L-Cysteine hydrochloride monohydrate
[CAS# 7048-04-6]

Identification

• Classification: Biochemical >> Amino acids and their derivatives >> Cysteine derivative
• Name: L-Cysteine hydrochloride monohydrate
• Synonyms: alpha-Amino-beta-mercapto-propionic acid hydrochloride monohydrate
• Molecular Formula: C₃H₈ClNO₂S.H₂O
• Molecular Weight: 175.63
• Protein Sequence: C
• CAS Registry Number: 7048-04-6
• EC Number: 615-117-8
• SMILES: C([C@@H](C(=O)O)N)S.O.Cl

Properties

• Solubility: 1 M (H2O 20 °C) (Expl.)
• Melting point: 175 °C (decomp,) (Expl.)

Safety Data

• Hazard Symbols: GHS07 Warning
• Risk Statements: H315-H319-H335-H341-H372
• Safety Statements: P203-P260-P261-P264-P264+P265-P270-P271-P280-P302+P352-P304+P340-P305+P351+P338-P318-P319-P321-P332+P317-P337+P317-P362+P364-P403+P233-P405-P501

Hazard Classification

Hazard	Class	Category Code	Hazard Statement
Eye irritation	Eye Irrit.	2	H319
Skin irritation	Skin Irrit.	2	H315
Specific target organ toxicity - single exposure	STOT SE	3	H335

Discovery and Applications

L-Cysteine hydrochloride monohydrate is the monohydrate form of the hydrochloride salt of L-cysteine, an optically active, naturally occurring amino acid with the molecular formula C₃H₈ClNO₂S·H₂O. It is a white crystalline powder that is freely soluble in water and slightly soluble in ethanol. The compound is widely used in biochemical, pharmaceutical, and industrial applications due to its chemical reactivity and biological significance.

L-Cysteine was first isolated from animal horn in 1884 by Eugen Baumann, and its hydrochloride salt forms, including the monohydrate, were developed to improve its stability and solubility. The hydrochloride monohydrate variant is commonly used in both laboratory and industrial processes where a more water-soluble and easily handled form of L-cysteine is required. The salt form also reduces the tendency of the thiol (-SH) group to oxidize, thus enhancing shelf life.

In the pharmaceutical industry, L-cysteine hydrochloride monohydrate is used as a precursor or active agent in formulations aimed at treating acetaminophen (paracetamol) overdose, where its thiol group serves to replenish intracellular glutathione. While N-acetylcysteine is more commonly used for this purpose clinically, L-cysteine hydrochloride is employed in related therapeutic and research settings. It is also used as a component in parenteral nutrition solutions to provide sulfur-containing amino acids.

In the food industry, the compound is used as a dough conditioner and flavor enhancer. L-Cysteine hydrochloride monohydrate is approved as a food additive and is particularly useful in baking, where it reduces dough mixing time and improves the extensibility of gluten. It is also used to synthesize savory flavor compounds via Maillard-type reactions with reducing sugars, particularly in the production of meat flavors.

The compound is further applied in cell culture and microbiological media as a supplement to support the growth of cells and microorganisms that require reduced sulfur. In biochemical research, L-cysteine hydrochloride monohydrate is used in protein structure studies, disulfide bond reduction, and as a substrate in enzymatic reactions involving cysteine-dependent enzymes.

L-Cysteine hydrochloride monohydrate also plays a role in the synthesis of chemicals and materials. It can serve as a chiral auxiliary or starting material in the preparation of complex organic molecules. In some cases, it is used in the preparation of gold nanoparticles and metal complexes due to its metal-binding thiol group.

The compound is typically manufactured through hydrolysis of proteins such as human hair, feathers, or microbial fermentation, followed by acid treatment and crystallization to yield the hydrochloride monohydrate salt. Regulatory approval for its use varies by jurisdiction, but it is generally recognized as safe (GRAS) when used according to specified limits.

References

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1986. Effects of sulfhydryl agents on neuromuscular transmission in the presence or absence of cadmium. Naunyn-Schmiedeberg's Archives of Pharmacology, 333(4).
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2023. Theaflavin indicates protection on vascular endothelium via hydrogen sulfide production. Molecular & Cellular Toxicology, 19(4).
DOI: 10.1007/s13273-023-00365-y