Question

A dipeptide, “x”, on complete hydrolysis gives “y” and “z”; “y” on treatment with aqueous HNO$_2$, produces lactic acid. On the other hand, “z” on heating gives the following cyclic molecule.

Based on the information given, the dipeptide X is:

• A. valine-glycine
• B. alanine-glycine
• C. valine-leucine
• D. alanine-alanine

Hint:

The key to solving this question lies in recognizing that glycine reacts with HNO\(_2\) to produce lactic acid, and proline (formed from alanine) can form a cyclic structure when heated.

Answer 1

The Correct Option is B

To determine the dipeptide "x," we need to analyze the amino acids "y" and "z" obtained upon its hydrolysis:

1. Information about "y": The amino acid "y" on treatment with aqueous HNO₂ produces lactic acid. This transformation suggests that "y" is likely to be alanine. The reaction of alanine with HNO₂ results in the formation of lactic acid through the loss of the amino group and subsequent rearrangement.
2. Information about "z": The amino acid "z," upon heating, forms the given cyclic molecule. The structure presented corresponds to a diketopiperazine formed by condensation of glycine. Such a cyclization is characteristic of glycine.
3. Identifying the Dipeptide:
   • Since "y" is alanine and "z" is glycine, the dipeptide "x" must be alanine-glycine.

Hence, based on the transformations and information given, the dipeptide "x" is identified as alanine-glycine.

Answer 2

Let’s break down the key information provided: 
1. Hydrolysis of “x”: 
- The dipeptide "x" undergoes complete hydrolysis to produce two amino acids: y and z. 
- y is a compound that, when treated with aqueous HNO\(_2\), produces lactic acid. This strongly suggests that y is glycine, as glycine reacts with nitrous acid to form lactic acid. 
Therefore, glycine must be one of the products after hydrolysis. 
2. Heating of “z”: 
- z on heating forms a cyclic molecule. This strongly indicates that z is proline, as proline is an amino acid that can form a cyclic structure under heating conditions. 
3. Identifying the Dipeptide: 
- The dipeptide must be one that hydrolyzes to give glycine (which produces lactic acid upon treatment with HNO\(_2\)) and proline (which forms a cyclic structure upon heating). 
- The only dipeptide in the given options that fits this pattern is alanine-glycine (option 2), as alanine can undergo cyclization to form proline under heat. 
Thus, the correct dipeptide x is alanine-glycine.