Test for Proteins

Ninhydrin Test:

• Amino acids react with ninhydrin at pH = 4 → purple (violet) colour
• Proline and hydroxyproline → yellow colour (exception)

Xanthoproteic Test:

• \[ \text{Solution} \xrightarrow{\mathrm{HNO}_3} \text{Yellow colour} \xrightarrow{\mathrm{NaOH}} \text{orange - yellow color} \]
• Detects amino acids with phenolic or indole groups: tyrosine, tryptophan, phenylalanine, glutamic acid

Millon's Test:

• Millon's reagent = mixture of Hg(NO₃)₂ and Hg(NO₂)₂

• \[ \text{Solution} \xrightarrow{\text{Millon's agent}} \text{white precipitate} \xrightarrow{\text{boiling}} \text{brick - red color} \]

• Detects proteins/amino acids with phenolic –OH group (e.g., tyrosine, phenylalanine, β-naphthol)

• Not specific to proteins; positive for any phenolic compound

Biuret Test:

• \[ \text{Protien} + \mathrm{CuSO}_4 \xrightarrow{\mathrm{NaOH}} \text{Copper cation complex (purple to violet)} \]

• Detects peptide bonds (not for amino acids alone)

• Examples: gelatin, casein, albumin