1 research outputs found
Kinetic Modeling Assisted Analysis of Vitamin C‑Mediated Copper Redox Transformations in Aqueous Solutions
The kinetics of oxidation of micromolar concentrations
of ascorbic
acid (AA) catalyzed by Cu(II) in solutions representative of biological
and environmental aqueous systems has been investigated in both the
presence and absence of oxygen. The results reveal that the reaction
between AA and Cu(II) is a relatively complex set of redox processes
whereby Cu(II) initially oxidizes AA yielding the intermediate ascorbate
radical (A•–) and Cu(I). The rate constant
for this reaction was determined to have a lower limit of 2.2 ×
104 M–1 s–1. Oxygen
was found to play a critical role in mediating the Cu(II)/Cu(I) redox
cycle and the oxidation reactions of AA and its oxidized forms. Among
these processes, the oxidation of the ascorbate radical by molecular
oxygen was identified to play a key role in the consumption of ascorbic
acid, despite being a slow reaction. The rate constant for this reaction
(A•−+O2→DHA+O2•−) was determined for the first time with
a calculated value of 54 ± 8 M–1 s–1. The kinetic model developed satisfactorily describes the Cu/AA/O2 system over a range of conditions including different concentrations
of NaCl (0.2 and 0.7 M) and pH (7.4 and 8.1). Appropriate adjustments
to the rate constant for the reaction between Cu(I) and O2 were found to account for the influence of the chloride ions and
pH on the kinetics of the process. Additionally, the presence of Cu(III)
as the primary oxidant resulting from the interaction between Cu(I)
and H2O2 in the Cu(II)/AA system was confirmed,
along with the coexistence of HO•, possibly due
to an equilibrium established between Cu(III) and HO•