D-Cysteine methyl ester hydrochloride is a derivative of the naturally occurring amino acid cysteine, specifically the D-enantiomer of cysteine methyl ester in its hydrochloride salt form. It is a white to off-white crystalline powder with the molecular formula C4H9NO2S·HCl. This compound is characterized by the presence of a thiol (-SH) group and a methyl ester moiety, making it a protected and activated form of D-cysteine. Its primary applications are in the fields of biochemistry and pharmaceutical research. It serves as a crucial chiral building block for the synthesis of peptides and peptidomimetics, particularly those requiring the incorporation of the D-cysteine configuration. The methyl ester group facilitates peptide coupling reactions. Furthermore, due to its thiol functionality, it is used in studies involving disulfide bond formation, metal chelation, and as a precursor for various biochemical probes and enzyme inhibitors. As a derivative of a non-proteinogenic D-amino acid, it is a valuable tool for investigating biological processes and developing novel therapeutic agents. It is also utilized in the preparation of chelating agents and in materials science for surface functionalization. Proper handling is required as it may cause skin and eye irritation.