Approach to the aspects of driving force of inclusion by .alpha.-cyclodextrin

Abstract

A comprehensive model of the inclusion process of -cyclodextrin is presented herein, van der Waals interaction energy, Allinger's conformation energy, solvation energy of apolar solute in water, hydrogen bond energy of water molecules in the cavity of -cyclodextrin hexahydrate, and all other possible energies were taken into account for the calculation of freeenergy change by complexing an apolar guest molecule by -cyclodextrin. Motional freedoms of all of the particles relevant to the inclusion process were taken into consideration. The calculated values of the net stabilization free energy of a-cyclodextrin complexes with benzene, methyl orange, and p-iodoaniline were 4.50, 6.20, and 8.99 kcal mol-1, respectively, which were in good agreement with the observed values. The largest value of 10.09 kcal mol-1 was obtained as the van der Waals stabilization energy of the -cyclodextrin-p-iodoaniline complex which was most easily understood as a result of large atomic polarizability of iodine atom and/or better fit of the iodide substrate to -cyclodextrin's cavity than the other substrates. The value of the solvation energy (-A//scluster) ranges from 4.1 to 7.1 kcal mol-1, depending upon the size of the apolar moiety of the guest molecule, or the number of water molecules in the first layer of water clusters. The increase in entropy which accompanies breaking of the water clusters around the apolar guest has been found to be a nonnegligible term which contributes to the net stabilization of the inclusion complex. A true aspect that -cyclodextrin in the complex has a larger conformation energy by ca. 4 kcal mol-1 than that in the hexahydrate is also pointed out. Therefore, a speculation presented early by others that the strain relief is the main driving force of the formation of the inclusion complexes of -cyclodextrin is to be corrected based on the present results.

Propriedades e aplicações recentes das ciclodextrinas

“…Quanto mais fortemente ligadas se encontram as moléculas de água, mais vagarosamente serão removidas da esfera de hidratação do substrato 30 . Tabushi e colaboradores 40 afirmaram que a estabilização do complexo da CD com o substrato por forças de van der Waals e a remoção da água de solvatação no substrato apolar são fatores responsáveis pela entalpia de inclusão. Quanto maior a estabilização do complexo CD-substrato por forças de van der Waals maior o colapso da estrutura do agregado de água durante o processo de inclusão, e a contribuição relativa destes dois termos compensató-rios dita a estabilidade relativa do complexo receptor-substrato para uma série de substratos relacionados.…”

Stability and stoichiometry of some binary fluorophore–cyclodextrin complexes

“…Furthermore, it should be considered that when amino groups are protonated, they are able to hamper the cavity desolvation process that has always been considered to be one of the essential steps to promote inclusion complex formation. 19 The binding ability of both hosts is influenced by pH changes and, independent of the guest considered, they form less stable complexes at pH¼8.0 than at pH¼9.0.…”

Thermodynamics of Supramolecular Structure Formation in Water

“…It is generally very difficult to extract hydrophobic interactions in a pure form from these data. Large entropic cost to freeze the translational and rotational freedom of guest upon binding should be paied by a number of weak interactions (Knox, 1971;Tabushi et al, 1978;Mizutani et al, 1994). In this case, which pairwise interaction paid the entropic cost is not clear.…”