TMB, (3,3',5,5'-tetramethybenzidine) has been shown to be a safe-sensitive substrate for the assay of HRP. Initially, in the presence of HRP and hydrogen peroxide, a one-electron oxidation product is formed. This compound, a cation free radical, is blue in color with an adsorption maximum of 653 nm. Further reaction with HRP/H2O2 or acidification of the radical with acid yields the diimine terminal oxidation product adsorbing light at 450 nm. The extinction coefficient of the radical (E653nm=3.9 x 104 mol-1 cm-1) and diimine (E450nm=5.9 x 104 mol-1 cm-1) provide a remarkably sensitive system for the assay of HRP and HRP labeled probes. ES022 is a single component reagent stable at room temperature and not sensitive to normal laboratory light. It is optimized with respect to TMB and hydrogen peroxide concentrations and yields a linear response with the concentrations of HRP usually employed in immunological assays.
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