Study of the conformational and self-aggregation properties of 2I,3I-O-(o-xylylene)-per-O-Me-alpha- and -beta-cyclodextrins by fluorescence and molecular modeling.

Steady-state and time-resolved fluorescence techniques were used to study the behavior of 2I,3I-O-(o-xylylene)-per-O-Me-alpha- and -beta-cyclodextrins in aqueous solution, based on the fluorescence of the bidentate xylylene moiety. Fluorescence decay profiles obtained upon excitation of the xylylene group were fitted to three-exponential decay functions. In addition to a fast component due to stray and/or scattered light, two other components ascribed to the monomer and dimer species, respectively, were identified. The dimer/monomer ratio increases with concentration and decreases with temperature, which is in agreement with an enthalpy-driven association process. The corresponding dimerization equilibrium constants (KD) were obtained from nonlinear regression analysis of the plots of tau against [CD] in the 5-45 degrees C range. A linear van't Hoff analysis for KD allows us to obtain the DeltaH and DeltaS associated to dimer formation. Molecular mechanics as well as molecular dynamics calculations in the presence of water were also employed to study the conformational behavior of such secondary-face-substituted cyclodextrins and rationalize the dimerization processes.