Entropy complexation driven

The interaction between 4-(4-hydroxybut-2-ynyloxy)-3-(phenylsulfonyl)-l,2,5-oxadiazole-2-oxide 16 and bovine serum albumin (BSA) was studied by spectroscopic methods including fluorescence and UV-Vis absorption spectroscopy. The results indicate that molecules 16 bind with BSA forming 1 1 complex. Thermodynamic parameters, such as AH, AG, and A.Y, were calculated. The results indicate that the binding reaction is mainly entropy driven and hydrophobic forces play a major role in this reaction <2006CHJ1050>. 

As noted in table 11.1, the ability of THFTCA to separate LJO from trivalent lanthanide ions is mainly of enthalpic origin. Reaction 11.33 has a considerably more unfavorable enthalpic contribution than reaction 11.32. The complexation is, however, predominantly entropy driven because the T ArS° term dominates the ArH° contribution for all systems. The large positive entropy changes observed for reactions 11.32 and 11.33 result from the release of water molecules coordinated to the metal on complexation with the tridentate THFTCA2- ligand. Note that a negative entropy contribution would be expected if these reactions were truly 2 particle = 1 particle reactions [226]. 

The thennodynainics of complexation between hard cations and hard (O, N donor) hgands often are characterized by positive values of both the enthalpy and entropy changes. A positive AH value indicates that the products are more stable than the reactants, i.e., destabilizes the reaction, while a positive entropy favors it. If TAS > AH°, AG° will be negative and thus log(3 positive, i.e., the reaction occurs spontaneously. Such reactions are termed entropy driven since the favorable entropy overcomes the unfavorable enthalpy. 

That the entropy change is unfavourable could be confidently predicted, given the presence of two moles of gas on the left-hand side. As the temperature is increased, the TAS" term becomes more important neglecting the small temperature dependence of AH° and A5°, it can be easily shown that AG° will become zero at about 850 K, at which temperature the decomposition of the complex should be complete. Such decomposition can be achieved at lower temperatures if the partial pressure of ammonia is kept low, by pumping. Most thermal decompositions-which are often the reverse of acid-base reactions (see Section 9.2) - are entropy-driven. All substances containing chemical bonds can be decomposed by heating to a sufficiently high temperature. 

Metal complexes also can show contradictive behavior with benzo-15-crown-5 the complexation of lanthanide perchlorates is entropy-driven in acetonitrile, while the complexation of lanthanide nitrates with corresponding disubstituted derivatives is primarily enthalpy-driven, with small entropic differences.44... 

A similar complexation study supported by thermodynamic parameters was performed by the same author for mono-(6-anilino-6-deoxy)- 3-cyclodextrin and mono-[ 6-( 1 -pyridinio)-6-deoxy]-P-cyclodextrin complexation with several amino acids in zwitterionic form [24]. The inclusion complexation was enthalpy driven for the former and entropy driven for the latter host. 

Usually, there is a compensation effect, that is AHd —TAS11 so that AGd 0. Experimentally, AS1 is positive (especially for macrocyclic ligands), and so is very often A Hr (which means that usually, the Ln-L bonds are weaker than the Ln-OH2 ones) so that complexation reactions in water are entropy driven and, moreover, a linear relationship between AH and A. S 1 holds for the Ln(III) series of cations. One has, however, to be cautious when this approach is applied to polydentate ligands. The thermodynamic parameters may also reflect other factors such as the formation of stable 5-membered chelate rings. When another solvent is considered, the solvation enthalpy is much smaller than in water and the above considerations may no more hold. 

The complexation-decomplexation equilibrium of the polymer is strongly entropy driven. The monomer complex 19a (Fig. 30) is stable up to 100°C, since it... 

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