Isocytosine

Isocytosine or 2-aminouracil is a pyrimidine base that is an isomer of cytosine. The nucleoside form is called isocytidine (iC).^[1]

Isocytosine

Names
Preferred IUPAC name 2-Amino-3H-pyrimidin-4-one
Other names 2-Aminouracil
Identifiers
CAS Number	108-53-2 Y
3D model (JSmol)	Interactive image
ChEBI	CHEBI:55502 Y
ChEMBL	ChEMBL56260 Y
ChemSpider	60309 Y
ECHA InfoCard	100.003.266
PubChem CID	66950
CompTox Dashboard (EPA)	DTXSID00148350
InChI InChI=1S/C4H5N3O/c5-4-6-2-1-3(8)7-4/h1-2H,(H3,5,6,7,8) Y Key: XQCZBXHVTFVIFE-UHFFFAOYSA-N Y InChI=1/C4H5N3O/c5-4-6-2-1-3(8)7-4/h1-2H,(H3,5,6,7,8) Key: XQCZBXHVTFVIFE-UHFFFAOYAD
SMILES O=C1/C=C\N=C(\N)N1
Properties
Chemical formula	C4H5N3O
Molar mass	111.104 g·mol−1
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Y verify (what is YN ?) Infobox references

It is used in combination with isoguanine in studies of unnatural nucleic acid analogues of the normal base pairs in DNA.^[2] In particular, it is used as a nucleobase of hachimoji RNA with the abbreviation rS.^[3]

Isoguanine-Isocytosine-base-pair

Synthesis

It can be synthesized from guanidine and malic acid.^[4]

Synthesis of isocytosine from malic acid

Isocytosine can also be obtained by condensing guanidine hydrochloride with 3-oxopropanoic acid. However, the C₃ component is not storable in this case and was, therefore, replaced with malic acid. This is decarbonylated in concentrated sulfuric acid with elimination of water, thus losing carbon monoxide. The 3-oxopropanoic acid formed in situ condenses with the guanidine in the sulfuric acid solution with double elimination of water.^[5]

Uses

It is also used in physical chemical studies involving metal complex binding, hydrogen bonding, and tautomerism and proton transfer effects in nucleobases.^[6]

Tautomerism of isocytosine