The Sanger method for N-terminus determination is a less common alternative to the Edman degradation. In the Sanger method, the peptide is treated with the Sanger reagent, 2, 4-dinitrofluorobenzene, and then hydrolyzed by reaction with 6 M aqueous HCl. The N-terminal amino acid is recovered as its 2, 4-dinitrophenyl derivative and identified.
The Sanger method

(a) Propose a mechanism for the reaction of the N terminus of the peptide with 2,4-dinitrofluorobenzene.
(b) Explain why the Edman degradation is usually preferred over the Sanger method.

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O,N F H2N-CH-C NH peptide No peptide 2.4-dinitrofluorobenzene (Sanger reagent) 6 M HCI, heat O,N NH CHC NH NH CH COOH R1 R1 NO derivative NO2 2,4-dinitrophenyl derivative +amino acids