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

The dipeptide "x" is identified by analyzing its constituent amino acids, "y" and "z," obtained after hydrolysis:

1. Analysis of "y": Treatment of amino acid "y" with aqueous HNO₂ yields lactic acid. This indicates that "y" is likely alanine, as its reaction with HNO₂ leads to the formation of lactic acid via deamination and rearrangement.
2. Analysis of "z": Upon heating, amino acid "z" forms the presented cyclic molecule. This structure is a diketopiperazine, characteristic of the cyclization of glycine.
3. Dipeptide Identification:
   • Given that "y" is alanine and "z" is glycine, the dipeptide "x" is determined to be alanine-glycine.

Therefore, the dipeptide "x" is confirmed as alanine-glycine based on the provided data.