Antioxidant and redox properties of supramolecular complexes of carotenoids with β-glycyrrhizic acid

Supramolecular complexes between carotenoids and a triterpene glycoside, β-glycyrrhizic acid (GA), were found to exhibit unusual antioxidant activity. Complexation with GA increases a scavenging rate of canthaxanthin and 7′,7′-dicyano-7′-apo-β-carotene toward OOH radicals more than 10 times, but has no effect on the scavenging rate of zeaxanthin. Scavenging rate constants were measured in DMSO solution of carotenoids using the EPR spin-trapping technique. EPR parameters of spin adducts were determined as a(H) = 2.3 G, a(N) = 13.9 G for PBN (N-tert-butyl-α-phenylnitrone)–OOH, and a(H) = 3.4 G, a(N) = 14.9 G for the PBN–CH3 adduct. Taking into account the previously measured dependence of the scavenging rate constants toward OOH radicals on the oxidation potential of carotenoids, this result can be explained by the hypothesis that the complexation with GA affects the value of oxidation potentials. This hypothesis was confirmed by CV measurements.