Thermodynamics of the complexation of mono- and bis-cyclodextrin derivatives with a polarity sensitive probe: Fluorescence, Induced Circular Dichroism and molecular modelling

Steady-state, time-resolved fluorescence, Induced Circular Dichroism (ICD), Molecular Mechanics and Molecular Dynamics techniques have been employed in the study of the complexation of dimethyl 2,6-naphthalenedicarboxylate (DMN), a fluorescent probe, with mono- and bis beta CD derivatives whose appended group, or beta CD spacer, contain the 1,3-diphenoxy moiety (OB), i.e., 6[4-(3-(prop-2-ynyloxy)phenoxy)methyl]-1H-1,2,3-triazol-1-yl]6-deoxy-beta CD (m beta CD) and 1,3-bis((1-6'-beta CD-6'-yl)-1H-1,2,3-triazol-4-yl)methoxy)benzene (b beta CD). The study also considers complexation with the native beta CD partner. The DMN emission spectrum shows two overlapping electronic bands whose ratio of intensities, R, is very sensitive to the polarity of the medium surrounding it. The stoichiometry, the formation constants of the complexes and the Delta H and Delta S parameters upon inclusion were obtained from the change in R and weighted average lifetime, with CD concentration and temperature. DMN forms 1:1 and 2:1 stoichiometry complexes with b beta CD, but does not with m beta CD. Molecular modelling was also used to emulate the complexation processes in the presence of water. The 1:1 and 2:1 DMN/b beta CD complex structures agree with the signs of enthalpy and entropy changes. Quenching, Rat infinite b beta CD concentration, fluorescence depolarization measurements and ICD spectra also support the proposed structures. Inclusion is mostly dominated by van der Waals interactions. (C) 2012 Elsevier B.V. All rights reserved.