Yn. Wang et La. Eriksson, Theoretical studies of electron and hydrogen transfer reactions between semiquinone radicals and oxygen, THEOR CH AC, 106(1-2), 2001, pp. 158-162
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.