Which colorimetric method is used to detect peptide bonds in proteins?

• A. Biuret
• B. Kjeldahl
• C. Folin-Ciocalteau
• D. Ninhydrin

Answer: (A)

Peptide bonds are detected colorimetrically by the Biuret reaction. In alkaline conditions, the peptide bonds in proteins chelate copper(II) ions to form a violet-colored copper-peptide complex, and the intensity of this color increases with the number of peptide bonds, reflecting protein concentration. Free amino acids or small peptides don’t produce the same complex, so they don’t give the same color. The absorbance is measured in the visible range, typically around 540–560 nm. The other options don’t detect peptide bonds directly: Kjeldahl measures total nitrogen content after digestion; Folin-Ciocalteau is used in other protein assays (like Lowry) after copper reduction but isn’t a direct peptide-bond detector; Ninhydrin reacts with free amino groups of amino acids to give Ruhemann’s purple, not dependent on peptide bonds.