The Fe^II(citrate) Fenton reaction under physiological conditions

The Fenton reaction of Fe^II(citrate) in the presence and absence of bicarbonate (HCO₃ ⁻) is studied. It is found that the rate constant of the Fenton reaction (k_obs) increases with increasing [citrate]. k_obs also increase with increasing [HCO₃ ⁻]; this effect is most significant at biological citrate concentrations. Methane and ethane gases are formed from (CH₃)₂SO when the Fenton reaction is carried out in the presence of large [citrate] due to the reaction of the citrate radical, (⁻ ₂OC)CH₂C(OH)(CO₂ ⁻)CH(CO₂ ⁻)[rad]/(⁻ ₂OC)CH₂C(O)(CO₂ ⁻)CH₂(CO₂ ⁻)[rad] with (CH₃)₂SO. In the absence of citrate (CH₃)₂SO₂ is the main product of the Fenton reaction. However, in the presence of 0.10 mM citrate, no (CH₃)₂SO₂ is formed, some (CH₃)SOOH is formed, along with a low yield of beta-ketoglutaric acid. Formation of (CH₃)SOOH and beta-ketoglutaric acid are due to the citrate radical and Fe^IV(citrate). In the presence of bicarbonate formation of abundant beta-ketoglutaric acid confirms the formation of carbonate radical anion (CO₃[rad]⁻). Thus, bicarbonate affects the mechanism and kinetics of the reaction dramatically. Hydroxyl radicals (OH[rad]) are not formed in the presence of bicarbonate and probably also not in its absence. These results point out that hydroxyl radicals, formed by the Fenton reaction, do not initiate oxidative stress in biological systems.