(2S)-1-[(2S)-2-Methyl-3-sulfanylpropanoyl]pyrrolidine-2-carboxylic acid; 1-[(2S)-3-Mercapto-2-methylpropionyl]-l-proline;(S)-1-((S)-3-mercapto-2-methylpropanoyl)pyrrolidine-2-carboxylic acid;