Guanine is one of the four main nucleobases found in the nucleic acids DNA and RNA, the
others being adenine, cytosine, and thymine. In DNA, guanine is paired with cytosine. With
the formula C5H5N5O, 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 major keto form and rare enol form. It
binds to cytosine through three hydrogen bonds. 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.
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; guanine was named in 1846. Between 1882 and
1906, 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 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.
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 is unknown whether
the presence of guanine was not simply a resultant contaminant of the reaction.
10NH3 + 2CH4 + 4C2H6 + 2H2O → 2C5H8N5O + 25H2
A Fischer-Tropsch synthesis can also be used to form guanine, along with adenine, uracil,
and thymine. Heating an equimolar gas mixture of CO, H2, and NH3 to 700 °C for 15 to 24
minutes, followed by quick cooling and then sustained reheating to 100 to 200 °C for
16 to 44 hours with an alumina catalyst, yielded guanine and uracil:
10CO + H2 + 10NH3 → 2C5H8N5O + 8H2O Another possible abiotic route was explored
by quenching a 90% N2–10%CO–H2O gas mixture high-temperature plasma.
Traube's synthesis involves heating 2,4,5-triamino-1,6-dihydro-6-oxypyrimidine with formic acid for several hours.
Other uses The word guanine derives from the Spanish
loanword guano, which itself is from the Quechua word huanu or wanu, meaning 'dung.' As the
Oxford English Dictionary notes, guanine is "A white amorphous substance obtained abundantly
from guano, forming a constituent of the excrement of birds".
In 1656 in Paris, François Jaquin extracted from scales of some fishes the so-called pearl
essence, crystalline guanine forming G-quadruplexes. In the cosmetics industry, crystalline guanine
is used as an additive to various products, where it provides a 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. Facial treatments using the droppings, or guano,
from Japanese nightingales have been used in Japan and elsewhere, reportedly because
the guanine in the droppings produces a clear, "bright" skin tone that users desire. Guanine
crystals are rhombic platelets composed of multiple transparent layers, but they have
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 the eyes. Its alternatives are mica, faux pearl, and aluminium and bronze
particles. Spiders and scorpions convert ammonia, as
a product of protein metabolism in the cells, to guanine, as it can be excreted with minimal
water loss. Guanine is found in integumentary system of
many fish such as sturgeon. It is also present in the reflective deposits of the eyes of
deep-sea fish and some reptiles such as crocodiles. On August 8, 2011, a report, based on NASA
studies with meteorites found on Earth, was published suggesting building blocks of DNA
and RNA may have been formed extraterrestrially in outer space.
See also Cytosine
Guanine deaminase References
External links Guanine MS Spectrum
Guanine at chemicalland21.com