Carbon-13 nuclear magnetic resonance spectroscopy was used to monitor the preferential sulfoxidation of the two methionine residues (8 and 81) of glycophorin A. In urea Met-8 is readily oxidized. However, Met-81 can only be oxidized in trifluoroacetic acid containing hydrogen peroxide. Our results also give some insight into the reagent accessibility of different portions of the protein molecule and the general stability of this glycoprotein.
