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Structural formula
Structure of cytosine
Surname Cytosine
other names
  • 4-amino-1 H -pyrimidin-2-one
Molecular formula C 4 H 5 N 3 O
Brief description

colorless platelets

External identifiers / databases
CAS number 71-30-7
EC number 200-749-5
ECHA InfoCard 100,000,681
PubChem 597
ChemSpider 577
Wikidata Q178425
Molar mass 111.10 g mol −1
Physical state


Melting point
  • 333 ° C
  • 320-325 ° C (monohydrate)

moderately in boiling water and ethanol , insoluble in diethyl ether

safety instructions
GHS labeling of hazardous substances
07 - Warning


H and P phrases H: 315-319-335
P: 261-305 + 351 + 338
Thermodynamic properties
ΔH f 0

−221.3 kJ / mol

As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions .

Cytosine (C, Cyt) is one of the four nucleobases in DNA and RNA , along with adenine , guanine and thymine ( uracil in RNA). It is a heterocyclic organic compound with a pyrimidine backbone and two substituents ( amino group in position 4 and oxygen atom in position 2). The nucleoside of cytosine is the deoxycytidine in DNA and the cytidine in RNA. In the Watson-Crick base pairing , it forms three hydrogen bonds with guanine.

History, extraction and representation

The cytosine was first obtained from the thymus gland of calves in 1894 by the later Nobel Prize winner Albrecht Kossel and his assistant Albert Neumann .

In 1903 the chemical structure was clarified by Albrecht Kossel in collaboration with Hermann Steudel and the first successful synthesis was carried out.


Physical Properties

Cytosine forms colorless platelets with a melting point of over 300 ° C. It dissolves moderately in boiling water and ethanol ; it is insoluble in diethyl ether .

Cytosine is tautomeric , with the 1 H form predominating.

Cytosine.svg Cytosine 3H.svg
Cytosine (1 H tautomer) Cytosine (3 H tautomer)

Chemical properties

Due to its chemical instability cytosine can uracil deamination .

Deamination of cytosine to uracil

Biological importance

Cytosine can be part of DNA, RNA or various nucleosides and nucleotides .


Via the N 1 atom of the ring, cytosine can be bound N -glycosidically to the C 1 atom of the ribose ; one then speaks of a nucleoside , the cytidine . The nucleoside deoxycytidine is formed when it binds to deoxyribose . If, however, cytosine is C-glycosidically bound to the C 1 atom of the ribose via the C 5 atom of the ring , the synthetic pseudocytidine is formed . In contrast to most nucleosides, cytarabine contains arabinose instead of ribose .

Cytidine.svg Deoxycytidine, svg Pseudocytidine.svg Cytarabin.svg
Cytidine, C. Deoxycytidine, dC Pseudocytidine, ψC Cytarabine, araC


The phosphorylation of cytidine at the C 5 atom of ribose leads to the important nucleotides cytidine monophosphate (CMP), cytidine diphosphate (CDP) and cytidine triphosphate (CTP), or similarly for deoxycytidine to deoxycytidine monophosphate (dCMP), deoxycytiphosphate ( dCMP), deoxycytosphidiphosphate (dCTP).

Structural formula of CTP

As cytidine triphosphate (CTP) it serves as a cofactor for various enzymes and can give its phosphate group to ADP to build up ATP .

Part of DNA and RNA

In the DNA double helix , cytosine forms three hydrogen bonds with the corresponding guanine base of the complementary strand via the oxo group, the N 3 atom and the amino group .

Structural formula of a GC base pair

It is converted into the methylated form 5-methylcytosine by cytosine-specific methyltransferases .

Related links

Structural formula of 1-methylcytosine Structural formula of 3-methylcytosine Structural formula of 5-methylcytosine Structural formula of 5-hydroxymethylcytosine Structural formula of N4-methylcytosine Structural formula of N4, N4-dimethylcytosine Structural formula of isocytosine, 3H-tautomer Structural formula of fluorocytosine Structural formula of 5-chlorocytosine Structural formula of bromocytosine Structural formula of 5-iodocytosine Structural formula of 5-hydroxycytosine
1-methylcytosine 3-methylcytosine 5-methylcytosine 5-hydroxymethylcytosine N 4 -methylcytosine N 4 , N 4 -dimethylcytosine Isocytosine 5-fluorocytosine 5-chlorocytosine 5-bromocytosine 5-iodocytosine 5-hydroxycytosine

Individual evidence

  1. Entry on CYTOSINE in the CosIng database of the EU Commission, accessed on March 31, 2020.
  2. a b c Entry on cytosine. In: Römpp Online . Georg Thieme Verlag, accessed on November 11, 2014.
  3. A. Abdelaziz, DH Zaitsau, TA Mukhametzyanov, BN Solomonov, P. Cebe, SP Verevkin, C. Schick: Melting temperature and heat of fusion of cytosine revealed from fast scanning calorimetry ; in: Thermochim. Acta , 2017, 657, pp. 47-55 ( doi : 10.1016 / j.tca.2017.09.013 ).
  4. a b Cytosine data sheet from Sigma-Aldrich , accessed on November 23, 2013 ( PDF ).
  5. David R. Lide (Ed.): CRC Handbook of Chemistry and Physics . 90th edition. (Internet version: 2010), CRC Press / Taylor and Francis, Boca Raton, FL, Standard Thermodynamic Properties of Chemical Substances, pp. 5-25.
  6. Representation and cleavage products of nucleic acid (adenylic acid). Lecture in: Reports of the German Chemical Society , Volume 27, 1894, page 2215.
  7. A. Kossel , H. Steudel: Further investigations on cytosine ; in: Hoppe-Seyler's magazine for physiological chemistry , 1903 , 38  (1-2), pp. 49-59 ( doi : 10.1515 / bchm2.1903.38.1-2.49 ).
  8. Further research on cytosine. In: Hoppe-Seyler's magazine for physiological chemistry , Volume 38, 1903, page 49.
  9. A. Kossel , H. Steudel: About a basic component of animal cells ; in: Hoppe-Seyler's magazine for physiological chemistry , 1903 , 37  (2), pp. 177-180 ( doi : 10.1515 / bchm2.1903.37.2.177 ).
  10. HL Wheeler, TB Johnson: Synthesis of aminooxy-pyrimidines having the composition of cytosine, 2-amino-6-oxypyrimidine and 2-oxy-6-amino-pyrimidine ; in: Am. Chem. J. , 1903 , 29 , pp. 492-504.
  11. M. Dreyfus, O. Bensaude, G. Dodin, JE Dubois: Tautomerism in Cytosine and 3-Methylcytosine. A Thermodynamic and Kinetic Study ; in: J. Am. Chem. Soc. , 1976 , 98  (20), pp. 6338-6349 ( doi : 10.1021 / ja00436a045 ; PMID 965648 ).
  12. ^ GA Wagner: Introduction to Archaeometry , 1st edition, Springer Verlag, Berlin 2007, ISBN 3-540-71936-9 , p. 282.
  13. ^ JR Siewert, M. Rothmund, V. Schumpelick: Praxis der Viszeralchirurgie: Onkologische Chirurgie , 3rd edition, Springer Verlag, Berlin 2010, ISBN 3-642-03807-7 , p. 71.

Web links

Commons : Cytosine  - Collection of images, videos and audio files
Commons : Cytosine Derivatives  - Collection of images, videos and audio files
Wiktionary: Cytosine  - explanations of meanings, word origins, synonyms, translations
  • Entry on cytosines in the Human Metabolome Database (HMDB) , accessed November 18, 2013.