Question

Phenyl isothiocyanate efficiently reacts with amino groups of proteins

• A. under highly acidic conditions
• B. between pH 2 and pH 4
• C. in the presence of anhydrous HCl gas
• D. under alkaline conditions

Hint:

Coupling = Base (Alkaline); Cleavage = Acid.

Answer 1

The Correct Option is D

Step 1: Concept
This reaction is the first step of the Edman degradation process used for sequencing amino acids in a peptide.

Step 2: Meaning
Phenyl isothiocyanate (PITC) reacts with the N-terminal amino group of a protein to form a phenylthiocarbamoyl derivative.

Step 3: Analysis
For the amino group ($-NH_2$) to act as a nucleophile and attack the PITC, it must be in its deprotonated (basic) form. In acidic conditions, the amino group becomes protonated ($-NH_3^+$) and loses its nucleophilicity.

Step 4: Conclusion
Therefore, the coupling reaction is performed under mildly alkaline conditions (typically pH 8-9) to ensure the amino group is reactive. Final Answer: (D)