7-Ethyl-2,3,5,6,8-pentahydroxy-1,4-naphthoquinone (echinochrome A): A DFT study of the antioxidant mechanism. 1. Interaction of echinochrome A with hydroperoxyl radical

The molecular geometry and electronic structure of hydroxy-substituted naphthazarin (NZ)-7-ethyl-2,3,5,6,8-pentahydroxy-1,4-naphthoquinone (echinochrome A, (Et)NZ(β-OH)₃, 1) were calculated by the B3LYP/6-311G(d) method. The influence of the (i) character of the β-OH groups dissociation and (ii) conformational mobility of molecule 1 and the anions, radicals, and radical anions derived from 1 on the energy of their reactions with hydroperoxyl radical was studied by the (U)B3LYP/6-31G and (U)B3LYP/6-311G(d) methods. The enol-enolic tautomerism due to the transfer of hydrogen atoms of α-OH groups and rotational isomerism of the β-OH groups at the C(2) and C(3) atoms and of the α-OH groups at the C(5) and C(8) atoms were studied. The equilibrium in the gas-phase reaction 1 + ^•OOH ⇄ (Et)(HO-β)₂NZ(β-O^•) + HOOH (1) (quenching of hydroperoxyl radical) is shifted to the separated reagents. Heterolysis of the O—H bond in one of the three β-hydroxy groups considerably reduces the energy of subsequent O—H bond homolysis in either of the two remaining β-hydroxy groups. As a consequence, the reaction (Et)(HO-β)₂NZ(β-O⁻) + ^•OOH ⇄ (Et)(HO-β,⁻O-β)NZ(β-O^•) + HOOH (2) (quenching of hydroperoxyl radical) becomes exothermic and the equilibrium is shifted to the formation of hydrogen peroxide. The Gibbs energy gain in reaction (2) varies from −6.4 to −10.9 kcal mol⁻¹ depending on which β-hydroxy group is involved in the O—H bond homolysis. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 400–415, March, 2007.