Testing the effect of the cavity size and the number of molecular substitutions in host–guest complexes formed by 2-hydroxypropyl-cyclodextrins and n-octyl-β-d-glucopyranoside

• Thermodynamics of HP-CD:OGP complex formation has been studied by ITC.
• Binding parameters of CD:OGP complexes differ greatly for native and HP modified CDs.
• The substituted groups increase entropically the stability of 1:1 species based on α-CD.
• The competition of HP groups and OGP for the CD cavity is revealed for the modified β- and γ-CD.
• HP groups enhance the formation of noninclusion second-order complexes based on the modified β-CD.

The thermodynamic characterization of host–guest complex formation processes based on modified cyclodextrins (CDs) presents several difficulties that are absent when using native CDs. They are mainly due to the relatively wide distribution of the degree of molecular substitution and also to the indeterminacy in the location of the modified groups. A more common trouble would arise from the coexistence of coupled processes, such as CD-surfactant complexation and surfactant demicellization. In the present work a reasonable approach based on the analysis of isothermal titration calorimetric measurements is employed to assess the effect of the number of molecular substitutions as well as of the CD cavity size in supra-molecular complexes formed by 2-hydroxypropyl-CDs and n-octyl-β-d-glucopyranoside. The employed method considers both first- (1:1) and second-order (1:2 and 2:1) stoichiometries. The results were significantly different from those previously observed for native CDs. The substituted groups increase entropically the stability of 1:1 species based on α-CD, while compete with the surfactant for the CD cavity in the case of 1:1 and 1:2 species based on β- and γ-CD, respectively. Also, the modified β-CD seems to enhance the formation of non-inclusion second-order complexes.

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