Method for separating the n-terminal methionine.

The present invention provides a method for removing a methionine residue chemically N TERMINAL SELECTIVELY specifically and efficiently from a peptide or a salt thereof having a methionine residue optionally oxidized at its N-TERMINAL. IN THE METHOD REACTS a peptide or a salt thereof having a methionine residue optionally oxidized at its N-TERMINAL WITH A DERIVATIVE {AL} - diketone, followed by hydrolysis.