Hydrates water molecules

Crystals of uranyl perchlorate, U02(C10[13093-00-0] have been obtained with six and seven hydration water molecules. The uranyl ion is coordinated with five water molecules (4) in the equatorial plane with a U—O(aquo) distance of 245 nm (2.45 E). The perchlorate anion does not complex the uranyl center. The unit cells contain two [0104] and one or two molecules of hydration water held together by hydrogen bonding (164). 

The NMR study by Wiithrich and coworkers has shown that there is a cavity between the protein and the DNA in the major groove of the Antennapedia complex. There are several water molecules in this cavity with a residence time with respect to exchange with bulk water in the millisecond to nanosecond range. These observations indicate that at least some of the specific protein-DNA interactions are short-lived and mediated by water molecules. In particular, the interactions between DNA and the highly conserved Gin 50 and the invariant Asn 51 are best rationalized as a fluctuating network of weak-bonding interactions involving interfacial hydration water molecules. 

In one of the cages within which gas molecules are trapped in methane hydrate, water molecules form a pentagonal dodecahedron, a three-dimensional figure in which each of the 12 sides is a regular pentagon. 

For Ca and Ba, whose n values are larger than 10, however, it is thought that some hydrated water molecules not only in the first hydration shell but also in the second hydration shell are cotransferred into NB. Accordingly, it can be supposed that some water molecules in the first hydration shell (i.e., in the vicinity of the ion) are covered with the second hydration shell, so that they cannot be associated with outer solvent... 

Of special interest is the dehydration of polynuclear technetium bromide clusters, which contain hydroxonium cations with different numbers of hydration water molecules. Analysis of the results obtained leads us to conclude that at 140-200 °C dehydration occurs with a partial decomposition of the [H30(H20)3] + cations (30). 

Infrared spectra were obtained with a Perkin-Elmer 1800 and a Nicolet Magna-IR 750 FTIR spectrophotometer, and the absorption frequencies are reported in wave numbers (cm4). NMR spectra were obtained with BZH-300 and CA-F-300 Bruker FTNMR 300 MHz spectrometers. Chloroform-d was used as solvent, and all chemical shifts are reported in parts per million downfield (positive) of the standard. H-NMR and 13C-NMR chemical shifts are reported relative to internal tetramethylsilane, while 19F-NMR chemical shifts are reported relative to internal fluorotrichloromethane, Rf values were obtained from silica gel thin-layer chromatography developed with a mixture of 1.5 mL methylene chloride and three drops of acetone. The number of hydrate water molecules was calculated from the integration of H-NMR spectra. 

Surface catalysis affects the kinetics of the process as well. Saltzman et al. (1974) note that in the case of Ca -kaolinite, parathion decomposition proceeds in two stages with different first-order rates (Fig. 16.14). In the first stage, parathion molecules specifically adsorbed on the saturating cation are quickly hydrolyzed by contact with the dissociated hydration water molecules. In the second stage, parathion molecules that might have been initially bound to the clay surface by different mechanisms are very slowly hydrolyzed, as they reach active sites with a proper orientation. 

The effects specific to the coordination center and the cooperative effects between these emd the hydrated water molecules together determine the final arrangement in the hydrated complex ion and hence the hydration structure surrounding a given ion this structure is characteristic and specific for each species. 

When aluminum chloride salt is dissolved in water, aluminum (111) cations become surrounded by clusters of six water molecules to form a hexahydrated aluminum cation, Al(H20)g ". Being a Group lllA metal, aluminum easily gives up its valence electrons. The oxygen atom in water possesses two lone pairs. What kind of bonding most likely occurs between the aluminum and the hydrating water molecules ... 

A coordinate covalent bond forms between the aluminum and the hydrating water molecules. Aluminum is a Group lllA element, so the aluminum (111) cation (with a chcirge of -1-3) formally has no valence electrons. The oxygen of water has lone pairs. Therefore, water molecules most likely hydrate the cation by donating lone pairs to form coordinate covalent bonds. In this respect, you can call the water molecules ligands of the metal ion. 

Evaporites on Mars and Europa. The NASA s robotic explorers, Spirit and Opportunity, landed on at Mars and examined their landing sites for past environmental conditions. Kinds of minerals in a hot-spring environment and dried-up lake beds were photographed suggesting future use of ESR to date these evaporate with a portable ESR on the rover. Sulfate mineral precipitation, epsonite, MgS04 with 7 hydration water molecules in frozen ice, was studied by sampling the icy environment, especially icy fault on the surface of Europa, a satellite of Jupiter.61... 

