Extension of electrochemical methods of study of inclusion complexes to the case where the guest molecule undergoes a multielectron, multiproton electrode reaction: Study of phenolic acids forming inclusion complexes with α- and β-cyclodextrins

Cyclic voltammetry has been employed to determine formation constants for the inclusion complexes formed from α- and β-cyclodextrin and three phenolic acids, protocatechuic (PTC), caffeic (CAF) and chlorogenic (CHL) acids. The method required the prior determination of the mechanism of oxidation of the phenolic acids in the absence of cyclodextrin. The study was conducted in unbuffered aqueous 1.0 M KCl using a glassy carbon working electrode. The voltammetric results could be accounted for equally well by the electron transfer/proton transfer/electron transfer/proton transfer (eHeH mechanism) or the electron transfer/proton transfer/proton transfer/electron transfer (eHHe mechanism). Upon addition of cyclodextrin to solutions of one of the phenolic acids, the anodic peak current for oxidation of the free acid was depressed as was the associated cathodic peak. There was also an increase of the anodic-to-cathodic peak potential separation. Simulation of the cyclic voltammograms allowed the extraction of the formation constant for inclusion of the phenolic acid in the cyclodextrin, Kf,4, as well as estimation of the formation constant for the oxidized phenolic acid (o-quinone) with the cyclodextrin, Kf,5. The diffusion coefficients of the complexes were also determined. Values of Kf,4 were determined under the same conditions using microcalorimetry and these results, along with literature values, were compared with those obtained by cyclic voltammetry. Underlying causes of the differences are discussed.