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 bisβCD derivatives whose appended group, or βCD 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-βCD (mβCD) and 1,3-bis((1-(6′-βCD-6′-yl)-1H-1,2,3-triazol-4-yl)methoxy)benzene (bβCD). The study also considers complexation with the native βCD 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 ΔH and ΔS parameters upon inclusion were obtained from the change in R and weighted average lifetime, 〈τ〉, with CD concentration and temperature. DMN forms 1:1 and 2:1 stoichiometry complexes with bβCD, but does not with mβCD. Molecular modelling was also used to emulate the complexation processes in the presence of water. The 1:1 and 2:1 DMN/bβCD complex structures agree with the signs of enthalpy and entropy changes. Quenching, R at infinite bβCD concentration, fluorescence depolarization measurements and ICD spectra also support the proposed structures. Inclusion is mostly dominated by van der Waals interactions.