A close-up on doxorubicin binding to γ-cyclodextrin: an elucidating spectroscopic, photophysical and conformational study

The association of doxorubicin (DOX) with gamma-cyclodextrin (gamma-CyD) was studied in phosphate buffer of pH 7.4, at 22 degrees C by performing titration experiments monitored with circular dichroism (CD), UV-vis absorption, and fluorescence. Global analysis of multiwavelength spectroscopic data obtained at different DOX concentrations was performed by taking into account the DOX monomer-dimer equilibrium. Formation of gamma-CyD: DOX 1 : 1, 2 : 1, 1 : 2 and 2 : 2 complexes was evidenced. The stability constants and the absolute CD, UV-vis absorption and fluorescence spectra of all the complexes were determined. gamma-CyD did not prove to be able to disrupt the DOX dimer when the latter is the predominant form in solution. The triplet state absorption and kinetic properties of DOX in the presence of gamma-CyD and in ethanol were also determined by laser flash photolysis. The excited singlet and the triplet features indicated the environment experienced by DOX in the CyD complexes is ethanol-like. The structure of the gamma-CyD: DOX 1 : 1 complex was investigated by Molecular Mechanics (MM) and Molecular Dynamics (MD) for both the neutral and the positively charged (-NH3+ in the daunosamine moiety) DOX forms. The possibility of interaction of both the aglycone and the daunosamine parts with the gamma-CyD cavity was evidenced.