Everything about Guanine totally explained
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Guanine is one of the five main
nucleobases found in the
nucleic acids
DNA and
RNA, the others being
adenine,
cytosine,
thymine, and
uracil. With the formula C
5H
5N
5O, guanine is a derivative of
purine, consisting of a fused pyrimidine-imidazole ring system with conjugated double bonds. Being unsaturated, the bicyclic molecule is planar. The guanine
nucleoside is called
guanosine.
Basic principles
Guanine, along with adenine and cytosine, is present in both DNA and RNA, whereas thymine is usually seen only in DNA, and uracil only in RNA. Guanine has two tautomeric forms, the
keto form and enol form.
It binds to cytosine through three
hydrogen bonds. In cytosine, the amino group acts as the hydrogen donor and the C-2 carbonyl and the N-3 amine as the hydrogen-bond acceptors. Guanine has a group at C-6 that acts as the hydrogen acceptor, while the group at N-1 and the amino group at C-2 acts as the hydrogen donors.
The first isolation of guanine was reported in 1844 from the excreta of sea birds, known as guano, which was used as a source of fertilizer. About fifty years later, Fischer determined the structure and also showed that uric acid can be converted to guanine.
Guanine can be hydrolyzed with strong acid to
glycine,
ammonia,
carbon dioxide, and
carbon monoxide. Guanine is first deaminated to
XanthineGuanine 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's soluble in dilute acids and bases.
Syntheses
Trace amounts of guanine form by the polymerization of ammonium cyanide (NH
4CN). Two experiments conducted by Levy et al. showed that heating 10 mol·L
−1 NH
4CN at 80 °C for 24 hours gave a yield of 0.0007%, while using 0.1 mol·L
−1 NH
4CN 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 NH
3, CH
4, C
2H
6, and 50 mL of water, followed by a subsequent acid hydrolysis. However, it's unknown whether the presence of guanine wasn't simply a resultant contaminant of the reaction.
» 5NH
3 + CH
4 + 2C
2H
6 + H
2O → C
5H
8N
5O (guanine) + (25/2)H
2
A Fischer-Tropsch synthesis can also be used to form guanine, along with adenine, uracil, and thymine. Heating an equimolar gas mixture of CO, H
2, and NH
3 to 700 °C for 15 to 24 minutes, followed by quick cooling and then sustainted reheating to 100 to 200 °C for 16 to 44 hours with an alumina catalyst, yielded guanine and uracil:
» 5CO + (1/2)H
2 + 5NH
3 → C
5H
8N
5O (guanine) + 4H
2O
Traube's synthesis involves heating 2,4,5-triamino-1,6-dihydro-6-oxypyrimidine (as the sulfate) with formic acid for several hours.
Other uses
In 1656 in Paris, François Jaquin (a rosary maker) extracted from scales of some fishes the so-called pearl essence, crystalline guanine forming
G-quadruplexes: In cosmetic industry, crystalline guanine is used as an additive to various products (for example,
shampoos), where it provides the pearly
iridescent effect. It is also used in metallic paints and simulated pearls and plastics. It provides shimmering luster to eye shadow and
nail polish. Guanine crystals are rhombic platelets composed of multiple, transparent layers, but they've a high index of refraction 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 eyes. Its alternatives are
mica, synthetic
pearl, and
aluminium and
bronze particles.
Further Information
Get more info on 'Guanine'.
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