Host−Guest Complexes of Carotenoids with β-Glycyrrhizic Acid

The structure, stability, and reactivity of the host−guest complexes between a set of carotenoids and the triterpene glycoside, β-glycyrrhizic acid (GA), were investigated by different physicochemical techniques: high-performance liquid chromatography, optical absorption, and fluorescence spectroscopy. It has been demonstrated recently that the molecular complexes of GA with a number of drugs are characterized by reduced toxicity and increased therapeutic activity of these drugs. In the present work it was found that carotenoids form 1:2 complexes with GA in aqueous solutions as well as in polar organic solvents, methanol, acetonitrile, and dimethylsulfoxide. We assume that the structure of the complex is a cycliclike dimer of GA encapsulating a carotenoid molecule. The stability constants in all solvents are near 104 M-1. In addition, GA forms inclusion complexes with carotenoid radical cations, which results in their stabilization. Complex formation (a) decreases the rate of electron transfer from carotenoids to electron acceptors (Fe3+ or quinone) and (b) considerably increases the lifetime of the carotenoid−quinone charge-transfer complex and the yield of the major product (a carotenoid−quinone adduct). A thermodynamic study shows that hydrophobic interactions are the main driving force of the carotenoid−GA complex formation. These results are important for understanding both the nature of GA complexes and the influence of GA on the therapeutic activity of some drugs. Furthermore, carotenoid−GA complexes could be used for the design of artificial light-harvesting, photoredox, and catalytic systems.