Uracil

Structural formula
General
Surname	Uracil
other names	Pyrimidine-2,4 (1 H , 3 H ) -dione; 2,4-pyrimidinedione;
Molecular formula	C 4 H 4 N 2 O 2
Brief description	white, powdery solid
External identifiers / databases
EC number	200-621-9
ECHA InfoCard	100,000,565
PubChem	1174
DrugBank	DB03419
Wikidata	Q182990
properties
Molar mass	112.09 g mol −1
Physical state	firmly
Melting point	334 ° C
solubility	moderately soluble in cold water, easily soluble in hot water
safety instructions
GHS labeling of hazardous substances	no GHS pictograms
	no GHS pictograms
H and P phrases	H: no H-phrases
	P: no P-phrases
Thermodynamic properties
ΔH f 0	−429.4 kJ / mol
	As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions .

Uracil (U, Ura) is one of the four most important nucleobases in RNA , along with adenine , cytosine and guanine . Thymine takes its place in the DNA . It is a heterocyclic organic compound with a pyrimidine skeleton and two substituents ( oxygen atoms in positions 2 and 4). The nucleosides of uracil are the uridine in RNA and the very rare deoxyuridine in DNA. In the Watson-Crick base pairing , it forms two hydrogen bonds with adenine.

history

Uracil was isolated in Marburg in 1900 as a cleavage product of the nuclein from yeast by the Italian Alberto Ascoli , advised by his teacher Albrecht Kossel . However, the name for uracil goes to the Leipzig chemist Robert Behrend back of the structure of this compound from the 1885 methyl - derivative was derived - in his attempts descendants of uric acid to synthesize.

presentation

Uracil can be obtained by condensation of urea with 3-oxopropanoic acid ("formyl acetic acid", C ₃ H ₄ O ₃ ). In this case, however, the C ₃ component cannot be stored and was therefore replaced by malic acid . This is decarbonylated in concentrated sulfuric acid with elimination of water , so it loses carbon monoxide . The 3-oxopropanoic acid formed in situ condenses with the urea in the sulfuric acid solution with two-fold elimination of water.

properties

Uracil is a white, powdery solid that melts at 341 ° C.

The compound was discovered in the hydrolytic breakdown of nucleic acids. An X-ray crystal structure analysis showed that of the possible tautomeric forms in the solid state (several desmotropic forms exist) the dioxo form is present.

Uracil is a weak acid (pK _a = 9.45). The acidity constant is in the range of phenol (pK _a = 10.0) and other enols. In the uracil anion, the negative charge is delocalized (mesomeric boundary structures). Therefore, uracil dissolves in aqueous alkalis and aqueous ammonia.

Biological importance

Uracil is mainly bound to ribose phosphate in the body , either as one of the nucleotides uridine monophosphate (UMP), uridine diphosphate (UDP) or uridine triphosphate (UTP) or as a component of ribonucleic acid (RNA).

Part of the RNA

Uracil forms the base pairing to adenine with two hydrogen bonds via the 4-oxo group and the N – H group .

Structural formula of an AU base pair

Physiologically, uracil is only incorporated into the single-stranded RNA, but not into the double-stranded deoxyribonucleic acid (DNA). For mating with adenine occurs during the transcription , in the loops ( loops ) of the tRNA and during the translation ( protein synthesis ).

Nucleosides

Uracil forms two different nucleosides with riboses , uridine (U) (linked via the N ¹ atom) and pseudouridine (Ψ) (linked via the C ⁵ atom).

Pseudouridine (right) is formed from uridine (left) by a Ψ-synthase.

Nucleotides

The phosphorylation of uridine at the C ⁵ atom of ribose leads to the important nucleotides uridine monophosphate (UMP), uridine diphosphate (UDP) and uridine triphosphate (UTP).

Structural formula of UTP

Comparison of uracil and thymine

Deamination of cytosine (C) to uracil (U)

In RNA, in addition to the purine bases adenine (A) and guanine (G), the two pyrimidine bases cytosine (C) and uracil (U) occur as nucleobases ; in DNA instead of uracil, thymine (T) occurs. A uracil can be produced from a cytosine relatively easily by deamination and hydrolysis , whereby the base sequence is then changed (mutated) and the information genetically encoded in the nucleotide sequence can be changed.

Thymine (T)

Thymine differs from uracil through an additional methyl group ( 5-methyl-uracil ); therefore it cannot arise quite so easily from cytosine. Uracil occurring in a DNA can be recognized as a mutated base by specific repair enzymes, removed and exchanged for cytosine ( base excision repair ).

Biochemical degradation

Uracil is broken down into carbon dioxide (CO ₂ ), two ammonium ions (NH ₄⁺ ) and 3-oxopropanoic acid . The latter reacts further to form malonyl-CoA , which can be used, for example, in fatty acid synthesis .

Related compounds and derivatives


Dihydrouracil	1-methyluracil	3-methyluracil	5-methyluracil (= thymine)	5-fluorouracil	5-chloruracil	5-bromouracil	5-ioduracil


Glucopyranosyloxymethyluracil

Individual evidence

↑ ^a ^b ^c ^d data sheet Uracil, 99 +% at AlfaAesar, accessed on December 6, 2019 ( PDF )(JavaScript required) .
↑ ^a ^b ^c data sheet uracil (PDF) from Calbiochem, accessed on December 7, 2015.
↑ 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.
↑ Alberto Ascoli: About a new cleavage product of yeast nucleins. In: Hoppe-Seyler (ed.): Journal for physiological chemistry . tape 31 , no. 1-2 , October 2, 1900, pp. 161–164 , doi : 10.1515 / bchm2.1901.31.1-2.161 .
↑ See also Acknowledgments on p. 164 .
↑ Robert Behrend: Attempts to synthesize bodies of the uric acid series. In: Justus Liebig's Annals of Chemistry . tape 229 , no. 1-2 , March 4, 1885, pp. 1-44 , doi : 10.1002 / jlac.18852290102 .
↑ RF Stewart, LH Jensen: "Redetermination of the crystal structure of uracil", Acta Cryst , 1947 , 23 , pp. 1102-1105 ( doi: 10.1107 / S0365110X67004360 ).

Web links

Wikibooks: Pyrimidine metabolism - structure and breakdown of uracil - learning and teaching materials

Commons : Uracil - collection of images, videos and audio files

Commons : Uracilderivate - collection of images, videos and audio files

Entry for uracil in the Human Metabolome Database (HMDB) , accessed October 12, 2013.

[Alfa-1] ta sheet Uracil, 99 +% at AlfaAesar, accessed on December 6, 2019 ( PDF )(JavaScript required) .

[Merck-2] ta sheet uracil (PDF) from Calbiochem, accessed on December 7, 2015.

[CRC90_5_25-3] 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.

[4] Alberto Ascoli: About a new cleavage product of yeast nucleins. In: Hoppe-Seyler (ed.): Journal for physiological chemistry . tape 31 , no. 1-2 , October 2, 1900, pp. 161–164 , doi : 10.1515 / bchm2.1901.31.1-2.161 .

[5] See also Acknowledgments on p. 164 .

[6] Robert Behrend: Attempts to synthesize bodies of the uric acid series. In: Justus Liebig's Annals of Chemistry . tape 229 , no. 1-2 , March 4, 1885, pp. 1-44 , doi : 10.1002 / jlac.18852290102 .

[7] RF Stewart, LH Jensen: "Redetermination of the crystal structure of uracil", Acta Cryst , 1947 , 23 , pp. 1102-1105 ( doi: 10.1107 / S0365110X67004360 ).