Molecular determinants of singlet oxygen binding by anthraquinones in relation to their redox cycling activity

A series of model anthraquinones with varying symmetry of pi-electron density distribution have been examined to verify our previous hypothesis concerning the essential role of quinone-singlet oxygen complex formation by asymmetric anthraquinones in their peroxidating properties. Comparison of the results of enzymatic studies using NADH dehydrogenase with those of cyclovoltammetric measurements fully confirmed the assumption that one-electron transfer mediation is facilitated by the preceding quinone-oxygen complex formation. To extend the scope of the molecular determinants of oxygen binding found in our previous studies, CNDO/2 and molecular electrostatic field (MEF) calculations have been performed. It has been concluded that the analysis of molecular electrostatic field as well as the dipole moment components has to be taken into account to judge whether a mutual orientation of the quinone and oxygen molecule can be reached which enables binding to occur. The second important factor is the appropriate symmetry of the quinone outer filled orbitals which assures that binding is not forbidden by the Woodward-Hoffman rules. These characteristics also explain the lack of oxygen binding by some asymmetric anthraquinones. The efficient electron transfer mediation be anthraquinones requires, beside the formation of the intermediate quinone-oxygen complex, effective catalysis of this process by oxidoreductase enzyme. The results obtained with model anthraquinones indicated that compounds with more than one phenolic group and an unsubstituted quinone carbonyl are good NADH dehydrogenase substrates. Imino derivatives and compounds with a reduced number or without free phenolic groups exhibit low affinity towards the enzyme.