A 660 mg sample of a protein with quaternary structure (MW=132,000 g/mol) was treated with excess 1-fluoro-2,4-dinitrobenzene (FDNB) under slightly alkaline conditions until the reaction was complete. After FDNB treatment, the peptide bonds of the protein were then completely hydrolyzed by heating it with concentrated 6M HCl. The resulting solution was found to contain 5.5 mg of DNP-Val (shown in the figure below):

2,4-Dinitrophenyl derivatives of the α-amino groups of the other amino acids were not detected. Assuming 100% yield for the reaction of FDNB with the protein, determine the number of subunits in this protein.

• A. 1.
• B. 2.
• C. 3.
• D. 4.
• E. 5.

Answer: (D)

Verified step by step guidance

1. First, understand the reaction process: FDNB reacts with the N-terminal amino group of each subunit in the protein, forming a DNP derivative. The protein is then hydrolyzed to release these derivatives.
2. Calculate the number of moles of DNP-Val produced using its molecular weight (MW = 283 g/mol) and the mass of DNP-Val obtained (5.5 mg). Use the formula: \( \text{moles} = \frac{\text{mass}}{\text{molecular weight}} \).
3. Calculate the number of moles of the protein using its molecular weight (MW = 132,000 g/mol) and the mass of the protein sample (660 mg). Use the formula: \( \text{moles} = \frac{\text{mass}}{\text{molecular weight}} \).
4. Determine the number of subunits in the protein by dividing the moles of DNP-Val by the moles of protein. This ratio gives the number of subunits, as each subunit contributes one N-terminal amino group that reacts with FDNB.
5. Interpret the result: The calculated number of subunits indicates how many polypeptide chains are present in the quaternary structure of the protein.