Theoretical studies of electron and hydrogen transfer reactions between semiquinone radicals and oxygen

To explore the interactions between ubiquinones and oxygen in living organi sms, the thermodynamics of a series of electron and hydrogen transfer react ions between semiquinone radicals, as well as their corresponding protonate d forms, and oxygen, singlet or triplet, were studied using the hybrid Hart ree-Fock-density functional theory method Becke's three parameter hybrid me thod with the Lee, Yang, and Parr correlation functional. Effects of the so lvent and of the isoprenyl tail on the electron and hydrogen transfer react ions were also investigated. It is found that semiquinone radicals (semiqui none anion radicals or protonated semiquinone radicals) cannot react with t riplet oxygen to form the superoxide anion radical O-2(-.). In contrast, ne utral quinones can scavenge O-2(-.) efficiently. In the gas phase, only pro tonated semiquinone radicals can react spontaneously with singlet oxygen to produce peroxyl radical (HO2.). However, both semiquinone anion radicals a nd protonated semiquinone radicals can react with singlet oxygen to produce harmful oxygen radicals (O-2(-.) and HO2., respectively) in aqueous and pr otein environments. The free-energy changes of the corresponding reactions obtained for different ubiquinone systems are very similar. It clearly show s that the isoprenyl tail does not influence the electron and hydrogen tran sfer reactions between semiquinone radicals and oxygen significantly. Resul ts of electron affinities, vertical ionization potentials, and proton affin ities also show that the isoprenyl tail has no substantial effect on the el ectronic properties of ubiquinones.