Specific and nonspecific effects of biologically active inorganic salts on inclusion complex formation of cyclodextrins with aromatic carboxylic acids

Inclusion complex formation of α- and β-cyclodextrins with zwitterionic nicotinic and m-aminobenzoic acids in water and 0.2 M solutions of KCl, KBr, KH2PO4 and K2SO4 was studied by 1H NMR and UV-spectroscopy. We complemented the experiments with statistical mechanics calculations in the framework of the 3D-RISM approach to analyze the ion-binding between inorganic univalent anions and positively charged groups of the acids under study. It was detected that binding affinity of cyclodextrins to the acids is decreased in the presence of the salts. This is caused by specific and nonspecific action of the considered inorganic anions. The influence of the Cl− and SO42− was found to be nonspecific and insignificant. On the contrary, Br− and H2PO4− ions can considerably affect the inclusion complex formation. Insertion of Br− into the macrocyclic cavity and attraction between H2PO4− and the zwitterions are the main processes competing with cyclodextrin-acid binding. It has been demonstrated that the manifestation of the salt effects depends on the cyclodextrin cavity size, ionization state of the acid and other experimental conditions.