Extraction of Ligands

Mann, G., Prins, J., Hermans, J. Energetics of forced extraction of ligand Simulation studies of Xe in mutant T4 lysozyme as a simple test system. Bioohys. J., in preparation (1998)... 

Extraction of a ligand from the binding pocket of a protein. The force (represented by an arrow) applied to the ligand (shown in van der Waals spheres) leads to its dissociation from the binding pocket of the protein (a slice of the protein represented as a molecular surface is shown). 

Thus, we have found unexpected complexities and even in this simple system have not yet been unable to accurately extrapolate the results of simulations done over periods varying from 1 to several hundred ps, to the low-friction conditions of extraction experiments performed in times on the oi dc r of ms. The present results indicate that one should not expect agreement between extraction experiments and simulations in more complex situations typically found in experiments, involving also a reverse flow of water molecules to fill the site being evacuated by the ligand, unless the simulation times are prolonged well beyond the scope of current computational resources, and thereby strengthen the conclusion reached in the second theoretical study of extraction of biotin from it.s complex with avidin [19]. 

In a typical experiment 105 mg (0.50 mmol) of 3.8c, dissolved in a minimal amount of ethanol, and 100 mg (1.50 mmol) of 3.9 were added to a solution of 1.21g (5 mmol) of Cu(N03)2 BH20 and 5 mmol of ligand in 500 ml of water in a 500 ml flask. -Amino-acid containing solutions required addition of one equivalent of sodium hydroxide. When necessary, the pH was adjusted to a value of 5 ( -amino acids) and 7.5 (amines). The flask was sealed carefully and the solution was stirred for 2A hours, followed by extraction with ether. After drying over sodium sulfate the ether was evaporated. Tire endo-exo ratios were determined from the H-NMR spectra of the product mixtures as described in Chapter 2. 

Cupferron is a ligand whose strong affinity for metal ions makes it useful as a chelating agent in liquid-liquid extractions. The following distribution ratios are known for the extraction of Hg +, Pb +, and Zn + from aqueous solutions to an organic solvent. 

Hydrophobicity inflkence. Despite of the diphilic character of the NS-rich phase the efficiency of micellar extraction raises with general substrate hydrophobicity increase. The hydrophobicity of ligand is the main factor, which determines the cloud point extraction of complexes. 

Early examples of enantioselective extractions are the resolution of a-aminoalco-hol salts, such as norephedrine, with lipophilic anions (hexafluorophosphate ion) [184-186] by partition between aqueous and lipophilic phases containing esters of tartaric acid [184-188]. Alkyl derivatives of proline and hydroxyproline with cupric ions showed chiral discrimination abilities for the resolution of neutral amino acid enantiomers in n-butanol/water systems [121, 178, 189-192]. On the other hand, chiral crown ethers are classical selectors utilized for enantioseparations, due to their interesting recognition abilities [171, 178]. However, the large number of steps often required for their synthesis [182] and, consequently, their cost as well as their limited loadability makes them not very suitable for preparative purposes. Examples of ligand-exchange [193] or anion-exchange selectors [183] able to discriminate amino acid derivatives have also been described. 

Based on the above results they have concluded that the ligand groups circularly arranged on the lower rim of the calixarene cavity construct a novel cyclic metal receptor for selective extraction of transition metal cations. Results suggest that the fine tuning in molecular... 

Table 3 Extraction of Metal Cations with Ligands ...

The nature of the donor atoms in the chelating agent. Ligands which contain donor atoms of the soft-base type form their most stable complexes with the relatively small group of Class B metal ions (i.e. soft acids) and are thus more selective reagents. This is illustrated by the reagent diphenylthiocarbazone (dithizone) used for the solvent extraction of metal ions such as Pd2+, Ag+, Hg2+, Cu2+, Bi3+, Pb2+, and Zn2 +. ... 

Liquid-liquid extractions of metal ions by chelating ligands. E. Uhlig, Coord. Chem. Rev., 1982, 43, 299-312 (38). 

In general, a decrease in enantioselectivity was observed after the second reuse. The origin of this effect lies in the partial extraction of the chiral hgand in the hexane phase after each reaction. This was demonstrated by adding a small amount of ligand after the fourth reuse. In all cases, the original enantioselectivity was fully recovered (entries 2 and 3 in Table 6). 

A comprehensive study of the complex interfacial processes involved in the solvent extraction of cupric ion by oxime ligands represents one of the most detailed and successful studies carried out with the RDC [37,38]. Recently, the technique was also used to study the transfer of tetrabutylammonium cations [43] and the kinetics of partitioning of compounds between octanol and water [44]. In the latter study, Fisk and coworkers investigated the rates of partitioning of 23 compounds from octanol to an aqueous phase. The RDC arrangement used most frequently in this work is of the o/o/w type. So according to Eq. (15), and can be calculated from the gradient and intercept of... 

A growing-drop method has been reported [53] for measuring interfacial liquid-liquid reactions, in which mass transport to the growing drop was considered to be well-defined and calculable. This approach was applied to study the kinetics of the solvent extraction of cupric ions by complexing ligands. 

When a heptane solution of 5-Br-PADAP and an aqueous solution of Ni " " were stirred, the ligand in the organic phase was continuously consumed according to the complexation, but there was no extraction of the complex. The complex formed was completely adsorbed at the interface. On the other hand, in a toluene system the complex was extracted very slowly (Fig. 6). The complexation mechanism in the two solvent systems could be analyzed by taking into account the interfacial adsorption of the ligand. The next equation was derived for the initial rate of the consumption of HLq in the heptane system ... 

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