Effect of Werner-Type Complex Formation of Cu2+ and Fe2+ on Oxidative Potentials Assessed Using Ascorbic Acid Assay

The ascorbic acid (AA) assay is a widely recognized tool for assessing the oxidation potential (OP) of atmospheric particulate matter (PM), including PM2.5. OP quantified through the cell-free AA assay can be used to study the association between chemical properties and harmful biological effects, such as the degradation of AA in the lungs by PM sample. AA is oxidized and depleted in solutions containing redox-active species such as polycyclic aromatic hydrocarbon quinones and heavy metal ions (Cu2+ and Fe2+), which are potential PM components. The metal ions form a Werner-type complex with ligands; thus, the AA depletion rate changes with the co-existing ligands in the PM sample. However, how the coordination structure of the complexes affects the AA depletion rate is poorly understood. This study examined the impact of the Werner-type complex formation of Cu2+ and Fe2+ on the AA depletion rate. Cu2+ and Fe2+ complexes were prepared by mixing them with three ethyleneamine forms: ethylenediamine, diethylenetriamine, and triethylenetetramine. The AA depletion rate was determined by measuring the changes in absorbance at 265 nm in the reaction solutions. Results indicated that the AA depletion rates of Cu2+ and Fe2+ were suppressed by the formation of complexes, and the degree of suppression depended on the coordination number and stability constants of the ethyleneamines. Additionally, AA depletion rates decreased with decreasing oxidative reduction potential in the solutions and changes in the coordination structures of the metal ion complexes. These findings demonstrate that the formation of Werner-type complexes with Cu2+ and Fe2+ reduces the AA depletion rate. As the number of ligands coordinating to the metal ions increases, the ORP decreases, creating a reducing environment that suppresses the oxidation of AA.