SmethylLcysteine

S-methyl-L-cysteine is a non-proteinogenic amino acid in which the thiol group of L-cysteine is alkylated with a methyl group, yielding a thioether side chain (-CH2-S-CH3). Its chemical formula is C4H10NO2S, and the molecule is designated in the L configuration at the alpha carbon. Like other amino acids, it can exist as a zwitterion at physiological pH, with the amino and carboxylate groups contributing to its overall acid–base behavior. The thioether side chain makes it chemically distinct from cysteine, reducing thiol reactivity.

Occurrence and synthesis: In the chemical literature, S-methyl-L-cysteine can be prepared by methylation of L-cysteine or its derivatives. It has been reported in trace amounts in some plant extracts, but it is not a standard component of proteins or typical human metabolism. The enantiomeric purity depends on the synthetic route used to produce the compound.

Properties and applications: S-methyl-L-cysteine is soluble in water and is generally more chemically stable than cysteine due to the thioether linkage. It is primarily used in research as a reference compound for amino-acid analyses and for studying sulfur-containing, sulfur-alkylated amino acid–like structures. It is not incorporated into polypeptides by ribosomes, reflecting its status as a non-proteinogenic analog.

Safety and handling: As a low-toxicity amino-acid derivative, it is treated with standard laboratory chemical hygiene practices. Toxicology data are limited; consult safety data sheets and handle with appropriate care.