Entropy ligands

Conformational Adjustments The conformations of protein and ligand in the free state may differ from those in the complex. The conformation in the complex may be different from the most stable conformation in solution, and/or a broader range of conformations may be sampled in solution than in the complex. In the former case, the required adjustment raises the energy, in the latter it lowers the entropy in either case this effect favors the dissociated state (although exceptional instances in which the flexibility increases as a result of complex formation seem possible). With current models based on two-body potentials (but not with force fields based on polarizable atoms, currently under development), separate intra-molecular energies of protein and ligand in the complex are, in fact, definable. However, it is impossible to assign separate entropies to the two parts of the complex. 

Lewis acid and the ligand. By contrast, soft-soft interactions are mainly enthalpic in origin and are characterised by a negative change in entropy" ... 

NMR signals of the amino acid ligand that are induced by the ring current of the diamine ligand" ". From the temperature dependence of the stability constants of a number of ternary palladium complexes involving dipeptides and aromatic amines, the arene - arene interaction enthalpies and entropies have been determined" ". It turned out that the interaction is generally enthalpy-driven and counteracted by entropy. Yamauchi et al. hold a charge transfer interaction responsible for this effect. 

The enthalpies of complexation of 3.8c to the copper(lf) - amino acid ligand complexes have been calculated from the values of at 20 C, 25 1C, 30 1C, 40 1C and 50 1C using the van t Hoff equation. Complexation entropies have been calculated from the corresponding Gibbs energies and enhalpies. 

Ligand-Field Stabilization Energies 8.2.3 Contributions to the Chelate Effect - The Entropy... 

In more mathematical language, the favourable entropy term is associated with the release of a large number of monodentate ligands upon the formation of the chelate. 

There is also an entropy term associated with the desolvation of the ligands. This is much more difficult to assess, and may make for either favourable or unfavourable contributions to the overall entropy changes. 

When N-acetylcysteine and thiomalate were studied in a mixed ligand system, the respective activation entropies were found to be AS = —141 and —151J mol ... 

The driving force for the temperature-dependent spin crossover (SCO) is the entropy difference between the HS and the LS isomers which arises mainly from a shift of the vibrational frequencies when passing from the HS to the LS state [97-99]. This frequency shift has been studied by IR- and Raman-spectroscopy and recently also by NIS [23, 39, 87]. The NIS method is isotope ( Fe) selective and, therefore, its focus is on iron-ligand bond-stretching vibrations which exhibit the most prominent contribution to the frequency shift upon SCO [87]. 

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