L-Cysteine

A sulfur-containing amino acid with a thiol side chain.

Formula
C3H7NO2S
Functional group
Amino acids
Molar mass
121.16 g/mol
Also known as
(2R)-2-amino-3-sulfanylpropanoic acid, cysteine, Cystein, (R)-2-Amino-3-mercaptopropanoic acid, Thioserine, Half-cystine, L-(+)-Cysteine

Overview

A sulfur-containing amino acid with a thiol side chain.

Its CH2SH side chain adds sulfur chemistry to the amino-acid acid–base pattern. The thiol is chemically distinct from the amino and carboxyl groups. L-cysteine also illustrates why the L/D convention must not be substituted mechanically for R/S nomenclature.

Identifiers

Formula
C3H7NO2S
Functional group
Amino acids
Molar mass
121.16 g/mol
Also known as
(2R)-2-amino-3-sulfanylpropanoic acid, cysteine, Cystein, (R)-2-Amino-3-mercaptopropanoic acid, Thioserine, Half-cystine, L-(+)-Cysteine
Chemical identifiers
SMILES
C([C@@H](C(=O)O)N)S
InChI
InChI=1S/C3H7NO2S/c4-2(1-7)3(5)6/h2,7H,1,4H2,(H,5,6)/t2-/m0/s1
PubChem CID
5862

Properties

Melting / decomposition
260 °C decomposes
Water solubility
Water solubility (per g/100 g H2O): 28 g/100 mL of solution at 25 °C; 16 g/100 mL of solution at 20 °C
Data availability

Not published: Boiling Point, Density.

Related compounds

  • Amino acids

    A sulfur-containing amino acid with a thiol side chain.

References

  1. PubChem Compound Summary: L-Cysteine (CID 5862)National Center for Biotechnology Information · PubChem

    Identity, molecular formula, molecular weight and aliases retrieved via PUG REST on 2026-09-08.

  2. OpenStax Organic Chemistry: Structures of amino acidsJohn McMurry · OpenStax · 2023

    Supports the qualitative structural interpretation of the recorded molecular structure; no new physical measurement or verified preparative yield is claimed.

  3. Hazardous Substances Data Bank (HSDB): L-Cysteine — melting pointHazardous Substances Data Bank (HSDB)

    Retrieved through PubChem PUG View on 2026-09-08. https://pubchem.ncbi.nlm.nih.gov/compound/5862#section=Melting-Point. Source value, complete property clause or compact miscibility statement retained; absent pressure, temperature or phase conditions are not inferred. Original reference: Haynes, W.M. (ed.). CRC Handbook of Chemistry and Physics. 95th Edition. CRC Press LLC, Boca Raton: FL 2014-2015, p. 3-140

  4. Hazardous Substances Data Bank (HSDB): L-Cysteine — solubilityHazardous Substances Data Bank (HSDB)

    Retrieved through PubChem PUG View on 2026-09-08. https://pubchem.ncbi.nlm.nih.gov/compound/5862#section=Solubility. Source value, complete property clause or compact miscibility statement retained; absent pressure, temperature or phase conditions are not inferred. Original reference: Drauz K et al; Amino Acids. Ullmann's Encyclopedia of Industrial Chemistry. 7th ed. (1999-2016). New York, NY: John Wiley & Sons. Online Posting Date: 15 Apr 2007