Solvation-desolvation

The direct ligand-protein interactions and the net solvation-desolvation term together should give an energy contribution that strongly favors formation of the complex (large and negative), since the other two components favor its dissociation. 

Solvation—Desolvation Equilibrium. From the observation of migration of plasticizer from plasticized polymers it is clear that plasticizer molecules, or at least some of them, are not bound permanently to the polymer as iu an internally plasticized resia, but rather an exchange—equiHbrium mechanism is present. This implies that there is no stoichiometric relationship between polymer and plasticizer levels, although some quasi-stoichiometric relationships appear to exist (3,4). This idea is extended later ia the discussion of specific iateractions. 

Furthermore, the reaction scheme implies that the molecular weight distribution is Poisson-like — i.e. very narrow — as it had been shown earlier on theoretical basis by Flory 8), Gold 9), and Szwarc l0>. Even though two (or more) types of active species add monomer at very different rates, the polydispersity remains narrow, provided solvation/desolvation and ionic dissociation/association processes are fast U). 

In this connection, attention should be paid to an unusual NMR technique called nuclear magnetic relaxation dispersion (NMRD). In contrast with NMR spectroscopy, the NMRD signal arises from the nuclei of the abundant solvent molecules and not from the dissolved substances. The relaxation properties of the solvent molecules are profoundly modified if the solvent contains paramagnetic particles (see a review by Desreux 2005). A solvent molecule sails in the vicinity of an ion-radical and finds itself in the local magnetic field of this paramagnetic particle. Then, induced magnetism of the solvent molecule dissipates in the solvent bulk. This kind of relaxation seems to be registered by NMR. NMRD is applicable to studies on ion-radical solvation/desolvation, ion-pair dynamics, kinetics of ion-radical accumulation/consumption, and so on. 

The goal of this research is to develop a new class of bioresponsive materials that undergo rapid, large-magnitude, volume-phase transitions in response to specific biological stimuli. Our approach to these materials is based on two fundamental aspects of hydrogels (1) hydrogel solvation/desolvation thermodynamics can be perturbed... and... 

TABLE II. Representative Time Constants for Solvation-Desolvation of Hydrophobic Structures... 

In summary, the failure of solvation-desolvation theory to explain the observed changes of enzyme enantioselectivity may well be caused by the fact that several barriers of physically different character contribute to the E-value. For additional considerations the reader is referred to [107] and [30]. 

Extensive structural characterization of many different -ring heterocycles has not yet been done. Difficulties predicting relative potency of these compounds a priori stem from the lack of understanding of solvation/desolvation effects as well as difficulties in characterizing the low-intensity hydrophobic interactions. Consequently, it seems likely that new structure—activity relationships about the A-ring heterocycle will continue to be determined based on empirical findings. 

In addition to size and molecular weight, one of the most important factors which determines plasticizer efficiency is the rate of diffusion of the plasticizer in the polymer matrix. In view of the dynamic solvation-desolvation between the plasticizer molecules and the polymer chains, the higher the diffusion rate, the greater the efficiency of the compound as a plasticizer. However, high diffusion rates are usually encountered with small molecules the smaller the plasticizer molecule, the greater its volatility and, therefore, the rate at which it is lost from the plasticized product. 

The lack of correlation with enthalpic (i.e. bond length-related) data highlights the complexity of the system, and the need to consider solvation/desolvation-related effects as contributing significantly to the free energies of complexation. 

The effects of solvation/desolvation equilibria on the thermodynamics of complexation can be discerned in the pattern of formation constant values across a series of azaciyptand hosts or anionic guests. In the dianionic series thiosulfate, selenate, sulfate, formation constants, to a first approximation, decrease as the hydration energy of the anion increases, reflecting the enthalpic cost of desolvation. 

Solvation/desolvation effects in the cryptand also complicate the expected simple dependence of stability constant on host basicity. For example the aliphatic cryptand O-bistren shows lower formation constants than the less basic aromatic analogues such as R3F, which we attribute to the greater desolvation cost of complexation with the former, more hydrophilic host. 

Early in the development of the field, compounds were often prepared to define the limits of both the synthetic methods and the stable products that could be formed. To date, many thousands of heteromacrocycles have been prepared. The dominant application of the vast family of host or receptor molecules has been to bind or complex a guest structure. The guests can be metal cations, organic cations, neutral substrates, anions, or complementary molecules. The complexation process can be understood from the simple example of 18-crown-6 complexing K+Cl- in solution. Ignoring structural and solvation/desolvation issues, the process can be described simply as... 

The analyte solvation-desolvation equilibrium inside the column could be written in the following form ... 

Assuming that solvation-desolvation equilibrium is fast, we can express the overall retention factor of injected analyte as a sum of the retention factor of solvated form multiplied by the solvated fraction (9) and the retention factor of the desolvated form multiplied by the desolvated fraction (1-9), or... 

The direct simulation of the aqueous binding process is difficult because changes in solvation/desolvation that accompany association are slow and hard to sample, especially when hydrogen bonding patterns are coupled to conformational changes in the protein, or for recessed binding sites, where the associating substrate may hinder solvent escape. Another important kinetic factor is the differential stabilization (by enzyme vs. solvent) of the transition state of the... 

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