Question

Biuret test is not positive for:

• A. Protein
• B. Alanine
• C. Peptides
• D. Polypeptides

Hint:

The Biuret test requires a minimum of two peptide bonds to show a positive violet color. Individual amino acids (except histidine under certain conditions) always give a negative result.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
The Biuret test is a chemical test used to detect the presence of peptide bonds in a given sample.
A positive result is indicated by a color change from blue to violet or purple.

Step 2: Key Formula or Approach:
For a compound to give a positive Biuret test, it must contain at least two peptide bonds (i.e., it must be a tripeptide or larger molecule).
The peptide bonds form a coordination complex with $Cu^{2+}$ ions in an alkaline solution, which produces the characteristic violet color.

Step 3: Detailed Explanation:
Proteins, peptides, and polypeptides are polymer chains made of amino acids linked by multiple peptide bonds.
Therefore, they all give a positive Biuret test.
Alanine is a single, free amino acid.
Since it does not contain any peptide bonds, it cannot form the required coordination complex.
Thus, alanine gives a negative result (the solution remains blue).

Step 4: Final Answer:
The Biuret test is not positive for Alanine.