Fmoc-L-cysteic acid disodium salt is a protected amino acid derivative where the sulfonic acid group of L-cysteic acid is in its disodium salt form, and the α-amino group is protected by a 9-fluorenylmethoxycarbonyl (Fmoc) group. This white to off-white solid is highly soluble in water and polar organic solvents, making it suitable for solution-phase chemistry. Its primary and most significant application is in solid-phase peptide synthesis (SPPS) using the Fmoc strategy. The Fmoc group serves as a base-labile protecting group for the amino terminus, while the cysteic acid moiety introduces a permanently charged, hydrophilic sulfonate group into the peptide sequence. This is crucial for synthesizing peptides that mimic post-translationally modified proteins, particularly those containing sulfotyrosine, or for engineering peptides with enhanced solubility, altered binding properties, or specific ionic characteristics for research and therapeutic development. As a specialized, protected amino acid building block, it is a fundamental reagent in peptide chemistry and chemical biology. Its use extends to the preparation of peptide-based probes, antigens, and potential drug candidates where the incorporation of a negatively charged, sulfonated residue is required.