Thermodynamics and Molecular Mechanics Studies on α- and β-Cyclodextrins Complexation and Diethyl 2,6-naphthalenedicarboxylate Guest in Aqueous Medium

• Published in: The journal of physical chemistry. B Vol. 106; no. 8; pp. 1995 - 2003
• Main Authors: Pastor, Isabel, Di Marino, Antonio, Mendicuti, Francisco
• Format: Journal Article
• Language: English
• Published: American Chemical Society 28-02-2002
• ISSN: 1520-6106; 1520-5207
• DOI: 10.1021/jp013118q

Steady-state, time-resolved fluorescence spectroscopy and Molecular Mechanics (MM) were used to study the inclusion complexes of Diethyl 2,6-naphthalenedicarboxylate (DEN) with α- and β-cyclodextrins (CDs). The ratio, R, of intensities of two bands that are sensitive to the medium polarity and the average of lifetime, <τ>, which is more sensitive to the medium microviscosity surrounding the guest molecule, were obtained as a function of the CD concentration and temperature. Stoichiometries, formation constants, and the changes of enthalpy and entropy upon inclusion were also obtained. The complexes prefer 1:2 (DEN:CD) stoichiometries. MM calculations were employed to study the formation of different complexes of DEN with both α- and βCDs. For the most stable structure of 1:1 complexes, an important portion of DEN is outside CD cavity, making it possible to stabilize them by adding another CD. Driving forces for 1:2 inclusion processes are dominated by nonbonded van der Waals DEN−CD interactions. Nevertheless, due to the different geometry of the 1:2 complexes, an important electrostatic interaction appears between both βCDs in the DEN:βCD2 complex that does not exist between αCDs in the DEN:αCD2 complex. Most of this contribution is due to the intermolecular hydrogen bonding formation between secondary hydroxyl groups of both βCDs.