Question

Identify the pair of reactants that upon reaction, with elimination of HCl will give rise to the dipeptide Gly-Ala.

• A. $\mathrm{NH}_{2}-\mathrm{CH}_{2}-\mathrm{COCl}$ and $\mathrm{NH}_{2}-\mathrm{CH}-\mathrm{COOH}$
• B. $\mathrm{NH}_{2}-\mathrm{CH}_{2}-\mathrm{COCl}$ and $\mathrm{NH}_{3}-\mathrm{CH}-\mathrm{COCl}$
• C. $\mathrm{NH}_{2}-\mathrm{CH}_{2}-\mathrm{COOH}$ and $\mathrm{NH}_{2}-\mathrm{CH}-\mathrm{COCl}$
• D. $\mathrm{NH}_{2}-\mathrm{CH}_{2}-\mathrm{COOH}$ and $\mathrm{NH}_{2}-\mathrm{CH}-\mathrm{COOH}$

Hint:

The formation of a dipeptide involves the reaction between an amino acid and its chloride derivative with the elimination of HCl.

Answer 1

The Correct Option is A

To identify the correct reactants for forming the dipeptide Gly-Ala with HCl elimination, we consider peptide formation principles and functional group reactivity:

• Dipeptides are synthesized via condensation of two amino acids, typically reacting a carboxylic acid group with an amine group.
• While water is usually eliminated in peptide bond formation, HCl is eliminated when using acyl chlorides (acid chlorides) as reactants.

Evaluation of the options proceeds as follows:

1. The combination of \(NH_{2}-CH_{2}-COCl\) and \(NH_{2}-CH-COOH\) is the most suitable. One reactant is a reactive acid chloride, and the other is an amine with an intact carboxylic acid group, facilitating subsequent reactions.

The formation reaction proceeds as:

• The acid chloride end of Glycine reacts with the amine group of Alanine, eliminating HCl.
• This reaction mechanism aligns with the condensation process for forming peptide bonds between amino acids.

Consequently, the correct reactant pair is \(NH_{2}-CH_{2}-COCl\) and \(NH_{2}-CH-COOH\).

Reasons for the inadequacy of other options:

• \(NH_{2}-CH_{2}-COCl\) and \(NH_{3}-CH-COCl\): Both are acid chlorides. The absence of a free amine group prevents peptide bond formation.
• \(NH_{2}-CH_{2}-COOH\) and \(NH_{2}-CH-COCl\): Glycine in this configuration lacks a reactive site. The COOH group, unlike COCl, requires activation for reaction.
• \(NH_{2}-CH_{2}-COOH\) and \(NH_{2}-CH-COOH\): Both reactants are in their free acid forms, exhibiting low reactivity and an inability to form a peptide linkage spontaneously.

Therefore, the pair \(NH_{2}-CH_{2}-COCl\) and \(NH_{2}-CH-COOH\) is confirmed as correct for synthesizing the dipeptide Gly-Ala with HCl elimination.