Fmoc-Cys-OSu)2 is a bis(activated) cysteine derivative in which cysteine side chains are converted into N-(9H-fluorenylmethoxycarbonyl) protected amino acid units bearing an O-succinimidyl (OSu) ester functionality. The molecule contains an Fmoc-protected α-amino group, a free carboxyl group masked as part of the OSu-activated ester motif, and thiol-bearing cysteine side chains that are poised for chemoselective coupling through the succinimidyl ester handle while the Fmoc group provides orthogonal protection for stepwise assembly. In peptide chemistry and bioconjugation workflows, (Fmoc-Cys-OSu)2 is used as an activated cysteine building block to introduce cysteine-derived linkages and to support preparation of peptide derivatives and labeled conjugates via nucleophilic amine capture by the OSu ester.
Fmoc-Cys-OSu is a cysteine-based amino acid derivative bearing an Fmoc-protected amino group and an N-hydroxysuccinimide (OSu) activated ester on the side-chain carboxylate, enabling efficient amide bond formation with primary amines. The product is supplied as (Fmoc-Cys-OSu)2, providing two OSu-activated sites for downstream coupling workflows while retaining the Fmoc handle for controlled incorporation into protected peptide intermediates. In peptide chemistry and chemical biology, this activated cysteine form is used when a cysteine-containing segment needs to be functionalized or coupled under conditions that preserve the Fmoc-protected backbone.
1. Peptide Segment Coupling
Fmoc-Cys-OSu is used by peptide synthesis groups to prepare cysteine-containing building blocks for segment condensation and side-chain functionalization steps, where OSu activation supports rapid formation of amide-linked side products with amine-bearing partners. The Fmoc group provides a protected amino handle for subsequent peptide assembly workflows, while the activated ester allows coupling to lysine-like amines, amino-functional linkers, or other amine-bearing reagents used in custom peptide manufacturing and research-grade peptide libraries. This format is particularly useful when cysteine side-chain derivatization is required as part of the synthesis plan rather than after peptide assembly.
2. Bioconjugation Linker Installation
Fmoc-Cys-OSu is applied in bioconjugation development to install cysteine-derived amide linkages onto proteins, peptides, or biomolecule scaffolds that present primary amines. Researchers use the OSu-activated ester to drive efficient coupling to amine-functional targets such as antibody fragments, peptide carriers, or engineered protein tags, enabling attachment of linkers that can later be carried through peptide-based workflows. The presence of the Fmoc-protected moiety supports compatibility with downstream protected intermediate handling, which is valuable when conjugates must be produced as defined peptide conjugation constructs rather than heterogeneous post-modification mixtures.
3. Functionalized Peptide Library Synthesis
Fmoc-Cys-OSu is commonly used in peptide chemistry laboratories to generate functionalized cysteine variants for library construction, where OSu activation enables controlled attachment of diverse amine-containing substituents to the cysteine side chain. This approach supports structure-property studies that require systematic variation of side-chain functionality while maintaining an Fmoc-protected framework for iterative synthesis and purification. By enabling amide coupling to a range of amine-bearing motifs, (Fmoc-Cys-OSu)2 supports the rapid preparation of derivatized peptide intermediates used in screening campaigns and method development for custom peptide synthesis.
4. Amine-Activated Intermediate Building Block
Fmoc-Cys-OSu is used as an intermediate building block in medicinal chemistry and specialty chemical development when an activated cysteine unit is required for amide bond formation with primary amines under peptide-compatible conditions. Process and R&D teams leverage the OSu ester reactivity to couple amine-containing fragments that will later be incorporated into larger synthetic sequences, including cysteine-containing peptidomimetic structures and amide-linked scaffolds. The Fmoc functionality supports downstream handling as a protected amino component, making (Fmoc-Cys-OSu)2 a practical reagent for assembling well-defined, amide-linked intermediates rather than relying on less controlled activation strategies.
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