Practical Protein Chemistry - A. Darbre 1989

Chemical fragmentation of polypeptides
Cleavage at tryptophan residues
Cleavage at tryptophan residues using cyanogen bromide in heptafluorobutyric acid

The interaction of Cyanogen bromide with the Tryptophan residue in Proteins can lead to peptide bond Cleavage involving the carboxyl group. A moderate reaction yield (~20%) was observed for ß-lactoglobulin [17] and Neurospora crassa Glutamate dehydrogenase [210]. It has been shown [134–136] that the reaction yield increases when using a large excess of the reagent (10,000 mol/mol of protein) and a higher medium acidity (achieved by replacing commercial 70% formic acid with heptafluorobutyric acid).

It was recently demonstrated [51] that the reaction mechanism involves the Oxidative Halogenation of the tryptophan Nucleus. The model compound N-acetyltryptophanamide is converted by the action of excess cyanogen bromide into N-acetyl-5-bromodioxoindolyllactone (along with other products), which exists in equilibrium with its open form (analogously to the reaction with N-Bromosuccinimide) [137]. Under the reaction conditions, partial bromination of free Tyrosine occurs with The formation of bromotyrosine, alongside The oxidation of free Methionine to methionine S-oxide. It is suggested [51] that in the presence of an acid, a nucleophile (indolyl, phenol, or thioether) attacks the bromine atom of the polar "pseudohalogen" CNBr molecule, yielding HCN and the corresponding bromo derivative, which ultimately rearranges into Other Compounds (hydroxy- and dioxoindole, sulfoxide):

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However, it cannot be ruled out that the bromination is caused by traces of bromine present in the cyanogen bromide.

2.5.4.1. Procedure [135]. Apocytochrome (0.1–0.5 µmol) is dissolved in 2 mL of a mixture (1:1) of 88% formic and heptafluorobutyric acids, 700 mg of cyanogen bromide is added, and the mixture is incubated in the dark for 24 h. The reagent and Solvents are evaporated under a stream of nitrogen, the residue is dissolved in 10 mL of Water and freeze-dried. The residue is dissolved in 9 M acetic acid and chromatographed on a Sephadex G-50 Column.