Purine

A purine is a heterocyclic aromatic organic compound, consisting of a pyrimidine ring fused to an imidazole ring. Purines, including substituted purines and their tautomers, are the most widely distributed kind of nitrogen-containing heterocycle in nature.^[1]

Purines and pyrimidines make up the two groups of nitrogenous bases, including the two groups of nucleotide bases. Two of the four deoxyribonucleotides and two of the four ribonucleotides, the respective building-blocks of DNA and RNA, are purines.

Notable purines

The quantity of naturally occurring purines produced on earth is huge. Two of the four 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, respectively. This is called complementary base pairing. In RNA, the complement of adenine is uracil instead of thymine.

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

Functions

Aside from the crucial role of purines (adenine and guanine) in DNA and RNA, purines are also 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.^[2] The starting material for the reaction sequence was uric acid (8), which had been isolated from kidney stones by Scheele in 1776.^[3] 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). The product was reduced to purine (1) using zinc-dust. Purines are also much larger than pyrimidines.

Metabolism

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

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

Sources

Purines are found in high concentration in meat and meat products, especially internal organs such as liver and kidney. In general, plant-based diets are low in purines.^[4] Examples of high-purine sources include: sweetbreads, anchovies, sardines, liver, beef kidneys, brains, meat extracts (e.g., Oxo, Bovril), herring, mackerel, scallops, game meats, beer (from the yeast) and gravy.

A moderate amount of purine is also contained in beef, pork, poultry, other fish and seafood, asparagus, cauliflower, spinach, mushrooms, green peas, lentils, dried peas, beans, oatmeal, wheat bran, wheat germ, and hawthorn.^[5]

Higher levels of meat and seafood consumption are associated with an increased risk of gout, whereas a higher level of consumption of dairy products is associated with a decreased risk. Moderate intake of purine-rich vegetables or protein is not associated with an increased risk of gout.^[6]

In August 2011, a report, based on NASA studies with meteorites found on Earth, was published suggesting purine and related organic molecules (including the DNA and RNA components, adenine and guanine) may have been formed extraterrestrially in outer space.^[7][8]

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 °C for 28 hours.^[9]

This remarkable reaction and others like it have been discussed in the context of the origin of life.^[10]

Procedure:^[9] Formamide (45 grams) was heated in an open vessel with a condenser for 28 hours in an oil bath at 170-190 °C. After removing excess formamide (32.1 grams) 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 grams (71% yield from formamide consumed). Crystallization from acetone afforded purine as colorless crystals; melting point 218 °C.

Oro, Orgel and co-workers have shown that four molecules of HCN tetramerize to form diaminomaleodinitrile (12), which can be converted into almost all natural-occurring purines.^{[11][12][13][14][15]}

The Traube purine synthesis (1900) is a classic reaction (named after Wilhelm Traube) between an amine-substituted pyrimidine and formic acid.^[16]

See also

• Simple aromatic rings
• Pyrimidine

References

1. ^ Rosemeyer, H. Chemistry & Biodiversity 2004, 1, 361.
2. ^ Fischer, E. Berichte der Deutschen Chemischen Gesellschaft 1899, 32, 2550.
3. ^ Scheele, V. Q. Examen Chemicum Calculi Urinari, Opuscula, 1776, 2, 73.
4. ^ http://www.dietaryfiberfood.com/purine-food.php
5. ^ Gout Diet: Limit High Purine Foods
6. ^ NEJM - Purine-Rich Foods, Dairy and Protein Intake, and the Risk of Gout in Men
7. ^ Callahan; Smith, K.E.; Cleaves, H.J.; Ruzica, J.; Stern, J.C.; Glavin, D.P.; House, C.H.; Dworkin, J.P. (11 August 2011). "Carbonaceous meteorites contain a wide range of extraterrestrial nucleobases". PNAS. doi:10.1073/pnas.1106493108. http://www.pnas.org/content/early/2011/08/10/1106493108. Retrieved 2011-08-15. 
8. ^ Steigerwald, John (8 August 2011). "NASA Researchers: DNA Building Blocks Can Be Made in Space". NASA. http://www.nasa.gov/topics/solarsystem/features/dna-meteorites.html. Retrieved 2011-08-10. 
9. ^ ^{a b} Yamada, H.; Okamoto, T. (1972). "A One-step Synthesis of Purine Ring from Formamide". Chemical & Pharmaceutical Bulletin 20 (3): 623. doi:10.1248/cpb.20.623. http://www.journalarchive.jst.go.jp/english/jnlabstract_en.php?cdjournal=cpb1958&cdvol=20&noissue=3&startpage=623. 
10. ^ Saladino et al.; Crestini, Claudia; Ciciriello, Fabiana; Costanzo, Giovanna; Mauro, Ernesto (2006). "About a Formamide-Based Origin of Informational Polymers: Syntheses of Nucleobases and Favourable Thermodynamic Niches for Early Polymers". Origins of Life and Evolution of Biospheres 36 (5–6): 523–531. doi:10.1007/s11084-006-9053-2. PMID 17136429. 
11. ^ Sanchez, R. A.; Ferris, J. P.; Orgel, L. E. Journal of Molecular Biology, 1967, 30, 223.
12. ^ Ferris, J. P.; Orgel, L. E. Journal of the American Chemical Society, 1966, 88, 1074.
13. ^ Ferris, J. P.; Kuder, J. E.; Catalano, O. W. Science, 1969, 166, 765.
14. ^ Oro, J.; Kamat, J. S. Nature, 1961, 190, 442.
15. ^ Houben-Weyl, Vol . E5, p. 1547
16. ^ Organic Syntheses Based on Name Reactions, Alfred Hassner, C. Stumer ISBN 008043259X 2002

External links

• Purine Content in Food