How ions affect the structure of water.
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Hribar B, Southall NT, Vlachy V, Dill KA. Faculty of Chemistry and Chemical Technology, University of Ljubljana, Askerceva 5, 1000 Ljubljana, Slovenia.
We model ion solvation in water. We use the MB model of water, a simple two-dimensional statistical mechanical model in which waters are represented as Lennard-Jones disks having Gaussian hydrogen-bonding arms. We introduce a charge dipole into MB waters. We perform (NPT) Monte Carlo simulations to explore how water molecules are organized around ions and around nonpolar solutes in salt solutions. The model gives good qualitative agreement with experiments, including Jones-Dole viscosity B coefficients, Samoilov and Hirata ion hydration activation energies, ion solvation thermodynamics, and Setschenow coefficients for Hofmeister series ions, which describe the salt concentration dependence of the solubilities of hydrophobic solutes. The two main ideas captured here are (1) that charge densities govern the interactions of ions with water, and (2) that a balance of forces determines water structure: electrostatics (water's dipole interacting with ions) and hydrogen bonding (water interacting with neighboring waters). Small ions (kosmotropes) have high charge densities so they cause strong electrostatic ordering of nearby waters, breaking hydrogen bonds. In contrast, large ions (chaotropes) have low charge densities, and surrounding water molecules are largely hydrogen bonded. PMID:12371874
Charge density-dependent strength of hydration and biological structure.
The major intracellular anions (phosphates and carboxylates) are kosmotropes, whereas the major intracellular monovalent cations (K+; arg, his, and lys side chains) are chaotropes; together they form highly soluble, solvent-separated ion pairs that keep the contents of the cell in solution. PMID:8994593
Unraveling water's entropic mysteries: a unified view of nonpolar, polar, and ionic hydration.
Acc Chemical Research. 2008 Aug;41(8):957-67. Ben-Amotz D, Underwood R. Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, USA.
Most chemical processes on earth are intimately linked to the unique properties of water, relying on the versatility with which water interacts with molecules of varying sizes and polarities. These interactions determine everything from the structure and activity of proteins and living cells to the geological partitioning of water, oil, and minerals in the Earth's crust. The role of hydrophobic hydration in the formation of biological membranes and in protein folding, as well as the importance of electrostatic interactions in the hydration of polar and ionic species, are all well known. However, the underlying molecular mechanisms of hydration are often not as well understood. This Account summarizes and extends emerging understandings of these mechanisms to reveal a newly unified view of hydration and explain previously mystifying observations. For example, rare gas atoms (e.g., Ar) and alkali-halide ions (e.g., K+ and Cl-) have nearly identical experimental hydration entropies, despite the significant charge-induced reorganization of water molecules. Here, we explain how such previously mysterious observations may be understood as arising from Gibbs inequalities, which impose rigorous energetic upper and lower bounds on both hydration free energies and entropies. These fundamental Gibbs bounds depend only on the average interaction energy of a solute with water, thus providing a deep link between solute-water interaction energies and entropies. One of the surprising consequences of the emerging picture is the understanding that the hydration of an ion produces two large but nearly perfectly cancelling, entropic contributions: a negative ion-water interaction entropy and a positive water reorganization entropy. Recent work has also clarified the relationship between the strong cohesive energy of water and the free energy required to form an empty hole (cavity) in water. Here, we explain how linear response theory (whose roots may also be traced to Gibbs inequalities) can provide remarkably accurate descriptions of the process of filling aqueous cavities with nonpolar, polar, or charged molecules. The hydration of nonpolar molecules is well-described by first-order perturbation theory, which implies that turning on solute-water van der Waals interactions does not induce a significant change in water structure. The larger changes in water structure that are induced by polar and ionic solutes are well-described by second-order perturbation theory, which is equivalent to linear response theory. Comparisons of the free energies of nonpolar and polar or ionic solutes may be used to experimentally determine electrostatic contributions to water reorganization energies and entropies. The success of this approach implies that water's ability to respond to solutes of various polarities is far from saturated, as illustrated by simulations of acetonitrile (CH 3CN) in water, which reveal that even such a strongly dipolar solute only produces subtle changes in the structure of water. PMID:18710198
High and low density intracellular water.
Cell Molecular Biological (Noisy-le-grand). 2001 Jul;47(5):735-44. Wiggins PM. Genesis Research and Development Corp. Ltm., Auckland, New Zealand.
The proposal that liquid water consists of microdomains of rapidly-exchanging polymorphs of high and low density is examined for its impact upon roles of water in biology. It is assumed that the two polymorphs persist in solution and adjacent to surfaces and that solutes partition asymmetrically between them. It transpires that chaotropes are solutes which partition preferentially into low density water and displace the water equilibrium toward the high density polymorph. Kosmotropes. both ionic and non-polar, partition into high density water and induce low density water. Displacement of the water equilibrium at constant temperature and pressure has a thermodynamic cost which can be high. This appears to be a dominant factor in folding of proteins and DNA, aggregation of biopolymers and insolubility of non-polar kosmotropes. Cells control both the concentration of proteins and the selection of small solutes to produce an intracellular environment most conducive to co-ordinated enzyme function. Intracellular water has similar microdomains to bulk water, but surfaces and solutes redistribute them. Average properties, as measured by NMR are similar, but local properties on a nm scale may differ widely. Enzymes apparently use these local differences to activate cations for transport, induce movement and for synthesis. PMID:11728089
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