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Stability and Structure of Inclusion Complexes of Zaleplon with Natural and Modified Cyclodextrins

The interaction between zaleplon (ZAL) and different cyclodextrins in aqueous solutions was investigated by spectrofluorimetric and phase solubility studies. Stability constants determined by both methods showed that among natural cyclodextrins, β-cyclodextrin (βCD) formed the most stabile complex but its solubilizing efficiency was limited. Among βCD derivatives, the complex stability and solubilisation efficiency decreased in order: randomly methylated-βCD (RAMEB) > sulphobutylether-βCD (SBEβCD ) > hydroxypropyl-βCD (HPβCD). The inclusion complexes of ZAL with βCD and RAMEB were further characterised by 1H-NMR spectroscopy and the inclusion complex formation was confirmed in both cases. ROESY spectra showed two binding modes between ZAL and βCD which exist simultaneously in the solution. The first binding mode occurs by the inclusion of the phenyl ring of ZAL into the βCD central cavity via the wider rim of the cyclodextrin cone and is dominant. The second one is formed by the inclusion of pyrazolo[1, 5- a]pyrimidine ring of ZAL

cyclodextrins; zaleplon; inclusion complexes; stability constants; fluorescence; 1H-NMR

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