Which of the following reagents would be most useful in determining the N-terminal amino acid of a polypeptide?
**Core Concept**
The N-terminal amino acid of a polypeptide can be identified using specific reagents that selectively react with the free amino group at the N-terminus. This is a crucial step in protein sequencing and structural analysis.
**Why the Correct Answer is Right**
The correct answer is **D. Fluorodinitrobenzene (FDNB)**. FDNB reacts with the free amino group at the N-terminus of a polypeptide to form a stable N-phenylthiocarbamoyl derivative, which can be easily identified and isolated. This reaction is a key step in the Edman degradation method, a widely used technique for protein sequencing.
**Why Each Wrong Option is Incorrect**
**Option A:** **Dinitrofluorobenzene (DNFB)** is similar to FDNB but is not typically used for protein sequencing due to its higher reactivity and potential for side reactions.
**Option B:** **2,4-Dinitrofluorobenzene (2,4-DNFB)** is another reagent that can react with amino groups, but it is not as specific or useful for identifying the N-terminal amino acid as FDNB.
**Option C:** **Carbodiimide** is a reagent used for peptide coupling, but it is not suitable for identifying the N-terminal amino acid of a polypeptide.
**Clinical Pearl / High-Yield Fact**
When working with proteins, it's essential to remember that the N-terminal amino acid can be identified using specific reagents like FDNB, but the C-terminal amino acid is typically identified using other methods, such as carboxypeptidase digestion.
**Correct Answer:** D. Fluorodinitrobenzene (FDNB).