Bound solvent

An alternative in writing this last result would be to use Pj since the density of the bound solvent is expected to be different from that in bulk. Rather than pursue this, however, we shall assume instead that Pi 5 Pi basis,... 

From the viewpoint of molecular interactions, the number of fundamentally distinct chromatographic stationary phases is very limited.17 One mechanism for adsorption to the stationary phase is solvophobic, or mobilestationary phase transfer free energy effects, in which the adsorption of an analyte to the stationary phase liberates bound solvent. There is often an accompanying enthalpic component to such binding through dispersion interactions. Another mechanism for adsorption is that of specific interactions,... 

Most authors have accounted for the mutual influence of ions and solvent molecules only by assuming a firmly bound solvent sheath around the ion. The structure of the bulk solvent and the influence of electrolyte concentration on this structure are not taken into consideration. 

Thioethers lack the capacity to neutralize positive charge and display weak donor properties. Consequently, they do not readily displace strong donor solvents (water) or strongly bonding anions (such as halides) from the coordination sphere. As a consequence, many thioether complex syntheses employ aprotic or alcoholic solvents and precursor complexes with weakly bound solvents (such as DMSO or acetone) or anions (such as C+3S03 ). Despite the synthetic challenges, a wide range of complexes has been reported, particularly with the cyclic poly-thioethers, where the macrocyclic effect overcomes many of the above difficulties. 

This review deals largely with work from my own laboratory. It attempts to show the reader some of the recent developments in the field and the breadth of the scientific questions which are being addressed through investigations of the kinetics and dynamics of ion-molecule reactions as mediated through the presence of bound solvent molecules. 

Mechanistic interpretation of activation volumes on square-planar complexes is complicated by the geometry. The sterically less crowded complexes may have loosely bound solvent molecules occupying the axial sites above and below the plane. Replacing them in the formation of a five-coordinate transition state or intermediate may result by compensation in relatively small volume effects. It is therefore difficult to distinguish between Ia and A mechanisms from the value of the activation volume. Nevertheless, the AV values are negative and together with the second-order rate laws observed, point to an a-activation for those solvent exchange reactions. 

Zinc may function to promote the nucleophilicity of a bound solvent molecule in both small-molecule and protein systems. The p/Ca of metal-free H2O is 15.7, and the p/Ca of hexaaquo-zinc, Zn (OH2)6. is about 10 (Woolley, 1975) (Table III). In a novel small-molecule complex the coordination of H2O to a four-coordinate zinc ion reduces the to about 7 (Groves and Olson, 1985) (Fig. 2). This example is particularly noteworthy since it has a zinc-bound solvent molecule sterically constrained to attack a nearby amide carbonyl group as such, it provides a model for the carboxypeptidase A mechanism (see Section IV,B). To be sure, the zinc ligands play an important role in modulating the chemical function of the metal ion in biological systems and their mimics. 

Fig. 21. Indirect carboxylate-zinc interactions through bridging hydroxyl groups (Fig. 20) orient the nucleophilic lone electron pair (stippled dumbbell) of zinc-bound solvent. This reduces the conformational disorder about the Zn -O axis, and thereby reduces the entropic barrier to catalysis (Merz, 1990).

Another contrast between the zinc proteases and the carbonic an-hydrases concerns the zinc coordination polyhedron. The carbonic an-hydrases ligate zinc via three histidine residues, whereas the zinc proteases ligate the metal ion through two histidine residues and a glutamate (bidentate in carboxypeptidase A, unidentate in thermolysin). Hence, the fourth ligand on each catalytic zinc ion, a solvent molecule, experiences enhanced electrostatic polarization in carbonic anhydrase II relative to carboxypeptidase A. Indeed, the zinc-bound solvent of carbonic anhydrase II is actually the hydroxide anion [via a proton transfer step mediated by His-64 (for a review see Silverman and Lindskog, 1988)]. 

