Fmoc-L-Cys(StBu)-OH

Fmoc-L-Cys(StBu)-OH is an Fmoc-protected L-cysteine derivative bearing a tert-butylthio (StBu) group on the side-chain thiol, preserving the native stereochemical configuration at the α-carbon while masking the highly nucleophilic sulfur functionality. The molecule contains an Fmoc carbamate at the amino terminus and a free carboxylic acid, enabling controlled peptide coupling at the C-terminus while keeping the side-chain thiol protected against premature oxidation or disulfide scrambling. The thioether StBu substituent is stable under many peptide synthesis conditions yet can be removed under appropriate deprotection strategies to reveal a reactive cysteine thiol for subsequent functionalization. The combination of an orthogonally protected thiol and a robust Fmoc handle makes the compound a chiral amino acid intermediate and peptide building block suited to sulfur chemistry workflows and downstream synthetic elaboration.

1. Peptide Synthesis

Fmoc-L-Cys(StBu)-OH is used in peptide synthesis to introduce a cysteine residue with orthogonal side-chain protection, where the Fmoc group supports standard N-terminal deprotection and coupling cycles and the StBu thioether protects the sulfur from side reactions. The free carboxylic acid participates in amide bond formation, while the α-stereocenter ensures incorporation of the L-configuration into growing peptide chains. The protected thiol can later be unmasked to enable disulfide bond formation, thioether/thiol conjugation, or selective post-coupling modifications. Peptide building block preparation based on this structure supports assembly of cysteine-containing sequences for biochemical research and solid-phase or solution-phase peptide chemistry.

2. Side-Chain Functionalization

Fmoc-L-Cys(StBu)-OH is applied in side-chain functionalization chemistry because the masked thiol can be converted into reactive sulfur species after StBu removal, enabling thiol-specific derivatization. The presence of the Fmoc-protected amine and carboxylic acid supports incorporation into protected peptide fragments or intermediate building blocks prior to thiol reveal. The resulting cysteine thiol can be used to generate disulfides, thioethers, thioester-like linkages, or electrophile-reactive handles for subsequent conjugation steps. Downstream molecular modification using this protected cysteine scaffold supports synthesis of sulfur-containing analogs for chemical biology, biomolecule labeling, and peptidomimetic construction.

3. Bioconjugation Chemistry

Fmoc-L-Cys(StBu)-OH is suitable for bioconjugation workflows that require defined cysteine chemistry, where controlled exposure of the thiol enables site-selective coupling to maleimides, activated halides, or other electrophiles under conditions compatible with biomolecular functional groups. The Fmoc handle and carboxylic acid provide a means to build peptide conjugates or linker-containing intermediates with predictable orientation and spacing before conjugation. The StBu-protected sulfur reduces oxidation risk during synthesis and handling, supporting reproducible downstream conjugate generation. Bioconjugation-oriented use of this amino acid derivative supports preparation of cysteine-bearing peptide linkers, protein labeling reagents, and structured conjugation scaffolds used in biochemical research and applied chemical manufacturing.

4. Process Chemistry Intermediate

Fmoc-L-Cys(StBu)-OH is employed as a chiral amino acid intermediate in process chemistry and fine chemical synthesis where orthogonal protection of the thiol and Fmoc group supports scalable peptide-building operations. The compound's functional group set, including the Fmoc carbamate and free acid, enables predictable conversion into activated coupling forms and controlled incorporation into protected intermediates. The StBu thioether protection strategy can be leveraged to minimize thiol oxidation and side-product formation during manufacturing steps that involve base/organic solvent exposure typical of protected amino acid handling. Industrial relevance is reflected in its role as a manufacturable cysteine building block for producing protected peptide intermediates and sulfur-functionalized downstream products.

5. Analytical Research Standards

Fmoc-L-Cys(StBu)-OH is applied in analytical research as a structurally defined reference material for method development and characterization of cysteine-containing peptides and sulfur-protected intermediates. The presence of Fmoc and the protected side-chain thiol provides characteristic chromatographic and spectrometric signatures that can support identification of coupling products, monitoring of deprotection events, and verification of side-chain integrity. The L-configuration and orthogonal protection pattern help distinguish cysteine derivatives from other thioether or protected amino acid analogs during analytical workflows. Analytical standard development using this compound supports quality control of peptide synthesis streams and characterization of sulfur-containing molecular scaffolds in research and industrial laboratories.

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