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Guanine () (symbol G or Gua) is one of the four main found in the DNA and RNA, the others being adenine, cytosine, and thymine (uracil in RNA). In DNA, guanine is paired with cytosine. The guanine nucleoside is called guanosine.
With the formula C5H5N5O, guanine is a derivative of purine, consisting of a fused pyrimidine-imidazole ring system with conjugated double bonds. This unsaturated arrangement means the bicyclic molecule is planar.
Guanine has multiple forms. For both the imidazole and pyrimidine rings, the proton can reside on either of the ring nitrogens. But a single tautomeric form dominates in nucleoside or nucleotide versions of guanine.
Guanine binds to cytosine through three . In cytosine, the amino group acts as the hydrogen bond donor and the C-2 carbonyl and the N-3 amine as the hydrogen-bond acceptors. Guanine has the C-6 carbonyl group that acts as the hydrogen bond acceptor, while a group at N-1 and the amino group at C-2 act as the hydrogen bond donors.
Guanine can be hydrolyzed with strong acid to glycine, ammonia, carbon dioxide, and carbon monoxide. First, guanine gets Deamination to become xanthine. Guanine oxidizes more readily than adenine, the other purine-derivative base in DNA. Its high melting point of 350 °C reflects the intermolecular hydrogen bonding between the oxo and amino groups in the molecules in the crystal. Because of this intermolecular bonding, guanine is relatively insoluble in water, but it is soluble in dilute acids and bases.
Trace amounts of guanine form by the polymerization of ammonium cyanide (). Two experiments conducted by Levy et al. showed that heating 10 mol·L−1 at 80 °C for 24 hours gave a yield of 0.0007%, while using 0.1 mol·L−1 frozen at −20 °C for 25 years gave a 0.0035% yield. These results indicate guanine could arise in frozen regions of the primitive earth. In 1984, Yuasa reported a 0.00017% yield of guanine after the electrical discharge of , , , and 50 mL of water, followed by a subsequent acid hydrolysis. However, it is unknown whether the presence of guanine was not simply a resultant contaminant of the reaction. - quotes the Yuasa paper and cites the possibility of there being a contaminant in the reaction.
Another possible abiotic route was explored by quenching a 90% N2–10%CO–H2O gas mixture high-temperature plasma.
Traube's synthesis involves heating 4-hydroxy-2,4,5-triaminopyrimidine with formic acid for several hours.
In 1656 in Paris, a Mr. Jaquin extracted from the scales of the fish Common bleak so-called "pearl essence",Johann Rudolf von Wagner, Ferdinand Fischer, and L. Gautier, Traité de chimie industrielle (Treatise on industrial chemistry), 4th ed., (Paris, France: Masson & Co., 1903), vol. 2, pp. 64–65. which is crystalline guanine.In 1861 the French chemist Charles-Louis Barreswil (1817–1870) found that "pearl essence" was guanine. See: Barreswil (1861) "Sur le blanc d'ablette qui sert à la fabrication des perles fausses" (On the white of ablette that's used in making imitation pearls), Comptes rendus, 53 : 246. In the cosmetics industry, crystalline guanine is used as an additive to various products (e.g., shampoos), where it provides a pearly iridescence effect. It is also used in metallic paints and simulated pearls and plastics. It provides shimmering luster to eye shadow and nail polish. Facial treatments using the droppings, or guano, from Japanese nightingales have been used in Japan and elsewhere, because the guanine in the droppings makes the skin look paler. Guanine crystals are rhombic platelets composed of multiple transparent layers, but they have a high Refractive index that partially reflects and transmits light from layer to layer, thus producing a pearly luster. It can be applied by spray, painting, or dipping. It may irritate the eyes. Its alternatives are mica, faux pearl (from ground shells), and aluminium and bronze particles.
Guanine has a very wide variety of biological uses that include a range of functions ranging in both complexity and versatility. These include camouflage, display, and vision among other purposes.
Spiders, scorpions, and some amphibians convert ammonia, as a product of protein metabolism in the cells, to guanine, as it can be excreted with minimal water loss.
Guanine is also found in specialized skin cells of fish called (e.g., the sturgeon), as well as being present in the reflective deposits of the eyes of deep-sea fish and some , such as and .
On 8 August 2011, a report, based on NASA studies with meteorites found on Earth, was published suggesting building blocks of DNA and RNA (guanine, adenine and related organic molecules) may have been formed extra-terrestrially in outer space.
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