Electron transfer is coupled with proton transfer involving the metal-bound solvent molecule. For Eqs (14) and (15) to be thermodynamically favorable, the potential of the Mn / -SOD couple must lie between the values for superoxide oxidation and reduction (02 — O2 + e ,... 

Solvent or ligand Interactions with tight Ion pairs produce externally complexed tight Ion pairs and/or ligand separated Ion pairs. The stability of the complexes depends on solvent, temperature, type of crown and the nature of the cation. For example, In ethereal solvents benzo-15-crown-5 and fluorenyl sodium (Fl-.Na ) form the two Isomeric complexes I and II depicted In reaction 1, but the ratio I/II Is highly solvent sensitive (9) (If the bound solvent In II Is Included In the structure of II, the two complexes of course can actually not be considered Isomeric). 

For complex III, the Na+ Is probably as accessible to solva-tlon by solvent molecules as is the Na In the tight Fl-,Na+ Ion pair. Hence, no externally bound solvent molecules need to be removed. This may be different In other systems. For example, the formation constant of a loose Ion pair complex between FI", Na+ and tetraglyme (tetraethylene glycol dimethyl ether) Is nearly four times lower In dloxane than In THF (10). This may be caused by specific solvent effects rather than by the difference In solvent dielectric constant. The flexible glyme ligand wraps Itself around the Na+ Ion, and this may make It more difficult for solvent molecules to remain bound to Na+ In the glyme-separated Ion pair. 

We have already seen that the ratio f/f0 describes the effect on the friction factor of either solvation, ellipticity, or both. This ratio equals unity for an unhydrated sphere and increases with both the amount of bound solvent and the axial ratio of the particles. We are now in a position to see how this ratio may be evaluated experimentally. The steps of the procedure are the following ... 

The solvation of a sphere swells its volume above the dry volume, which is presumably what is used to evaluate . Therefore whatever factor describes the increase in particle volume due to solvation is absorbed into [rj]. This is easily seen as follows. Suppose the mass of colloidal solute in a solution is converted to the volume of unsolvated material using the dry density. In this way, mass/volume units are converted to volume/volume units, which we label dry since the unsolvated density was used in evaluating this quantity. If the solvation is uniform throughout the particle —as would be the case for, say, aqueous proteins —then the solvated particle exceeds the unsolvated particle in volume by the factor [1 + (m,, b/m2)(p2/ Pi)] as shown by Equation (2.38). Recall that in this expression, muh is the mass of bound solvent, m2 is the mass of the solute particle, and p, is the density of solvent or solute, as appropriate. Still assuming dry has been used to evaluate [17], we see... 

The sample is disrupted completely and distributed over the surface as a function of interactions with the support, the bonded phase, and the tissue matrix components themselves. The solid support acts as an abrasive that promotes sample disruption, whereas the bonded phase acts as a lipophilic, bound solvent that assists in sample disruption and lysis of cell membranes. The MSPD process disrupts cell membranes through solubilization of the component phospholipids and cholesterol into the Cis polymer matrix, with more polar substituents directed outward, perhaps forming a hydrophilic outer surface on the bead. Thus, the process could be viewed as essentially turning the cells inside out and forming an inverted membrane with the polymer bound to the solid support. This process would create a pseudo-ion exchange-reversed-phase for the separation of added components. Therefore, the Cis polymer would be modified by cell membrane phospholipids, interstitial fluid components, intracellular components and cholesterol, and would possess elution properties that would be dependent on the tissue used, the ratio of Cis to tissue employed and the elution profile performed (99-104). 

As can be seen from the Table 15, 70 and 30% of the solvent are respectively in the bound and free state. The mass fractions of the bound and free solvents are in rough accordance with those of the crystalline-amorphous interphase and the amorphous phase in the two noncrystalline phases of the polymer. This result suggests that the solvent exists in the two noncrystalline phases of the polymer, as the bound solvent in the crystalline-amorphous interphase and as the free solvent in the amorphous phase, leaving the crystalline phase pure. It is concluded that the sPP/o-dichlorobenzene gel involves three phases, (1) the pure crystalline... 

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