Water molecules and metal ions

RNA carries out its enzymic roles with many fewer chemical resources than its protein counterparts it has a ribose-phosphate backbone (Chap. 3), with four rather similar heterocyclic nucleobases, plus attendant water molecules and metal ions. They carry out the water-elimination condensation reactions in splicing certain classes of introns (Chap. 7) and in the site-specific cleavage and ligation of small replicating RNA circles in some species of organism. 

FIGURE 19.6 Space-filling model of a micelle formed by association of carboxylate ions derived from a long-chain carboxylic acid. The hydrocarbon chains tend to be on the inside and the carboxylate ions on the surface where they are in contact with water molecules and metal cations. 

The finding above that many coordination complexes between organic molecules and metal ions are essential to life raises interest in both their quantitative binding strengths and their exchange rates. While the monovalent ions Na+ and K+ bind very weakly in water, they are present in the environment and in cells in higher... 

Figure 7. A schematic representation of the microscopic model for the metal/electrolyte solution interface. Shown from top to bottom are an ion that is contact adsorbed with partial loss of its hydration shell, an ion whose hydration shell partially consists of first layer of water molecules, and an ion that is not contact adsorbed.

Figure 5-11 shows a simple model of the compact double layer on metal electrodes. The electrode interface adsorbs water molecules to form the first mono-molecular adsorption layer about 0.2 nm thick next, the second adsorption layer is formed consisting of water molecules and hydrated ions these two layers constitute a compact electric double layer about 0.3 to 0.5 nm thick. Since adsorbed water molecules in the compact layer are partially bound with the electrode interface, the permittivity of the compact layer becomes smaller than that of free water molecules in aqueous solution, being in the range from 5 to 6 compared with 80 of bulk water in the relative scale of dielectric constant. In general, water molecules are adsorbed as monomers on the surface of metals on which the affinity for adsorption of water is great (e.g. d-metals) whereas, water molecules are adsorbed as clusters in addition to monomers on the surface of metals on which the affinity for adsorption of water is relatively small (e.g. sp-metals). 

The AGch represents a coverage and potential-independent term for the different interactions of the water molecule and the ion during the adsorption process and it can be split into AG°h = AG°hd + AG°h w. These interactions include dispersion and electronic forces (Section 6.8.2.1) of the ion and water molecules, as well as partial dehydration of the ion and the metal (Section 6.8.2.2). 

Macromolecular conformations and reversible order-disorder and disorder-order transitions are highly sensitive to solvent, temperature, pressure, pH, water activity, and metal ions. Polyanions are distinguished from neutral molecules by their sensitivity to electrolytes. Whereas synthetic polymers do not normally dissolve or disperse spontaneously, some polysaccharides may do so in water (hydration), given their strong hydrophilicity. 

Mn (0.97, 0.81) Co (0.72) Mg (0.86), and Al (0.58). The solvent exchange rates (Kex S 1) for inner-sphere water molecules in metal ions are Al(10°) Mg (105) Co (105 5), and Mn(106 7) [21].The order of increasing rate constants in acidic solution is, Al Ligand exchange rates take on special importance for Al because they are slow and the system may not be at equilibrium. Mg, Mn, and Co have around 105 faster exchange rate over Al. These differential characteristics of the metals play a crucial role in metal-DNA interactions. 

Table 11.1 Actinide aqua ions - numbers of bound water molecules and metal-water distances"...

It has been mentioned in Section III that [CoCl4] is unstable in water even in the presence of large amounts of alkali metal chlorides. In contrast [CoCl4] is quantitatively formed in concentrated aqueous solutions of hydrochloric acid (66). Outer-sphere interactions between water molecules and hydronium ions will lead to the formation of highly aggregated clusters [H(H20) ]. In this way the concentration of free... 

Electrocatalysis in metallic corrosion may be classified into two groups Adsorption-induced catalyses and solid precipitate catalyses on the metal surface. In general, the bare surface of metals is soft acid in the Lewis acid-base concept and tends to adsorb ions and molecules of soft base forming the covalent binding between the metal surface and the adsorbates. The Lewis acidity of the metal surface however may turn gradually to be hard as the electrode potential is made positive, and the bare metal surface will then adsorb species of hard base such as water molecules and hydroxide ions in aqueous solution. Ions and molecules thus adsorbed on the metal surface catalyze or inhibit the corrosion processes. Solid precipitates, on the other hand, are produced by the combination of hydrated cations of hard acid and anions of hard base forming the ionic bonding between the cations and the anions on the metal surface. 

When metal ions are dissolved in water, metal-bound water molecules and metal-bound hydroxide ions are also produced and play important catalytic roles. 

The standard partial molar volume of an ion in aqueous solution, Vf, is the actual volume to be assigned to the ion in the solution (at infinite dilution). It is the sum of its intrinsic volume, Tnntr", and the electrostriction that the ion has caused in the water around it, yieiec , the latter being a negative quantity. The volume of a bare unhydrated ion, A%Np,l i)r, cannot represent its intrinsic volume and must be enlarged to account for the void spaces between the water molecules and the ion and among themselves in order to represent the intrinsic volume of the ion in the solution. A factor of A = 1.213 was proposed by Mukerjee (1961) for the alkali metal and the halide ions, producing ... 

---