Question

Which of the following molecules is eliminated during peptide bond formation?

• A. \(\mathrm{H_2O}\)
• B. \(\mathrm{NH_3}\)
• C. \(\mathrm{CH_3OH}\)
• D. \(\mathrm{CO_2}\)

Hint:

Peptide bond formation is a condensation reaction because a water molecule is eliminated during bond formation.

Answer 1

The Correct Option is A

Step 1: Recall how a peptide bond forms.
A peptide bond joins two amino acids by linking the carboxyl group of one to the amino group of the other.
Step 2: Identify the reacting groups.
One amino acid offers its $-COOH$ group and the other offers its $-NH_2$ group for the reaction.
Step 3: See which atoms are removed.
The $-OH$ leaves from the carboxyl group and an $-H$ leaves from the amino group. These two pieces combine.
Step 4: Form the by-product.
An $-OH$ plus an $-H$ together make $H_2O$, so a water molecule is eliminated during the condensation.
Step 5: Write the reaction.
\[ R\text{-}COOH + H_2N\text{-}R' \rightarrow R\text{-}CONH\text{-}R' + H_2O \] where $-CONH-$ is the peptide linkage.
Step 6: Reject the other choices.
$NH_3$, $CH_3OH$ and $CO_2$ are not released here, so the eliminated molecule is water, option 1.
\[ \boxed{H_2O} \]