The Dansyl Method for Identifying N-Terminal Amino Acids

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The reagent 1-dimethylaminonaphthalene-5-sulfonyl chloride (dansyl chloride, DNS-Cl) reacts with the free amino groups of pep-tides and proteins as shown in Fig. 1. Total acid hydrolysis of the substituted peptide or protein yields a mixture of free amino acids plus the dansyl derivative of the N-terminal amino acid, the bond between the dansyl group and the N-terminal amino acid being resistant to acid hydrolysis. The dansyl amino acid is fluorescent under UV light and is identified by thin-layer chromatography on polyamide sheets. This is an extremely sensitive method for identifying amino acids and in particular has found considerable use in peptide sequence determination when used in conjunction with the Edman degradation (see Chapter 36). The dansyl technique was originally introduced by Gray and Hartley (1), and was developed essentially for use with peptides. However, the method can also be applied to proteins (see Note 1).