1H NMR and calorimetric study on binding ability of cyclodextrins to isomeric pyridinecarboxylic acids in aqueous solution

Complex formation of alpha- and beta-cyclodextrins with isomeric pyridinecarboxylic acids (picolinic, nicotinic and isonicotinic acids) in water were studied by calorimetry and 1H NMR at 298.15 K. The obtained results revealed the weak 1:1 complex formation governed by the cavity dimensions and position of the carboxylic group in the pyridine ring. It was found that selective inclusion complex formation of alpha- and beta-cyclodextrins with nicotinic and isonicotinic acids takes place. In the case of picolinic acid, the considerable role of external interactions and formation of less stable complexes were detected. The location of the carboxylic group in the meta-position of pyridine ring is more favorable for the effective binding. The pyridinecarboxylic acids are shallow inserted into alpha-cyclodextrin cavity possessing the smaller diameter, while they are deeply included into beta-cyclodextrin cavity. Thermodynamic parameters of complex formation (K, Delta(c)G0, Delta(c)H0 and Delta(c)S0) were calculated and discussed in terms of influence of reagents structure.