Animated Solution for Chemistry - Organic Chemistry: The peptide that gives positive ceric ammonium nitrate and carbylamine tests is

One of the most fascinating aspects of studying chemistry for competitive exams like JEE is when different domains of the subject collide. This problem is a perfect example of such a collision. It seamlessly weaves together the structural biology of amino acids with the classic qualitative analysis tests of organic chemistry.

To solve this, we aren't just looking at abstract names of peptides; we are hunting for specific functional groups hiding within their molecular architecture. The question demands a peptide that yields a positive result for two distinct chemical tests: the Ceric Ammonium Nitrate test and the Carbylamine test.

Before we even look at the options, we must establish what these tests are actually searching for.

1. The Ceric Ammonium Nitrate Test: This is a classic laboratory test used to detect the presence of an alcoholic hydroxyl group ($-OH$). When an alcohol reacts with the yellow-orange ceric ammonium nitrate reagent, it forms a complex that causes a distinct color change to red. Therefore, for a peptide to pass this test, at least one of its constituent amino acids must possess an alcoholic $-OH$ group in its side chain.

2. The Carbylamine Test: Also known as the Hofmann isocyanide synthesis, this test is highly specific for primary amines ($-N{H}_2$). When a primary amine is heated with chloroform ($CHC{l}_3$) and alcoholic potassium hydroxide ($KOH$), it produces an isocyanide (or carbylamine), which is infamous for its incredibly foul and intolerable odor. Thus, our target peptide must contain a free primary amine group.

Let's break down the amino acids provided in the options to see what functional groups they bring to the table:

Lysine (Lys): This is a basic amino acid. Its defining feature is a long, flexible alkyl chain that terminates in an extra primary amine group: $-(C{H}_2{)}_4-N{H}_2$. This makes it a prime candidate for the carbylamine test. Aspartic Acid (Asp): An acidic amino acid. Its side chain contains an extra carboxylic acid group: $-C{H}_2-COOH$. It lacks both an alcohol and an extra amine. Serine (Ser): This is the golden ticket for the ceric ammonium nitrate test. Serine's side chain is a hydroxymethyl group: $-C{H}_2-OH$. It contains the crucial alcoholic hydroxyl group. Glutamine (Gln): This amino acid features an amide group in its side chain: $-C{H}_2-C{H}_2-CON{H}_2$. It is a common trap to confuse the $-N{H}_2$ of an amide with a primary amine. Amides do not give a positive carbylamine test.

Now, let's evaluate the given dipeptides. A critical nuance to remember is that every standard dipeptide has a free N-terminal primary amine group (unless it's modified or involves proline). Because of this N-terminal $-N{H}_2$, all the given options could technically give a positive carbylamine test.

Therefore, the true differentiator—the ultimate deciding factor—is the Ceric Ammonium Nitrate test.

(a) Lys-Asp: Contains an extra $-N{H}_2$ and an extra $-COOH$. It lacks an $-OH$ group. It will fail the ceric test. (c) Gln-Asp: Contains an amide and an extra $-COOH$. It lacks an $-OH$ group. It will fail the ceric test. (d) Asp-Gln:* Contains an extra $-COOH$ and an amide. It lacks an $-OH$ group. It will fail the ceric test.

Ser-Lys

This leaves us with option (b) Ser-Lys. Let's appreciate its structure:

1. The Serine Contribution: The serine residue provides the $-C{H}_2-OH$ side chain. This alcoholic hydroxyl group will readily react with the ceric ammonium nitrate reagent, yielding the characteristic red complex. 2. The Lysine Contribution: The lysine residue provides a long side chain ending in a free primary amine, $-(C{H}_2{)}_4-N{H}_2$. Combined with the N-terminal amine, this peptide is exceptionally well-equipped to undergo the carbylamine reaction, producing the foul-smelling isocyanide.

By perfectly satisfying the functional group requirements of both qualitative tests, Ser-Lys emerges as the undeniable correct answer. This problem beautifully illustrates why memorizing the side chains of the 20 standard amino acids is absolutely non-negotiable for mastering biomolecules!