Q.17 Match the entries in Group I with that in Group II

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Introduction

Fehling’s test, Ninhydrin reaction, Biuret reaction and Nitroprusside reaction are classic qualitative tests used to identify functional groups such as reducing sugars, α‑amino acids, peptide linkages and sulfhydryl groups in biomolecules. Mastering these reactions is essential for solving biochemistry and CSIR‑NET style matching MCQs accurately.

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Step‑by‑step matching of Group I and Group II

Fehling’s test (P) → Reducing sugar (2)

• Fehling’s test uses an alkaline copper(II) solution that is reduced by reducing sugars (free or potentially free aldehyde/ketone groups), producing a red precipitate of cuprous oxide.

• Therefore Fehling’s test is a specific test for reducing sugars such as glucose, fructose and lactose, so P corresponds to 2 (reducing sugar).

Ninhydrin reaction (Q) → α‑Amino acid (1)

• Ninhydrin reacts with compounds having a free α‑amino group, especially free amino acids, giving a deep blue or purple product (Ruhemann’s purple).

• Because this reaction is routinely used to detect and estimate amino acids and fingerprints (due to free amino groups), Q matches with 1 (α‑amino acid).

Biuret reaction (R) → Peptide linkage (4)

• In the Biuret reaction, cupric ions in alkaline medium form a violet complex with peptide bonds; any compound having at least two peptide linkages gives a positive test.

• Proteins and polypeptides therefore show a purple colour in the Biuret test, so R corresponds to 4 (peptide linkage).

Nitroprusside reaction (S) → Sulfhydryl group (3)

• Sodium nitroprusside reacts specifically with free sulfhydryl (‑SH) groups, such as those in cysteine, producing a red or purple coloured complex in alkaline solution.

• Hence the Nitroprusside reaction is used as a test for thiol groups, so S matches with 3 (sulfhydryl group).

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Analysis of all answer options

• Option (A): P‑1, Q‑2, R‑3, S‑4

  • P‑1 is incorrect because Fehling’s test detects reducing sugars, not α‑amino acids.

  • Q‑2 is wrong because Ninhydrin is not a carbohydrate test; it detects α‑amino groups.

  • R‑3 is wrong; Biuret does not detect sulfhydryl groups.

  • S‑4 is wrong; Nitroprusside is not used for peptide bonds.

• Option (B): P‑3, Q‑4, R‑1, S‑2

  • P‑3 misassigns Fehling’s test to sulfhydryl groups, which are not involved in this redox reaction.

  • Q‑4 is incorrect because Ninhydrin does not depend on peptide bonds; it works with single amino acids as well.

  • R‑1 and S‑2 are both mismatched for reasons noted above.

• Option (C): P‑2, Q‑1, R‑4, S‑3

  • Each reaction is paired with the correct functional group, matching standard biochemical descriptions; hence this option is correct.

• Option (D): P‑4, Q‑1, R‑2, S‑3

  • P‑4 falsely links Fehling’s test to peptide bonds, which do not give the characteristic brick‑red precipitate.

  • R‑2 wrongly treats the Biuret test as a carbohydrate test; it is a protein/peptide assay.

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Final answer

• P) Fehling’s test → 2) Reducing sugar

• Q) Ninhydrin reaction → 1) α‑Amino acid

• R) Biuret reaction → 4) Peptide linkage

• S) Nitroprusside reaction → 3) Sulfhydryl group

So, the correct option is (C).