Question

The average molecular weight of amino acid in a peptide is

• A. 138 daltons
• B. 128 daltons
• C. 110 daltons
• D. 120 daltons

Hint:

To estimate the number of amino acid residues in a protein, divide its total molecular weight (in Daltons) by 110.
This is a standard biochemistry rule of thumb.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
When amino acids link together to form a peptide or protein chain, they undergo a condensation reaction.
In this reaction, a covalent peptide bond is formed with the loss of a water molecule.
Key Formula or Approach:
The average molecular weight of a free amino acid must be adjusted for the mass of the water molecule lost during polymerization: \[ M_{\text{residue}} = M_{\text{free amino acid}} - M_{\text{water}} \]

Step 2: Detailed Explanation:
- The weighted average molecular weight of the 20 standard amino acids based on their natural abundance in proteins is approximately 128 Daltons.
- During the formation of each peptide bond, one molecule of water (\(\text{H}_2\text{O}\)) is eliminated: \[ M_{\text{water}} = 18 \text{ Daltons} \] - Therefore, the average molecular weight of an amino acid residue in a polypeptide chain is: \[ M_{\text{residue}} = 128 \text{ Daltons} - 18 \text{ Daltons} = 110 \text{ Daltons} \]

Step 3: Final Answer:
The average molecular weight of an amino acid residue in a peptide is 110 Daltons, corresponding to option (C).