Polymer complexation frequently leads to dehydration and precipitation of polymer [15-32] as shown in Fig, 2. To cause polymer complexation, attractive forces between polymers are needed, and the force must overwhelm the strength of interaction between polymer chains and hydrating water molecules. In this process, water molecules must be replaced by competing polymer contacts. 

Numerical data are much more interesting—and lead to deeper understanding—when we can find patterns in the numbers. When we examine the data in Table 8.8, we see that the transfer of an ion from the gas phase into water is more exothermic the greater the charge of the ion and the smaller its radius. There are exceptions Ag+ is bigger than Na+, but its hydration is more exothermic. The explanation of this anomaly may be that the Ag+ ion can form covalent bonds with the hydrating water molecules. The low solubility of silver salts supports this explanation. 

These metal cations are too large or have too low a charge to have an appreciable polarizing effect on the hydrating water molecules that surround them, so the water molecules do not readily release their protons. These cations are sometimes called neutral cations, because they have so little effect on the pH. 

FIGURE 17.5 When NaCl dissolves in water, the crystal breaks up, and the Na+ and Cl- ions are surrounded by hydrating water molecules. The polar H20 molecules are oriented such that the partially negative O atoms are near the cations and the partially positive H atoms are near the anions. Disruption of the crystal increases the entropy but the hydration process decreases the entropy. For dissolution of NaCl, the net effect is an entropy increase. 

This is one distinguishing feature between hydrates and ice water molecules diffuse two orders of magnitude slower in hydrates than in ice. As shown in Table 2.8, ice water molecules diffuse almost an order of magnitude faster than they reorient about a fixed position in the crystal structure. In direct contrast, hydrate water molecules reorient 20 times faster than they diffuse. As for all... 

The dielectric constant values in Table 2.8 also suggest that, while hydrate water molecules reorient rapidly compared to molecules in other solids, reorientation rates are only one-half those in ice. The hydrate value is lower than that of ice due to the lower density of hydrogen-bonded water molecules. 

Historically, two periods occurred for the determination of the number of hydrate water molecules per guest molecule. In the first century (1778-1900) after the discovery of hydrates, the hydration number was determined directly. That is, the amounts of hydrated water and guest molecules were each measured via various methods. The encountered experimental difficulties stemmed from two facts (1) the water phase could not be completely converted to hydrate without some occlusion and (2) the reproducible measurement of the inclusion of guest molecules was hindered by hydrate metastability. As a result, the hydrate numbers differed widely for each substance, with a general reduction in the ratio of water molecules per guest molecule as the methods became refined with time. After an extensive review of experiments of the period, Villard (1895) proposed Villard s Rule to summarize the work of that first century of hydrate research ... 

After 1900 the direct determination of hydrate number was abandoned in favor of the second, indirect method. The indirect method is still in use today and is based on calculation of the enthalpies of formation of hydrate from gas and water, and from gas and ice. This method was originally proposed by de Forcrand (1902) who used the Clapeyron equation to obtain the heat of dissociation from three-phase, pressure-temperature data, as in the below paragraph. With this more accurate method many exceptions were found to Villard s Rule. The historical summary provided in Chapter 1 indicates that while the number of hydrated water molecules was commonly thought to be an integer, frequently that integer... 

M1 = K+, NH4+, Rb +, and Cs+) the hydrated M1 ions are partially coordinated with chloride ions, water molecules may be easily replaced with chloride ions to form [M Clg]5- as a part of the crystal formation process, because the MI-OH2 bond is rather weak. The rate constant for substitution may be in the order of 10 s 1, which is the rate constant for replacement of a hydrated water molecule of the alkali metal ion (59). Since the rate of formation of clusters from hydrated magne-sium(II) ions and partially hydrated chloro complexes of alkali metal... 

A hydrated water molecule is often referred to as H+ (aq), so the above equation may be rewritten as the following reaction that defines Kw. As with Kb, the concentration of water is nearly constant. 

We recently developed a systematic method that uses the intrinsic tryptophan residue (Trp or W) as a local optical probe [49, 50]. Using site-directed mutagenesis, tryptophan can be mutated into different positions one at a time to scan protein surfaces. With femtosecond temporal and single-residue spatial resolution, the fluorescence Stokes shift of the local excited Trp can be followed in real time, and thus, the location, dynamics, and functional roles of protein-water interactions can be studied directly. With MD simulations, the solvation by water and protein (residues) is differentiated carefully to determine the hydration dynamics. Here, we focus our own work and review our recent systematic studies on hydration dynamics and protein-water fluctuations in a series of biological systems using the powerful intrinsic tryptophan as a local optical probe, and thus reveal the dynamic role of hydrating water molecules around proteins, which is a longstanding unresolved problem and a topic central to protein science. 

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