Inclusion complexes of dimethyl 2,6-naphthalenedicarboxylate with alpha- and beta-cyclodextrins in aqueous medium: Thermodynamics and molecular mechanics studies

Steady-state fluorescence and molecular mechanics have been used to study t he inclusion complexes of dimethyl 2,6-naphthalenedicarboxylate (DMN) with alpha- and beta-cyclodextrins (CDs). Emission spectra of DMN show two bands whose ratio is very sensitive to the medium polarity. From the change of t his ratio with CD concentration and temperature, the stoichiometry, the for mation constants, and the changes of enthalpy and entropy upon inclusion of complexes formed were obtained. Stoichiometry depends on the host CD used. The estimated formation constants at 25 degrees C were (8.2 +/- 0.6) x 10( 5) M-2 for DMN:alpha CD2 and 1311 +/- 57 M-1 for DMN:PCD. A dependence of t he thermodynamic parameters Delta H degrees and Delta S degrees on the temp erature was also found. Both complexes showed a negative Delta C-p(degrees) . In addition, DMN seems to be a good probe for estimating microenvironment al polarity. Molecular mechanics calculations were also employed to study t he formation of 1:1 and 1:2 complexes of DMN with both alpha- and beta CDs. The study was mainly performed in the presence of water as a solvent. Resu lts seem to explain the stoichiometries for both complexes. Only a small po rtion of DMN penetrates into the alpha CD cavity, but it does penetrate alm ost totally into beta CD. This fact makes it possible to stabilize the form er 1:1 complex by adding other aCD. The driving forces for both 1:1 and 1:2 inclusion processes are dominated by nonbonded van der Waals host guest in teractions. Nevertheless, head-to-head hydrogen bonding formation between s econdary hydroxyl groups of alpha CDs can also contribute to the stability of the DMN:alpha CD2 complex.