H-Cys-OH · HCl · H2O

H-Cys-OH · HCl · H2O is cysteine hydrochloride monohydrate, consisting of the chiral amino acid cysteine in its salt form, with the free α-amino group and α-carboxylic acid present as protonated/ion-paired functionalities and a thiol side chain that can participate in redox and nucleophilic chemistry. The stereogenic center at the α-carbon fixes the L-configuration, which is critical for stereospecific peptide coupling and for compatibility with enzyme-recognition motifs in biochemical workflows. The thiol group can undergo alkylation, oxidation to disulfides, or formation of thioether conjugates, while the salt state and hydration influence solubility and handling during synthesis. As an amino acid building block and chiral intermediate, cysteine hydrochloride monohydrate is commonly used to introduce sulfur-containing functionality into peptides, protein labeling reagents, and downstream synthetic targets that require controlled thiol reactivity.

1. Peptide Synthesis

H-Cys-OH · HCl · H2O supports peptide building block preparation and coupling chemistry in solid-phase or solution-phase workflows where the L-cysteine stereocenter must be retained. The amino and carboxyl groups enable standard amide bond formation after appropriate protection/deprotection, while the thiol side chain typically requires orthogonal protection strategies (commonly thioether or disulfide-protected forms) to prevent side reactions during coupling and deprotection cycles. The salt and hydration state can improve aqueous compatibility for certain pre-activation and handling steps, while thiol reactivity can be leveraged for post-coupling disulfide formation or selective conjugation. Downstream, cysteine-containing peptides and cysteine-rich analogs can be generated for structural studies, ligand scaffolds, and disulfide-bridged peptide architectures.

2. Chemical Biology Labeling

H-Cys-OH · HCl · H2O is used in chemical biology research to introduce thiol-bearing handles for site-selective bioconjugation and molecular labeling strategies. The side-chain thiol can be converted into reactive intermediates for maleimide/thioether formation, disulfide exchange, or controlled oxidation to disulfide-linked constructs, enabling conjugation to biomolecule surfaces and affinity tags. The presence of a defined L-configuration at the α-carbon supports incorporation into cysteine-containing peptides and linkers that maintain stereochemical fidelity for biomolecular recognition. The resulting thiol-functional derivatives can serve as intermediates for probe synthesis, biomolecule modification, and mapping of sulfur-dependent interactions in biochemical assays.

3. Protein Engineering

H-Cys-OH · HCl · H2O functions as a chiral amino acid precursor for constructing cysteine variants and engineered protein segments where sulfur chemistry governs folding, stability, and intermolecular crosslinking. The amino acid backbone supports incorporation into peptide fragments used for recombinant protein expression workflows or for chemical protein modification, while the thiol side chain can be directed toward disulfide formation or thioether conjugation depending on the protection and oxidation state. Salt-form handling can facilitate aqueous-compatible intermediate preparation prior to thiol activation or protected-amino-acid incorporation. Downstream, cysteine-derived crosslinkers, redox-responsive linkages, and site-specific conjugation reagents can be generated for protein structure probing and engineered biomolecule assembly.

4. Side-Chain Functionalization

H-Cys-OH · HCl · H2O is applicable to side-chain functionalization routes that exploit cysteine's thiol nucleophilicity and redox behavior to build sulfur-containing motifs. The thiol can undergo alkylation to form thioethers, oxidation to form disulfides, or selective derivatization to introduce functional groups used in linker synthesis and heteroatom-rich scaffold construction. The α-amino and α-carboxyl functionalities enable conversion into protected derivatives for orthogonal chemistry, allowing selective transformations at the sulfur center while maintaining the stereogenic configuration. Downstream synthetic utility includes preparation of functionalized cysteine analogs, thioether linkers, and sulfur-containing intermediates used in fine chemical synthesis and peptide-mimetic construction.

5. Pharmaceutical Manufacturing Intermediates

H-Cys-OH · HCl · H2O serves as a manufacturing-relevant chiral intermediate for producing cysteine-based building blocks that feed into API-adjacent synthesis and process chemistry operations. The amino acid salt form provides a defined starting material for conversion into protected cysteine derivatives suitable for controlled peptide coupling and for generating thiol-reactive intermediates under protected conditions. The thiol functionality can be managed through protection-group strategies to minimize oxidation and side reactions during scale synthesis, while deprotection enables downstream conjugation or incorporation into cysteine-containing intermediates. Industrially, cysteine-derived fragments can be used to construct disulfide-linked motifs, thiol-reactive linkers, and sulfur-containing intermediates that align with established amino acid derivative manufacturing practices.

6. Analytical Research Standards

H-Cys-OH · HCl · H2O is suitable for analytical research as a reference material and derivatization substrate in amino acid profiling and thiol quantification workflows. The defined L-cysteine stereochemistry and the thiol group enable consistent derivatization chemistry for chromatographic detection, including conversion to stable derivatives that reflect thiol content and redox state. The hydrochloride monohydrate form supports reproducible preparation of calibration solutions and can be used to standardize sample handling where thiol oxidation sensitivity is a concern. Downstream, cysteine-based standards and thiol-derivatized products can support method development, quality control of peptide intermediates, and verification of sulfur-containing functional group incorporation.

1. Myotropic activity and immunolocalization of selected neuropeptides of the burying beetle Nicrophorus vespilloides (Coleoptera: Silphidae)

2. Crystal Structure of BamB from Pseudomonas aeruginosa and Functional Evaluation of Its Conserved Structural Features

3. Olfactory receptor based piezoelectric biosensors for detection of alcohols related to food safety applications

4. C-Peptide replacement therapy and sensory nerve function in type 1 diabetic neuropathy

5. Zeta-potential-changing nanoparticles conjugated with cell-penetrating peptides for enhanced transfection efficiency