L-cysteine ethyl ester hydrochloride is an amino acid ester derivative of L-cysteine in which the carboxyl group is converted to an ethyl ester while the amino functionality is present as a hydrochloride salt, yielding a zwitterionic/ionic form under typical conditions. The molecule contains a thiol side chain characteristic of cysteine and an amino group, with the ethyl ester masking the carboxylate and the hydrochloride counterion associated with the protonated amine. It is used as a protected/derivatized cysteine building block for peptide and thiofunctional chemistry, including preparation of cysteine-containing peptide intermediates and thiol-directed labeling or conjugation workflows where controlled handling of the ester and thiol is required.
L-cysteine ethyl ester hydrochloride contains the L-cysteine backbone with a thiol side chain and an ethyl ester at the carboxyl terminus, present as a hydrochloride salt that improves handling and solubility for synthetic work. The molecule bears a chiral center at the alpha-carbon, enabling stereochemically defined incorporation into peptide and thiofunctionalized building blocks. The ester and thiol functionalities participate in standard amino acid derivatization chemistry, while the salt form supports controlled reactivity during coupling and subsequent transformations. As a chiral amino acid ester intermediate, it can be converted into protected cysteine derivatives, activated for peptide bond formation, or used as a precursor for thioether, thioester, and disulfide-related motifs.
1. Peptide Synthesis
L-cysteine ethyl ester hydrochloride serves as a cysteine-containing amino acid ester intermediate for peptide coupling workflows where the carboxyl group is temporarily masked as an ethyl ester. The L-configuration at the alpha-carbon and the free thiol functionality can be managed through thiol protection strategies prior to amide bond formation, supporting compatibility with common peptide coupling conditions. Ester-to-activated-acid conversion and subsequent deprotection steps can be aligned with N- and S-protection schemes to control chemoselectivity and minimize side reactions such as thiol oxidation. Downstream, the resulting cysteine-bearing peptide segments and peptide analogs can be assembled for structure-activity relationship studies and biochemical probe construction.
2. Side-Chain Functionalization
L-cysteine ethyl ester hydrochloride is applied in synthetic organic chemistry and chemical biology for side-chain derivatization that leverages the thiol group as a reactive handle. The thiol can be transformed into thioethers, thioesters, disulfides, or conjugation-ready intermediates, while the ethyl ester can be retained or converted to other carboxyl derivatives depending on the target scaffold. The hydrochloride salt form can facilitate controlled handling during nucleophilic substitution or coupling steps that install sulfur-containing motifs into larger molecules. Generated derivatives can feed into peptidomimetics, redox-responsive linkers, and sulfur-functionalized ligands used to interrogate biomolecular interactions.
3. Bioconjugation Chemistry
L-cysteine ethyl ester hydrochloride functions as a chiral sulfur-containing precursor for bioconjugation workflows that require defined stereochemistry and a thiol-based attachment point. The amino acid ester framework can be converted into amide, activated ester, or linker modules, while the thiol enables chemoselective conjugation strategies after appropriate protection and deprotection. The L-stereocenter can be preserved through linker synthesis routes to maintain structural fidelity in conjugates designed for molecular recognition studies. Downstream products include cysteine-derived conjugation reagents for labeling biomolecules, preparing affinity probes, and generating immobilizable constructs for analytical or materials applications.
4. Protected Amino Acid Building Blocks
L-cysteine ethyl ester hydrochloride supports the preparation of protected amino acid derivatives used as building blocks in peptide chemistry and fine chemical synthesis. The ethyl ester allows controlled carboxyl manipulation, enabling conversion to activated acids or acids for coupling, while the thiol can be protected using standard sulfur-protecting groups to suppress oxidation and side reactions. The hydrochloride salt form can be advantageous for reproducible handling during protection, ester conversion, and subsequent coupling steps that require consistent reactivity. Protected cysteine intermediates derived from this compound can be used to construct peptide libraries, generate cysteine-rich sequences, and enable stereochemically defined synthesis of sulfur-bearing analogs.
5. Process Chemistry Intermediate
L-cysteine ethyl ester hydrochloride is suitable for process chemistry intermediate preparation where amino acid ester handling and downstream conversion to reactive derivatives are required at scale. The combination of an ethyl ester and a thiol functionality allows route design that separates protection, activation, and coupling operations into discrete manufacturing steps, supporting process control over chemoselectivity. The salt form can improve feed stability and facilitate consistent dosing into subsequent transformations that generate acids, activated intermediates, or protected cysteine derivatives. Resulting intermediates can be used for specialty chemical production, including sulfur-functional linker manufacture and cysteine-based reagent supply for industrial peptide and conjugate synthesis programs.
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5. SERS spectrum of the peptide thymosin‐β4 obtained with Ag nanorod substrate
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