Through molecular dynamics simulations and an analytic jump reorientation model, we identify the water reorientation mechanism next to a hydrophobic solute and provide evidence that no water molecules are immobilized by hydrophobic solutes Their moderate rotational slowdown compared to bulk water (eg, by a factor of less than 2 at low solute concentration) is mainly due to slower hydrogen-bond exchange The slowdown is quantitatively described by a solute excluded volume effect at the transition state for the key hydrogen-bond exchange in the reorientation mechanism We show that this picture is consistent with both ultrafast anisotropy and NMR experimental results and that the transition state excluded vol

Why Water Reorientation Slows without Iceberg Formation around Hydrophobic Solutes

The reorientation of a water molecule is important for a host of phenomena, ranging over--in an only partial listing--the key dynamic hydrogen-bond network restructuring of water itself, aqueous solution chemical reaction mechanisms and rates, ion transport in aqueous solution and membranes, protein folding, and enzymatic activity. This review focuses on water reorientation and related dynamics in pure water, and for aqueous solutes with hydrophobic, hydrophilic, and amphiphilic character, ranging from tetra-methylurea to halide ions and amino acids. Attention is given to the application of theory, simulation, and experiment in the probing of these dynamics, in usefully describing them, and in assessing the description. Special emphasis is placed on a novel sudden, large-amplitude jump mechanism for water reorientation, which contrasts with the commonly assumed Debye rotational diffusion mechanism, characterized by small-amplitude angular motion. Some open questions and directions for further research are also discussed.

Reorientation and allied dynamics in water and aqueous solutions.

Macroscopic characterizations of hydrophobicity (e.g., contact angle measurements) do not extend to the surfaces of proteins and nanoparticles. Molecular measures of hydrophobicity of such surfaces need to account for the behavior of hydration water. Theory and state-of-the-art simulations suggest that water density fluctuations provide such a measure; fluctuations are enhanced near hydrophobic surfaces and quenched with increasing surface hydrophilicity. Fluctuations affect conformational equilibria and dynamics of molecules at interfaces. Enhanced fluctuations are reflected in enhanced cavity formation, more favorable binding of hydrophobic solutes, increased compressibility of hydration water, and enhanced water-water correlations at hydrophobic surfaces. These density fluctuation–based measures can be used to develop practical methods to map the hydrophobicity/philicity of heterogeneous surfaces including those of proteins. They highlight that the hydrophobicity of a group is context dependent and is ...

Hydrophobicity of Proteins and Interfaces: Insights from Density Fluctuations

There are fundamental and not yet fully resolved questions concerning the impact of solutes, ions in particular, on the structure and dynamics of water, which can be formulated as follows: Are the effects of ions local or long-ranged? Is the action of cations and anions on water cooperative or not? Here, we investigate how the reorientation and hydrogen-bond dynamics of water are affected by ions in dilute and concentrated aqueous salt solutions. By combining simulations and analytic modeling, we first show that ions have a short-ranged influence on the reorientation of individual water molecules and that depending on their interaction strength with water, they may accelerate or slow down water dynamics. A simple additive picture combining the effects of the cations and anions is found to provide a good description in dilute solutions. In concentrated solutions, we show that the average water reorientation time ceases to scale linearly with salt concentration due to overlapping hydration shells and struct...

Mechanisms of Acceleration and Retardation of Water Dynamics by Ions

NMR studies on dynamic behavior of water molecule in aqueous denaturant solutions at 25 °C: Effects of guanidine hydrochloride, urea and alkylated ureas

Kenichi Yukiyasu

Papers

NMR studies on dynamic behavior of water molecule in aqueous denaturant solutions at 25 °C: Effects of guanidine hydrochloride, urea and alkylated ureas

NMR studies on dynamic behavior of water molecules in tetraalkylammonium bromide-D2O solutions at 5–25°C+☆