Title: Cysteine
CAS Registry Number: 52-90-4
CAS Name: L-Cysteine
Additional Names: Cys; C; b-mercaptoalanine; (R)-2-amino-3-mercaptopropanoic acid; 2-amino-3-mercaptopropionic acid; a-amino-b-thiolpropionic acid; half-cystine; thioserine
Molecular Formula: C3H7NO2S
Molecular Weight: 121.16
Percent Composition: C 29.74%, H 5.82%, N 11.56%, O 26.41%, S 26.47%
Literature References: A non-essential amino acid in human development. Readily oxides to form a dimeric amino acid, cystine, q.v., in which the two Cys are linked via a disulfide bridge, a common structural feature in proteins. Early chemistry and biochemistry: Amino Acids and Proteins, D. M. Greenberg, Ed. (Charles C. Thomas, Springfield, IL, 1951) 950 pp., passim; J. P. Greenstein, M. Winitz, Chemistry of the Amino Acids vol 1-3 (John Wiley and Sons, Inc., New York, 1961) pp. 1879-1928, passim. Simple synthesis of racemic cysteine: V. J. Martens et al., Angew. Chem. Int. Ed. 20, 668 (1981). Determn in proteins: J. G. Hoogerheide, C. M. Campbell, Anal. Biochem. 201, 146 (1992); D. Atherton et al., ibid. 212, 98 (1993). Review of biosynthesis: N. M. Kredich et al., Ciba Found. Symp. (Netherlands) 72, 87-99 (1980). Review of transport in mammalian cells: S. Bannai, Biochim. Biophys. Acta 779, 289-306 (1984). Review of effects on acrylonitrile toxicity: D. E. Nerland et al., Drug Metab. Rev. 20, 233-246 (1989). Review of thermodynamics and kinetics: T. R. Ralph et al., J. Electroanal. Chem. 375, 1-15 (1994); of electrosynthesis: eidem, ibid. 17-27. Review of role in chemo- and radioprotectant strategies: J. C. Roberts, Amino Acids 8, 113-124 (1995).
Properties: Crystals. [a]D25 +6.5° (5N HCl); [a]D25 +13.0° (glacial acetic acid). pK1 1.71; pK2 8.33; pK3 10.78. Absorption spectrum: Abderhalden, Rossner, Z. Physiol. Chem. 178, 160 (1928). Freely sol in water, alcohol, acetic acid, ammonia water. Insol in ether, acetone, ethyl acetate, benzene, carbon disulfide, carbon tetrachloride. In neutral or slightly alkaline aq solns it is oxidized to cystine by air. More stable in acidic solns.
pKa: pK1 1.71; pK2 8.33; pK3 10.78
Optical Rotation: [a]D25 +6.5° (5N HCl); [a]D25 +13.0° (glacial acetic acid)
Derivative Type: Hydrochloride
CAS Registry Number: 52-89-1
Molecular Formula: C3H7NO2S.HCl
Molecular Weight: 157.62
Percent Composition: C 22.86%, H 5.12%, N 8.89%, O 20.30%, S 20.34%, Cl 22.49%
Properties: Crystals, dec 175-178°. [a]D25 +5.0° (5N HCl); [a]D25 +10.0° (glacial acetic acid). Sol in water, alcohol, acetone; the aq soln is acid. Keep tightly closed. Decomposes and oxidizes slowly; hygroscopic.
Optical Rotation: [a]D25 +5.0° (5N HCl); [a]D25 +10.0° (glacial acetic acid)
Use: As dough conditioner.
Therap-Cat-Vet: Has been used as a detoxicant.
Cystine Cytarabine Cythioate Cytidine Cytisine

CAS number 52-90-4 YesY
52-89-1 (hydrochloride)
PubChem 5862
ChemSpider 574 (Racemic) YesY, 5653 (L-form) YesY
UNII K848JZ4886 YesY
EC-number 200-158-2
KEGG D00026 YesY
ChEBI CHEBI:15356 YesY
Jmol-3D images Image 1
Image 2
Molecular formula C3H7NO2S
Molar mass 121.16 g mol−1
Appearance white crystals or powder
Melting point 240 °C decomp.
Solubility in water soluble
Solubility 1.5g/100g ethanol 19 degC [1]
Chiral rotation [α]D +9.4° (H2O, c = 1.3)
Supplementary data page
Structure and
n, εr, etc.
Phase behaviour
Solid, liquid, gas
Spectral data UV, IR, NMR, MS
 YesY (verify) (what is: YesY/N?)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
Infobox references

Cysteine (abbreviated as Cys or C)[3] is an α-amino acid with the chemical formula HO2CCH(NH2)CH2SH. It is a semi-essential amino acid, which means that it can be biosynthesized in humans.[4] The thiol side chain in cysteine often participates in enzymatic reactions, serving as a nucleophile. The thiol is susceptible to oxidization to give the disulfide derivative cystine, which serves an important structural role in many proteins. When used as a food additive, it has the E number E920.