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Chapter 6: Problem 106

Explain any four colour reactions that serve as tests for proteins?

Short Answer

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Question: Explain the procedure and color change in the Biuret Test for detecting proteins. Answer: In the Biuret Test for detecting proteins, the sample is mixed with an alkaline solution, such as sodium hydroxide, and then the Biuret reagent (copper (II) sulfate) is added. If proteins are present, the solution will change to a violet color. The color change occurs due to the formation of a complex between the copper ions in the solution and the peptide bonds in the proteins.

Step by step solution

01

Biuret Test

Biuret Test is a colorimetric method for detecting proteins in a sample by detecting peptide bonds. In this test, the sample is mixed with an alkaline solution, such as sodium hydroxide, and then the Biuret reagent (copper (II) sulfate) is added. If proteins are present, the solution will change to a violet color. The color change occurs due to the formation of a complex between the copper ions in the solution and the peptide bonds in the proteins.
02

Ninhydrin Test

Ninhydrin Test, also known as the triketohydrindene hydrate test, is used primarily for detecting free amino acids in a sample. In this test, the solution containing amino acids is mixed with a solution of ninhydrin reagent and then heated. Upon heating, if amino acids are present, a color change occurs with the formation of a purple or blue color. The color change results from the reaction of ninhydrin with the free amino acids, leading to the formation of a colored complex called Ruhemann's purple.
03

Xanthoproteic Test

Xanthoproteic Test is used to detect aromatic amino acids, such as phenylalanine, tyrosine, and tryptophan, present in a protein sample. In this test, the sample is heated with concentrated nitric acid. If aromatic amino acids are present, the color of the solution changes to a yellowish-orange. The color change results from the nitration of the aromatic amino acid residues to form xanthoproteic (yellow) compounds. After adding an alkali like ammonia, the yellow color intensifies.
04

Millon's Test

Millon's Test is a specific test for proteins containing the amino acid tyrosine. The reagent used in this test is Millon's reagent, which is a solution of mercuric nitrate dissolved in nitric acid. In this test, the sample is mixed with Millon's reagent and heated. If tyrosine is present, the solution will change to a red or pink color. This color change occurs due to the formation of a complex between the tyrosine and mercury ions in the reagent.

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Most popular questions from this chapter

Amines can be separated using Hinsberg method. Amines undergo a wide range of reactions like alkylation, acylation, reaction with nitrous acid, carbylamine reaction and condensation reactions with aldehydes. Which of the following on reduction with LiAlH \(_{4}\) will give \(\mathrm{N}\) -methylaniline? (a) Methyl isocyanide (b) Acetamide (c) Phenyl carbylamine (d) Benzonitrile

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Which of the following gives a primary amine on hydrolysis? (a) \(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{CN}\) (b) \(\mathrm{CH}_{3}-\mathrm{CH}=\mathrm{NOH}\) (c) \(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{NO}_{2}\) (d) \(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{NC}\)
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