The Effect of Cupric Ions on the Indole Reaction for the Determination of Deoxyribonucleic Acid

The presence of cupric ions or ferric ions was found to enhance the development of color in the indole reaction for the colorimetric determination of deoxyribonucleic acid. The addition of calcium, cobaltous, magnesium, manganous, nickel, or zinc ions did not produce this effect. In the absence of added cupric ions, the indole reaction gave variable results, and the color developed to less than half of the optimum level. It is recommended that the reaction mixture for this assay include cupric ions at a concentration of 15 µm. Under these conditions, the molar extinction coefficient (based on the molarity of DNA phosphorus) of the colored product at 490 mµ is equal to 15,800 m-1 cm-1. The assay conforms to Beer's law until the concentration of DNA exceeds 25 µg per ml (A1 cm490 mµ = 1.25) in the reaction vessel.