Thermodynamic and Nuclear Magnetic Resonance Study of the Reactions of α- and β-Cyclodextrin with Acids, Aliphatic Amines, and Cyclic Alcohols

Abstract

Titration calorimetry was used to determine equilibrium constants and standard molar enthalpy, Gibbs energy, and entropy changes for the reactions of a series of acids, amines, and cyclic alcohols with α- and β-cyclodextrin. The results have been examined in terms of structural features in the ligands such as the number of alkyl groups, the charge number, the presence of a double bond, branching, and the presence of methyl and methoxy groups. The values of thermodynamic quantities, in particular the standard molar Gibbs energy, correlate well with the structural features in the ligands. These structural correlations can be used for the estimation of thermodynamic quantities for related reactions. Enthalpy−entropy compensation is evident when the individual classes of substances studied herein are considered, but does not hold when these various classes of ligands are considered collectively. The NMR results indicate that the mode of accommodation of the acids and amines in the α-cyclodextrin cavity is very similar, but that the 1-methyl groups in 1-methylhexylamine and in 1-methylheptylamine and the N-methyl group in N-methylhexylamine lie outside the α-cyclodextrin cavity. This latter finding is consistent with the calorimetric results. Many of the thermodynamic and NMR results can be qualitatively understood in terms of van der Waals forces and hydrophobic effects.

The association of anionic surfactants with β -cyclodextrin. An isothermal titration calorimeter study

“…A combination of hydrophobic effects and van der Waals forces and, in some cases, hydrogen bonding between the ligand and the cyclodextrin are involved. (1)(2)(3)(4)22) The resultant of all these interactions is described by the thermodynamic quantities in this study. Although, at present, it does not appear possible to obtain a rigorous, quantitative breakdown of the relative contributions of these various interactions for any specific (ligand + cyclodextrin) reaction, we believe that the following qualitative picture is useful for understanding these complexation reactions.…”

Propriedades e aplicações recentes das ciclodextrinas

“…Eles proporcionam informações valiosas relacionadas ao efeito hidrofóbico e às forças de dispersão de London 35,36 . As contribuições mais relevantes para a termodinâmica da complexação são originárias da entrada da parte hidrofóbica do substrato na cavidade da CD e da quebra da estrutura da água em torno do substrato orgânico 37 . Uma vez que a cavidade da CD é pronunciadamente lipofílica, a transferência da parte hidrofóbica do substrato, da água para a cavidade da CD, pode ser considerada como um processo típico de efeito hidrofóbico.…”

The influence of glycerol on ligand binding equilibria between monoalcohols and -cyclodextrin

“…This "enthalpy-entropy compensation" has been already referred to in reactions involving cyclodextrins and partly hydrophobic compounds in water. (12,13,19) As pointed out by Rekharsky et al, (19) in reactions with α-cyclodextrin the r H o m (int) values were found to be poorly correlated with r S o m (int), when all classes of compounds were taken together. However, some enthalpy-entropy compensation is apparent when individual classes of ligands are considered, although the pattern is not consistent (α, ω-diols (12) give a compensating slope of opposite sign to all other classes of compounds).…”