DFT study of mechanisms of the antioxidant effect of natural polyhydroxy-1,4-naphthoquinones. Reactions of echinamines A and B, metabolites of sea urchin Scaphechinus mirabilis, with hydroperoxyl radical

The molecular characteristics were calculated and the structures of monoamino-substituted polyhydroxy-1,4-naphthoquinones, 3-amino-7-ethyl-2,5,6,8-tetrahydroxy-1,4-naphthoquinone (echinamine A, 1) and 2-amino-7-ethyl-3,5,6,8-tetrahydroxy-1,4-naphthoquinone (echinamine B, 2), were studied by the B3LYP/6-311G(d) method in the gas phase. The enol-enol tautomerism of these compounds caused by the transfer of hydrogen atoms of the alpha-OH groups and rotamerism due to the rotation of the beta-OH groups around the C-O bonds are considered. The relative acidity of the beta-OH groups in compounds 1 and 2 was estimated. The gas-phase reactions of compounds 1 and 2 with the hydroperoxyl radical are exothermic. The heterolysis of the O-H bond of one of the two beta-OH groups in compounds 1 and 2 decreases the energy of the subsequent O-H bond homolytic dissociation of the remaining beta-hydroxy group, due to which the energy gain of the reaction with the hydroperoxyl radical increases.