Fmoc-L-Cys-OH*H2O

Fmoc-L-Cys-OH·H2O is a hydrated Fmoc-protected L-cysteine building block featuring an Fmoc carbamate on the amino group, a free carboxylic acid, and a thiol-containing side chain that is present in a hydrated solid form. The molecule bears the native L stereochemistry at the α-carbon, enabling stereochemically defined peptide bond formation while maintaining the sulfur functionality for subsequent chemoselective transformations. The Fmoc group supports base-labile protection strategies commonly used in stepwise solid-phase peptide synthesis, whereas the carboxylic acid enables coupling to activated amino acid derivatives under standard peptide coupling conditions. The thiol side chain can be managed through oxidation, alkylation, or orthogonal protection logic to control disulfide formation and to tune reactivity during synthesis and downstream derivatization.

1. Peptide Synthesis

Fmoc-L-Cys-OH·H2O is applied as a cysteine residue for peptide building block preparation in both solution-phase and Fmoc/t-Bu compatible solid-phase peptide synthesis workflows. The Fmoc-protected amine and the free carboxylic acid support iterative peptide coupling after Fmoc deprotection, while the L-configuration preserves stereochemical fidelity at the α-center. The thiol side chain can be directed toward controlled oxidation to disulfide-containing peptides or toward thiol-protected strategies to prevent side reactions during chain assembly. Downstream peptide products can include native disulfide architectures, cysteine-rich motifs, and peptide analogs that require sulfur chemistry compatible with standard protecting-group orthogonality.

2. Bioconjugation Chemistry

Fmoc-L-Cys-OH·H2O is used in chemical biology and bioconjugation development where cysteine thiols serve as handles for site-selective attachment to biomolecules. The presence of the thiol enables formation of thioether linkages through alkylation-based conjugation logic or disulfide exchange strategies when appropriate, while the Fmoc functionality supports controlled deprotection prior to conjugation steps. The α-carboxyl group and the protected amine can also be leveraged to generate cysteine-containing linkers and peptide conjugates with defined spacing and stereochemical identity. Resulting conjugates can be applied to labeling workflows, affinity probes, and modular construct generation for studying biomolecular interactions.

3. Side-Chain Functionalization

Fmoc-L-Cys-OH·H2O is suitable for amino acid derivatization and side-chain functionalization programs that transform the cysteine sulfur into chemically distinct motifs. The thiol can be oxidized to disulfides, converted to sulfenamides, or alkylated to introduce stable thioether substituents, enabling access to peptidomimetic sulfur patterns and redox-responsive linkers. The Fmoc-protected amino group and free carboxylic acid allow orthogonal manipulation sequences, where amino protection can be removed or retained depending on the targeted intermediate design. Downstream products include functionalized cysteine-containing fragments used for scaffold diversification, SAR studies, and synthesis of sulfur-bearing analogs with controlled electronic and steric properties.

4. Protein Engineering

Fmoc-L-Cys-OH·H2O supports protein engineering and recombinant protein modification strategies that require cysteine incorporation at defined positions. The L stereochemistry and the carboxyl functionality align with peptide-level assembly approaches used to generate cysteine-containing segments, which can subsequently be incorporated into larger constructs through ligation or conjugation chemistry. The thiol side chain can be managed to control oxidation state and to enable selective crosslinking, thereby supporting studies of structure-function relationships in cysteine-dependent interfaces. The resulting protein or protein-derived materials can be used for mechanistic investigations, domain mapping, and controlled assembly of multi-component biomolecular systems.

5. Pharmaceutical Intermediate Preparation

Fmoc-L-Cys-OH·H2O is relevant to pharmaceutical intermediate preparation where cysteine-derived motifs and thiol-bearing intermediates are used to build drug-like fragments and linker units. The protected amine and free carboxylic acid facilitate conversion into activated intermediates for subsequent coupling steps in fine chemical synthesis, while the thiol enables downstream functional group interconversions to thioethers, disulfides, or other sulfur functionalities used in medicinal chemistry. The hydration state can influence solid handling and reproducibility in manufacturing-oriented workflows, while the Fmoc group provides a controllable protection handle during multistep synthesis planning. Downstream utilization includes preparation of cysteine-containing building blocks for peptidomimetic construction, conjugatable scaffolds, and sulfur-functionalized intermediates used across process chemistry routes.

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