title: Use of N-chlorosuccinimide/urea for the selective cleavage of tryptophanyl peptide bonds in proteins. Cytochrome c.
abstract: The conditions and utility of the N-chlorosuccinimide/urea (NCS/urea) reagent for the selective cleavage of tryptophanyl peptide bonds in proteins is demonstrated with cytochrome c. At low concentrations of NCS/urea the oxidation of thioether side chains in cytochrome c is the predominant reaction. Methionyl residues are oxidized to sulfoxide and the heme-thioether bridge is partially cleaved. At 10-fold excess of NCS/urea reagent, cleavage of the tryptophanyl peptide bond is optimal at approximately 50% yield in several species of cytochrome c studied. Analytical data on isolated horse cytochrome c peptide fragments demonstrate lack of modification and cleavage at tyrosyl and histidyl residues. However, at high concentrations of NCS/urea reagent (30-fold) unexpected conversions of methionine to sulfone and cysteine to cysteic acid in intact proteins are observed. This is in contradistinction to the absence of sulfone in NCS/urea-reacted amino acid mixtures. The mechanisms of halogenation and cleavage by N-bromosuccinimide, N-iodosuccinimide, and N-chlorosuccinimide are discussed. It is porposed that the selectivity with respect to halogenation by N-chlorosuccinimide is due to the insignificant participation of molecular chlorine in the NCS/urea reaction. A mechanism of halogenation and cleavage by NCS at tryptophan is also offered.
mesh_terms: D000596:Amino Acids / Q000032:analysis; D002852:Chromatography, Ion Exchange; D003574:Cytochrome c Group; D007202:Indicators and Reagents; D008722:Methods; D013388:Succinimides; D014364:Tryptophan; D014508:Urea
chemical_list: D000596:Amino Acids; D003574:Cytochrome c Group; D007202:Indicators and Reagents; D013388:Succinimides; D014364:Tryptophan; D014508:Urea