A protein has been sequenced after cleavage of disulfide bonds. The protein is known to contain 3 Cys residues, located as shown here. Only one of the Cys has a free —SH group, and the other two are involved in an —S—S— bond. 

The only methionine and the only aromatic amino acid (Phe) in this protein are in the positions indicated. Cleavage of the intact protein (i.e., with disulfide bonds intact) by either cyanogen bromide or chymotrypsin does not break the protein into two peptides. Where is the —S—S— bond (i.e., AB, BC, or AC)?

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Phe Cys B A -Met- * N-terminus Cys C-terminus