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

Steady-state fluorescence and molecular mechanics have been used to study the 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 this ratio with CD concentration and temperature, the stoichiometry, the formation 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 the thermodynamic parameters Delta H degrees and Delta S degrees on the temperature was also found. Both complexes showed a negative Delta C-p(degrees). In addition, DMN seems to be a good probe for estimating microenvironmental polarity. Molecular mechanics calculations were also employed to study the 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. Results seem to explain the stoichiometries for both complexes. Only a small portion of DMN penetrates into the alpha CD cavity, but it does penetrate almost totally into beta CD. This fact makes it possible to stabilize the former 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 interactions. Nevertheless, head-to-head hydrogen bonding formation between secondary hydroxyl groups of alpha CDs can also contribute to the stability of the DMN:alpha CD2 complex.