Capillary zone electrophoresis study of cyclodextrin--lipoic acid host-guest interaction

Lipoic acid is a naturally occurring compound which is being widely investigated for its therapeutic effects in the treatment or prevention of a variety of diseases associated with oxidative injury, particularly diabetes. The diversity of therapeutic applications of lipoic acid requires an appropriate formulation to control its bioavailability, site-targeting delivery and to overcome its inherent chemical instability. In this regard, cyclodextrins (CDs) are ideally suitable due to their well-documented ability to include in their cavity proper guest molecules and protect them from physical or chemical damages. Lipoic acid forms 1:1 inclusion complexes with betaCD as shown in a previous report of an extended investigation that also indicated the suitability of capillary zone electrophoresis (CZE) for the study of such host-guest interactions. In view of these possible applications, we extended the CZE analysis to determine the strength of binding, in a pH 9 phosphate buffer, of lipoic acid with other CD derivatives such as alphaCD, gammaCD and the alkylated derivatives of betaCD, namely (2-hydroxypropyl)-beta-CD (HPbetaCD), and heptakis(2,3,6-tri-O-methyl)-beta-CD (TMbetaCD). Once established that the easily available betaCD is the most suitable receptor for lipoic acid, we set up and here describe a simple and reliable procedure for the quantitative determination of lipoic acid in commercial dietary supplement tablets containing also other active substances and excipients.