What color does the oxidized chromogen quinoneimine impart in Trinder-type assays?

• A. Red-purple
• B. Green
• C. Blue
• D. Yellow

Answer: (A)

In Trinder-type assays, horseradish peroxidase catalyzes the oxidation of a phenolic substrate with a coupling agent to form a colored dye called quinoneimine. This oxidized dye has a red to pinkish-red hue, often described as red-purple depending on concentration. That makes the red-purple description the best match for the color produced by quinoneimine. The other colors—green, blue, or yellow—are associated with other chromogens used in different systems, not with quinoneimine.