Question

The test used for identifying peptide linkage in proteins is:
(A) Borsche’s test
(B) Molisch’s test
(C) Ninhydrin test
(D) Biuret test

Answer 1

Hint: You must know what each of the above tests is used for. The concept of this question is taken from general organic chemistry, you might have performed some of the reactions practically on your own as well.

Complete Step by Step Solution:
Borsche’s test or brady’s test is used to detect ketones and aldehydes. Borsche’s reagent is prepared by dissolving DNPH in a solution of methanol and concentrated sulphuric acid. Positive test is shown by formation of yellow, red or orange precipitate of the dinitrophenylhydrazone. So, option (A) is incorrect.

Molisch’s test is used to check the presence of carbohydrate in a given solution. This test can also be shown by some organic acids like citric acids, lactic acids, oxalic acid, formic acid, etc. All carbohydrates be it monosaccharides, disaccharides and polysaccharides give positive reaction for molisch’s test. It is based on the dehydration of carbohydrate by \[{{H}_{2}}S{{O}_{4}}\] to produce aldehyde. So option (B) is incorrect.

Ninhydrin test is used to check whether a given solution contains amines or amino acids. It is a standard test for detecting amino acids. It is a characteristic test for proteins. Very small quantities of proteins or amino acids will give a positive ninhydrin test. So option (C) is incorrect.

Biuret test is used to check for the presence of peptide bonds among amino acids. This test is also used to indicate the presence and concentration of proteins in a test sample. In this peptide bonds react with copper ions and produce a violet or purple complex. More the number of peptide bonds, the more will be the intensity of the colour.
So option (D) is the correct answer

Note: Note that the above reactions are particularly used for the substrates discussed above, but these test and not exclusive only for the substrates discussed above, they can also be used for other substrates but in a narrow range for example molisch’s test is particularly used for detection of carbohydrates but sometimes it is also used for detection of organic acids.