Ademetionine  is a chiral molecule. In its endogenous form, it has an “S” configuration at the alpha carbon of the methionine moiety and also the “S” conformation at the sulfonium center.  The chirality of the alpha carbon of the methionine part of the molecule is stable, so, unless otherwise indicated, discussions of chirality, and “S” and “R” isomers are in reference to the sulfonium center.  The sulfonium center chirality is labile and has a thermodynamic tendency to racemize.  As such, Ademetionine preparations exist as enriched chiral mixtures.