Theoretical study of the role of hydrogen bonding and proton transfer in oxygen reduction by semiquinones

• Published in: Journal of molecular structure. Theochem 1997-06, Vol.398, p.445-449
• Main Authors: Liwo, Adam, Jeziorek, Danuta, Dyl, Dariusz, Ossowski, Tadeusz, Chmurzyński, Lech
• Format: Article
• Language: English
• Subjects: Ab initio methods; Anthracycline derivatives; Electron transfer; Peroxidation
• ISSN: 0166-1280; 1872-7999
• DOI: 10.1016/S0166-1280(96)05010-5

Electron transfer from the anion radicals of 1-hydroxy-, 1-amino- and 1-methoxy-5,8-naphthoquinone to molecular oxygen, resulting in the formation of the superoxide anion radical, was investigated by ab initio calculations. The energetics of the electron transfer was studied at the ROHF level, with the assumption that the oxygen molecule is in the ground triplet 3∑ g − state before the electron transfer and the whole system is always in the doublet state. For isolated reactants, electron transfer results in increasing the energy. However, for 1-hydroxy- and 1-aminonaphthoquinone, the formation of a hydrogen bond between the superoxide anion radical and the proton-donor group causes energy lowering, if the reactants are at a hydrogen-bonding distance. The energy barrier to the electron transfer was estimated by carrying out MCSCF calculations on a model system composed of the anion radical of 2-oxo-4-hydroxybuten-3-al and oxygen; the value was found to be about 6 kcal mol −1. The results obtained explain the high peroxidating activity of anthraquinones bearing hydroxy groups.