Desolvation, requirements

To improve the aqueous solubility of 5 we also synthesized a cyclopeptide 6 containing hydroxyproline subunits [24]. Although this compound is very water-soluble, and in solution adopts a similar conformation to 5, it only forms 1 1 complexes with anions. A reason for the inability of 7 to form 2 1 complexes could be that the hydroxyproline subunits in 6 are better solvated than prolines in aqueous solution, and the desolvation required for aggregation of two cyclopeptide molecules thus occurs less readily. Steric hindrance of hydroxyl groups from different cydopeptide moieties in the a dimeric complex of 6 could, moreover, also make aggregation difficult. Although peptide 6 cannot form sandwich-type complexes, it proved to be valuable for a quantitative determination of the anion affinity of 5, the results of which are summarized in Table 2.2.3 [24],... 

Control of product distribution. Simultaneously available reaction pathways may be characterised by widely differing pressure susceptibility. Pertinent choice of applied pressure and reaction solvent (i.e. controlling the dielectric constant, transport characteristics and desolvation requirements) has been shown to favour or disfavour particular products. According to transition-... 

Figure 3.3 Desolvation requirements of isophorone for head-to-tail dimerisation [58].

The thermospray device produces a wide dispersion of droplet sizes and transfers much of sample solution in unit time to the plasma flame. Therefore, it is essential to remove as great a proportion of the bigger droplets and solvent as possible to avoid compromising the flame performance. Consequently, the thermospray device usually requires both spray and desolvation chambers, especially for analyte solutions in organic solvents. 

A qualitative difference in the type of solvation (not simply in the strength of solvation) in a series of nucleophiles may contribute to curvature. Jencks has examined this possibility. " " An example is the reaction of phenoxide, alkoxide, and hydroxide ions with p-nitrophenyl thiolacetate, the Br insted-type plot showing Pnuc = 0.68 for phenoxide ions (the weaker nucleophiles) and Pnu = 0.17 for alkoxide ions. It is suggested that the need for desolvation of the alkoxide ions prior to nucleophilic attack results in their decreased nucleophilicity relative to the phenoxide ions, which do not require this desolvation step. 

In series with a desolvation energy barrier required to disrupt aqueous solute hydrogen bonds [14], the lipid bilayer offers a practically impermeable barrier to hydrophilic solutes. It follows that significant transepithelial transport of water-soluble molecules must be conducted paracellularly or mediated by solute translocation via specific integral membrane proteins (Fig. 6). Transcellular permeability of lipophilic solutes depends on their solubility in GI membrane lipids relative to their aqueous solubility. This lumped parameter, membrane permeability,... 

Measurements of thermal analysis are conducted for the purpose of evaluating the physical and chemical changes that may take place in a heated sample. This requires that the operator interpret the observed events in a thermogram in terms of plausible reaction processes. The reactions normally monitored can be endothermic (melting, boiling, sublimation, vaporization, desolvation, solid-solid phase transitions, chemical degradation, etc.) or exothermic (crystallization, oxidative decomposition, etc.) in nature. 

Differential thermal analysis (DTA) consists of the monitoring of the differences in temperature existing between a solid sample and a reference as a function of temperature. Differences in temperature between the sample and reference are observed when a process takes place that requires a finite heat of reaction. Typical solid state changes of this type include phase transformations, structural conversions, decomposition reactions, and desolvation processes. These processes may require either the input or release of energy in the form of heat, which in turn translates into events that affect the temperature of the sample relative to a nonreactive reference. 

It should be noted that application of the Marcus theory to these reactions is much more straightforward than application to reactions in solution. Since we are dealing with a single unimolecular step, namely, rearrangement of the reactant complex to the product complex, we need not be concerned with the work terms (2) which must be included in treatments of solution-phase reactions. These terms represent the work required to bring reactants or products to their mean separations in the activated complex, and include Coulombic and desolvation effects. 

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