Question

A dipeptide 'z' on complete hydrolysis gives 'y' and 'z'. y on treatment with aq. $HNO_2$ produces lactic acid on the other hand 'z' on heating gives the following the given dipeptide is:

• A. Alanine – Glycine
• B. Valine – Glycine
• C. Valine – Leucine
• D. Alamine – Alamine

Answer 1

Answer: (A)

Alanine – Glycine

Answer 2

1. Hydrolysis: The dipeptide "z" when hydrolyzed gives two different amino acids: one is "y" and the other is "z".

2. Nitrous Acid Reaction: It is given that "y" on treatment with aqueous $HNO_2$ gives lactic acid. Since lactic acid is derived on deamination of alanine ($CH_3CH(NH_2)COOH \rightarrow CH_3CH(OH)COOH$), "y" must be alanine.

3. Cyclization on Heating: The remaining amino acid "z" (which is not alanine) must be glycine because the dipeptide is unsymmetrical (giving two different products) and glycine ($NH_2CH_2COOH$) is known to undergo cyclization (by intramolecular nucleophilic attack resulting in a 2,5-diketopiperazine) more readily than more hindered amino acids.

4. Conclusion: Thus, the dipeptide "z" is made up of alanine and glycine.