Nitrophenylsulfenyl halides

Apart from the valuable data about the involvement of tryptophan in enzymic processes (Section VII.), which can be obtained by use of sulfenyl halides, the analytical utility of this reaction is another major application. By reaction with nitrophenylsulfenyl halides a chromophore is generated in a protein which absorbs in the visible part of the spectrum. This permits easy quantitation of the reaction, and thereby allows determination of the tryptophan content of a protein (see Section IV.4.2. for the analytical application of the reaction). 

The reaction of arylsulfenyl halides with proteins at pH< 3.5 is limited to the cysteinyl and tryptophanyl residues (Fontana and Scoffone 1972), as illustrated below for 2-nitrophenylsulfenyl chloride (NPS-Cl). 

Sulfenyl halides have been found to be specific, mild reagents for modification of tryptophan containing peptides and proteins 123, 124). By reaction with a sulfenyl halide like 2-nitrophenylsulfenyl chloride... 

At the present time the most widely used sulfenyl halide is 2-nitrophenylsulfenyl chloride 124). Other compounds tested are 4-nitro-phenyl- 354), 2,4-dinitrophenyl- 354), 2-nitro-4-carboxyphenyl- 416) and 2,4-dinitrophenyl-1,5-disulfenyl chloride 417). The bifunctional reagent has been used to introduce selectively intramolecular crosslinks between tryptophan units (140). 

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