Question

Why does cyanogen bromide not cleave the C-side of the cysteine?

Answer 1

Name	Type	Specificity
Cyanogen Bromide	Chemical	Carboxyl Side of Methionine

This cleaves the peptide bonds on the carboxyl side of Methionine

nucleophilic sulfur in methionine is in charge of assaulting BrCN, while the sulfur in cysteine does not carry and behave similarly.

In the event that the sulfur in cysteine attacked cyanogen bromide, the bromide particle would deprotonate the cyanide adduct, leaving the sulfur uncharged and the beta carbon of the cysteine not electrophilic