Peroxide-Induced Liberation of Iron from Heme Switches Catalysis during Luminol Reaction and Causes Loss of Light and Heterodyning of Luminescence Kinetics

The peroxidation of luminol yields bright luminescence when the reaction is catalyzed by heme proteins. However, an excess of peroxide leads to less light and altered luminescence kinetics, an effect commonly referred to as "suicide inactivation". The aim of this study is to present the molecular processes causing this effect. A comprehensive set of data reported here demonstrates that suicide inactivation is due to a peroxide-induced liberation of iron from its coordinating porphyrin. Liberated iron launches catalysis of the reaction at much lower efficiency. The light-yielding efficiencies of different organic and inorganic catalysts are precisely quantified and compared. It is shown that the catalysis by free iron involves superoxide. This is explained by the formation of a ferryl-oxo-iron complex. In this context, a complete reaction mechanism involving a modified Fenton-Haber-Weiss cycle is proposed for the first time. The switch from the highly efficient biogenically catalyzed luminescence to a less efficient inorganically catalyzed reaction is accompanied by a transition from "flash-type" to "glow-type" luminescence kinetics. Ethylenediaminetetraacetic acid-mediated chelation of iron is used to demonstrate this effect and to separate both kinetics. The explanation of kinetic heterodyning is underpinned by mathematical modeling. The results are able to explain the as yet unexplained phenomena discussed in the less recent literature and to settle disputes about them. It is concluded that peroxide concentrations exceeding the level tolerated by the catalyzing heme protein negatively impact performance and precision of luminol-based assays, where the light yield is used as a quantitative measure for analyte concentrations.