57. Which of the following protein modifying reagents is correctly defined? (A) 2-mercaptoethanol: readily oxidizes -SH group and forms disulfide bridges in proteins. (B) cyanogen bromide: the Br ion reacts with the nucleophilic sulfur atom of the target amino acid that leads to protein cleavage. (C) phenylisothiocyanate: reacts with free carboxyl groups in protein. (D) cyanogen bromide: specifically reacts with internal cysteine residues (E) guanidinium HCl: disrupts ionic interactions and hydrogen bonds