Fmoc-L-citrulline is a protected derivative of the non-proteinogenic amino acid L-citrulline. Its chemical structure features the citrulline residue with its side-chain ureido group and α-amino group protected by the 9-fluorenylmethoxycarbonyl (Fmoc) group. This white to off-white crystalline powder is a crucial building block in solid-phase peptide synthesis (SPPS) using the Fmoc strategy. The primary application of Fmoc-L-citrulline is in the research and production of peptides containing citrulline residues. Citrullination, the post-translational conversion of arginine to citrulline, is a key process studied in autoimmune diseases like rheumatoid arthritis. Therefore, this derivative is essential for synthesizing peptide antigens, epitopes, and inhibitors for immunological and biomedical research. It enables the precise incorporation of citrulline into custom peptide sequences. As a specialized, protected amino acid, it is a fundamental reagent in peptide chemistry laboratories. While its core use is in life science research and peptide drug development, its well-defined structure and reactivity also qualify it as a specialized organic synthesis building block for constructing complex molecules containing modified amino acid motifs.