Solvent-ligand interactions

Two repulsive contributions, osmotic and elastic contributions [31, 32], oppose the van der Waals attractive contribution where the osmotic potential depends on the free energy of the solvent-ligand interactions (due to the solvation of the ligand tails by the solvent) and the elastic potential results from the entropic loss due to the compression of ligand tails between two metal cores. These repulsive contributions depend largely on the ligand length, solvent parameters, nanopartide radius, and center-to-center distance ... 

UCW = capped water, TW = tethered water (see text), k = force constant for restraining potential (kcal/mol/A2). b Radius (A) of solvation sphere. 1 Numbers of dynamical water molecules within solvation sphere. d Mean and standard error for the forward (i.e. 8-methyl-N5-deazapterin —> 8-methylpterin) and reverse mutation of the electrostatic force field Cutoff for protein-ligand and solvent-ligand interaction all other interactions are subject to a 9 A cutoff. 

These results show that the metal ion in the complex ML+ is almost shielded from outside solvent, by being captured in the cavity of the cryptand. Thus, the solvent-complex interaction is nearly the same as the solvent-ligand interaction (see Table 4.3 in Chapter 4). However, there are cases when Eqs (2.12) and (2.13) are not valid. Then, the shielding is not complete and some influence of the metal ion-solvent interaction is observed. Equations (2.12) and (2.13) do not hold also when the solvent is protic, e.g. water. The oxygen and nitrogen atoms of the cryptand interact with the protic solvent and the interaction is stronger with the free cryptand than with the cryptand combined with the metal ion. 

Short alkanes such as decane and dodecane are completely miscible in sc-CO2 at moderate pressures and temperatures. Based on the earlier simulations, it would seem that nanocrystals capped with dodecanethiol should disperse in sc-CO2 at moderate conditions however, this is not the case. Gold and silver nanocrystals capped with dodecanethiol ligands were exposed to SC-CO2 at pressures as high as 483 bar and temperatures up to 80 C without any visible dispersibility. This indicates that ligands with better CO2 compatibility need to be developed to allow for effective dispersibility. The dodecanethiol capped nanocrystals were dispersible in sc-ethane, as the solvent-ligand interactions were much more compatible. 

It is interesting to compare this behaviour with the situation which occurs when a relatively inflexible ligand interacts with a very labile metal ion (that is, one in which the rate of solvent exchange is very high). For... 

Solvent extraction is a major industrial technique. The usual objective is to selectively remove one or more solutes from a complex mixture. Selectivity usually depends on strong specific solvent-solute interactions or on the formation of complexes between ions and ligands. Thus solvent extraction systems are likely to include a number of chemical reactions and to exhibit large deviations from ideality. The design of liquid extraction processes may require many kinds of data. References (31, 32, 55, 61, 81, and 118) are concerned specifically with solvent extraction. 

Simple thermodynamic considerations state that the reduction process is favoured (i.e. more positive cu(ii)/cu(p potential values are obtained) if the electron transfer is exothermic (AH° negative) and if the molecular disorder increases (AS° positive). It is therefore evident that the positive potential value for the reduction of azurin (as well as that of the most blue copper proteins) is favoured by the enthalpic factor. This means that the metal-to-ligand interactions inside the first coordination sphere (which favour the stability of the reduced form over the oxidized form) prevail over the metal complex-to-solvent interactions inside the second... 

We conclude that in this simple model there are two contributions to the ligand-ligand correlation, hence to the cooperativity one due to direct ligand-ligand interaction [/(1,1) and the other due to ligand-solvent interaction. The latter part of the indirect correlation is transmitted by solvent molecules. The extent of this correlation depends on the presence of the adsorbent molecule. 

Planet offers a 2-D representation (similar to that used in LIGPLOT [36]) of the protein-ligand interaction called a PLAID (Protein-Ligand Accessibility and Interaction Diagram, Figure 10.3). It is automatically calculated from the PDB data and shows van der Waals interactions, hydrogen bonds and solvent accessible areas. The interaction can be also viewed in 3 D or as the original PDB entry. 

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