H-D-Cys(Fm)-OH, with CAS number 257288-48-5, is a non-natural amino acid derivative. Its chemical name is S-9-Fluorenylmethyl-D-cysteine hydrochloride. The molecule features a D-cysteine backbone where the thiol (-SH) group is protected by a 9-fluorenylmethyl (Fm) group, and the carboxylic acid is in its free acid form. It is typically supplied as a hydrochloride salt to enhance stability and solubility. This compound is primarily used as a key building block in solid-phase peptide synthesis (SPPS). The Fm group serves as a thiol-protecting group, which is orthogonal to the standard Fmoc/tert-butyl strategy commonly used for amine and side-chain protection. This allows for the controlled, sequential assembly of peptides containing cysteine residues, which are crucial for forming disulfide bridges that dictate the three-dimensional structure and biological activity of many therapeutic peptides. Its main application lies in the research and development of peptide-based pharmaceuticals, including hormones, antimicrobial peptides, and enzyme inhibitors. The D-configuration of the cysteine can confer resistance to enzymatic degradation, potentially improving the metabolic stability of the resulting peptides. Handling requires standard precautions for laboratory chemicals, and it should be stored in a cool, dry place.