purine, type of organic base found in the nucleotides and nucleic acids of plant and animal tissue. The German chemist Emil Fischer did much of the basic work on purines and introduced the term into the chemical literature in the early 20th cent. The two major purines of almost universal distribution in living systems are adenine and guanine.

Any of a class of heterocyclic compounds with a two-ring structure composed of carbon and nitrogen atoms. The simplest member, purine itself (C₅H₄N₄), is not common, but its derivatives with the structure are. Examples are uric acid, caffeine, and two of the nucleotides in nucleic acids, guanine and adenine.

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Purine (1) is a heterocyclic aromatic organic compound, consisting of a pyrimidine ring fused to an imidazole ring. Purines and pyrimidines make up the two groups of nitrogenous bases. These bases make up a crucial part of both deoxyribonucleotides and ribonucleotides, and the basis for the universal genetic code.

The general term purines also refers to substituted purines and their tautomers.

The purine is the most widely distributed nitrogen-containing heterocycle in nature.

Notable purines

The quantity of naturally occurring purines produced on earth is enormous, as 50 % of the bases in nucleic acids, adenine (2) and guanine (3), are purines. In DNA, these bases form hydrogen bonds with their complementary pyrimidines thymine and cytosine. This is called complementary base pairing. In RNA, the complement of adenine is uracil (U) instead of thymine.

Other notable purines are hypoxanthine (4), xanthine (5), theobromine (6), caffeine (7), uric acid (8) and isoguanine (9).

Functions

Aside from DNA and RNA, purines are biochemically significant components in a number of other important biomolecules, such as ATP, GTP, cyclic AMP, NADH, and coenzyme A. Purine (1) itself, has not been found in nature, but it can be produced by organic synthesis.

They may also function directly as neurotransmitters, acting upon purinergic receptors. Adenosine activates adenosine receptors.

History

The name 'purine' (purum uricum) was coined by the German chemist Emil Fischer in 1884. He synthesized it for the first time in 1899. The starting material for the reaction sequence was uric acid (8), which had been isolated from kidney stones by Scheele in 1776. Uric acid (8) was reacted with PCl₅ to give 2,6,8-trichloropurine (10), which was converted with HI and PH₄I to give 2,6-diiodopurine (11). This latter product was reduced to purine (1) using zinc-dust.

Metabolism

Many organisms have metabolic pathways to synthesize and break down purines.

Purines are biologically synthesized as nucleosides (bases attached to ribose).

Food sources

Purines are found in high concentration in meat and meat products, especially internal organs such as liver and kidney. Plant based diet is generally low in purines

Examples of high purine sources include: sweetbreads, anchovies, sardines, liver, beef, kidneys, brains, meat extracts, herring, mackerel, scallops, game meats, and gravy.

A moderate amount of purine is also contained in beef, pork, poultry, fish and seafood, asparagus, cauliflower, spinach, mushrooms, green peas, lentils, dried peas, beans, oatmeal, wheat bran and wheat germ.

Moderate intake of purine-containing food is not associated with an increased risk of gout.

Laboratory synthesis

In addition to in vivo synthesis of purines in purine metabolism, purine can also be created artificially.

Purine (1) is obtained in good yield when formamide is heated in an open vessel at 170 ^oC for 28 hours.

Procedure: Formamide (45 gram) was heated in an open vessel with a condenser for 28 hours in an oil bath at 170-190 ^oC. After removing excess formamide (32.1 gram) by vacuum distillation, the residue was refluxed with methanol. The methanol solvent was filtered, the solvent removed from the filtrate by vacuum distillation, and almost pure purine obtained; yield 4.93 gram (71 % yield from formamide consumed). Crystallization from acetone afforded purine as colorless crystals; melting point 218 ^oC.

Oro, Orgel and co-workers have shown that four molecules of HCN tetramerize to form diaminomaleodinitrile (12), which can be converted into almost all important natural occurring purines.

The Traube purine synthesis (1900) is a classic reaction (named after Wilhelm Traube) between an amine substutited pyrimidine and formic acid

References

See also

• Simple aromatic rings
• Pyrimidine

External links

• Computational Chemistry Wiki
• Purine Content in Food