Sanger's reagent, used to label the N-terminal amino group of a peptide, is

A1-chloro-4-nitrobenzene
B1-fluoro-2,4-dinitrobenzene
C2,4,6-trinitrophenol
D2,4-dinitrophenylhydrazine

Answer: B

Sanger's reagent is an aryl fluoride activated by nitro groups ortho and para to fluorine. The free amino group of the peptide displaces fluoride by SNAr to give a yellow N-(2,4-dinitrophenyl) derivative. Brady's reagent (2,4-dinitrophenylhydrazine) and picric acid are different compounds. The answer is 1-fluoro-2,4-dinitrobenzene.

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