Cyclodextrin encapsulation of the functional group diminishes an antioxidant's free radical scavenging rates

Scavenging rates of cyclodextrin-solubilized lipophilic antioxidants, namely catechin, epicatechin, epigallocatechin gallate, and resveratrol, against alkoxyl (RO ^â¢) radical were measured with the use of electron paramagnetic resonance (EPR) spin-trapping method. Results indicated that the scavenging rates of catechin and resveratrol were notably dependent on the solubilizer used, i.e., native β-cyclodextrin (β-CD) or heptakis(2,6-di-O-methyl)-β-cyclodextrin (DM-β-CD). But, epicatechin and epigallocatechin gallate showed almost no dependence on the cyclodextrin used. Catechin's scavenging rate in β-CD was 66% lower than in DM-β-CD; in contrast, resveratrol in β-CD showed 45% higher rate than in DM-β-CD. Based on the reported solution-NMR structure of the inclusion complex of these antioxidants, it was concluded that the scavenging rate is decreased when the cyclodextrin cavity preferentially encapsulates the antioxidant-function bearing group, i.e., O- and p-quinolinol group in catechin and resveratrol, respectively. The depth of inclusion of the functional group determines the extent of the scavenging rate difference, suggesting that the cavity wall of the cyclodextrin acts like a barrier that hinders the approach of attacking free radicals.