Inclusion complex formation of α- and β-cyclodextrins with riboflavin and alloxazine in aqueous solution: thermodynamic study

摘要

Possibility of encapsulation of riboflavin and alloxazine by α- and β-cyclodextrins in aqueous solution was studied by ~1HNMR and solubility methods. Thermodynamic parameters of 1:1 inclusion complex formation (K, Δ_CG~0, Δ_CH~0 and Δ_CS~0) were obtained and analyzed in terms of influence of reagent's structure on complexation process. It was shown that a-cyclodextrin displays low binding affinity to riboflavin and alloxazine. On the contrary, β-cyclodextrin forms with riboflavin and alloxazine more stable inclusion complexes. Binding is accompanied by the negative enthalpy and entropy changes that are determined by predominance of van der Waals interactions and possible H-bonding. The presence of ribityl substituent in riboflavin molecule prevents the deep penetration of this compound into macrocyclic cavity. Proposed on the basis of ~1H NMR data the partial insertion of the hydrophobic part of riboflavin and alloxazine molecules into the β-cyclodextrin cavity causes the enhancement of aqueous solubility of the encapsulated substances. In comparison with α-cyclodextrin, the solubilizing effect of β-cyclodextrin is more pronounced due to its higher binding affinity to alloxazine and riboflavin.