Identification of L-methionine oxidation products in tripeptides, in Met-enkephalin and in the bovine basic pancreatic trypsin inhibitor: 1H and 13C NMR study

L-methionine (Met) oxidation products in tripeptides, in Met-enkephalin and in the bovine basic pancreatic trypsin inhibitor have been identified by ¹H and ¹³C NMR spectroscopy. The oxidation of Met residues by stoichiometric amounts of chloramine-B, H₂O₂ and I₂ yields a mixture of L-methionine sulfoxide (Met(O)) and dehydro-L-methionine (DH-Met), Met(O) and DH-Met, respectively, at pH 7.0 and 25.0°C. The formation of DH-Met occurs only if the amino-group of Met is not derivatized. The analysis of ¹H and ¹³C NMR spectra allows us to quantitate Met oxidation products, to ascertain the relative proportion of the R and S forms of Met(O) and DH-Met, and to reveal the presence of a Met residue at the N-terminal position in peptides and proteins.