Fmoc-D-Cys(StBu)-OH

Fmoc-D-Cys(StBu)-OH is an Fmoc-protected D-cysteine derivative bearing a tert-butylthio (StBu) protected thiol on the side chain, with the amino acid retaining the D stereochemical configuration at the alpha carbon. The molecule combines an N-(9-fluorenylmethoxycarbonyl) protecting group for orthogonal peptide coupling with a thioether-protected cysteine side chain that can be selectively unmasked to reveal a reactive thiol for further functionalization. The carboxylic acid remains available for activation and amide bond formation, while the bulky StBu group modulates sulfur reactivity during synthesis and storage. The overall structure functions as a chiral, sulfur-containing peptide building block and a protected amino acid intermediate compatible with standard peptide chemistry workflows.

1. Peptide Synthesis

Fmoc-D-Cys(StBu)-OH is used in peptide synthesis where Fmoc protection supports base-mediated N-deprotection and subsequent coupling cycles under typical protected amino acid strategies. The carboxylic acid enables amide bond formation at the C-terminus, while the D-configured alpha stereocenter allows incorporation of D-cysteine motifs into peptide sequences for stereochemical control and stability tuning. The StBu thioether protection helps manage side-chain sulfur reactivity during chain assembly, reducing unwanted oxidation or side reactions that can occur with free thiols. Downstream deprotection can generate a cysteine thiol handle for forming disulfides, thioether linkages, or thiol-reactive conjugates, supporting peptide analog construction and sulfur-specific structure generation in synthetic libraries.

2. Side-Chain Functionalization

Fmoc-D-Cys(StBu)-OH serves as a side-chain functionalization precursor in synthetic organic chemistry and chemical biology workflows that require controlled introduction of sulfur functionality. The StBu-protected thiol can be converted to a free thiol under appropriate conditions, enabling selective attachment of electrophiles such as alkylating agents, activated aromatics, maleimide-type partners, or haloacetamides for thioether formation. The presence of the Fmoc group allows orthogonal handling during multi-step derivatization, supporting strategies where the peptide N-terminus or intermediate amine must remain protected while the side chain is modified. The resulting thiol-derived products can be used to generate conjugation-ready intermediates, install redox-active disulfide motifs, or prepare chemically defined cysteine-containing probes for downstream molecular recognition studies.

3. Bioconjugation Chemistry

Fmoc-D-Cys(StBu)-OH is applicable to bioconjugation chemistry where cysteine thiol chemistry is used to construct site-selective linkages to biomolecules or biomolecule-derived scaffolds. The protected thiol form supports manufacturing and multi-step synthesis by limiting premature oxidation, while the D stereochemistry can be leveraged to tune local conformation and reduce susceptibility to enzymatic processing associated with L-cysteine motifs. The Fmoc-protected amino acid format aligns with workflows that incorporate cysteine into peptide tags, linkers, or protein-binding domains prior to thiol unmasking and conjugation. The thiol generated from the StBu group can then participate in controlled coupling to electrophilic conjugation partners, supporting the preparation of defined thioether or disulfide-linked bioconjugates used in biochemical research and applied molecular labeling.

4. Process Chemistry Intermediate

Fmoc-D-Cys(StBu)-OH functions as a chiral, sulfur-containing protected amino acid intermediate for process chemistry and specialty chemical production routes that require reliable protection-deprotection logic. The Fmoc group provides a robust N-protecting strategy compatible with iterative synthesis operations, while the StBu thio protection supports handling of a sulfur functionality that would otherwise be prone to oxidation and side reactions. The molecule's defined stereochemistry at the alpha carbon supports reproducible incorporation into peptide building blocks and downstream intermediates, which is important for scale-up consistency in fine chemical manufacturing. The compound can be employed as a feedstock for producing cysteine-containing protected fragments, peptide intermediates, and conjugation-ready derivatives that require sulfur stability during bulk synthesis and subsequent controlled activation.

5. Analytical Research Standards

Fmoc-D-Cys(StBu)-OH can be applied in analytical research as a structurally defined reference material for method development and characterization of cysteine-containing peptides and thioether/disulfide derivatives. The combination of Fmoc and StBu protection provides characteristic mass spectrometric and chromatographic signatures that can assist in monitoring protection state, side-chain integrity, and thiol deprotection outcomes in synthetic sequences. The D stereochemistry enables differentiation from L-cysteine analogs in stereospecific analytical workflows, supporting studies that track stereochemical incorporation and epimerization control. The compound's well-defined functional groups also make it suitable for preparing calibration or verification standards used in peptide synthesis analytics, sulfur functional group quantification, and quality-oriented characterization of amino acid derivative intermediates.

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