Guanine: Overview, Properties and Synthesis

Introduction

Guanine is an organic compound that is one of the nucleobases found in nucleic acids. It is denoted by a G or Gua. The nucleoside of guanine is known as guanosine. It is a purine derivative that is composed of a two-ringed structure of carbon and nitrogen atoms. It exists in diverse sources such as guano (collected excrement and dead bodies of bats, birds and seals) , yeast, fish scales and sugar beets.

Properties of Guanine

• The chemical formula of guanine is C₅H₅N₅O. Its molecular weight is 151.13 g/mol.
• It is 2-aminopurine having a 6-oxo substituent.
• Guanine is found in both DNA and RNA, unlike uracil that is found only in RNAs.
• It has two tautomeric forms: a major keto form and a rare enol form.
• It binds with cytosine by forming three hydrogen bonds. The C-6 carbonyl group is a hydrogen bond acceptor, while N-1 and amino group at C-2 act as hydrogen bond donor.
• Guanine gets hydrolysed with strong acids into ammonia, glycine, carbon monoxide and carbon dioxide.
• It oxidises more readily than adenine.
• It has a high melting point of 350℃ because it has a strong intermolecular hydrogen bonding between the amino and oxo groups in the guanine molecule.

History

Guanine was first isolated by a German chemist named Julius Bodo Unger in 1844 in mineral form from the excreta of sea birds. The term guanine was coined in 1846. Emil Fischer illustrated the structure of guanine and also showed that uric acid could be converted to guanine.

Synthesis of Guanine

Levy et al. conducted two experiments using ammonium cyanide for the synthesis of guanine. He showed that heating NH₄CN at 80℃ for 24 hours gave a yield of 0.0007% of guanine. In the second experiment, he could yield 0.0035% of guanine by keeping NH₄CN frozen at -20℃ for 25 years. This experiment indicates the possibility of synthesis of guanine in the primitive frozen regions of earth.

The Fischer-Tropsch method is another process which heats a mixture of H₂, CO, and NH₃ for 15-24 minutes at 700℃ followed by quick cooling and again reheating at 100-200℃ for 16-44 hours along with an alumina catalyst. This yielded a mixture of uracil and guanine.

Traube’s synthesis can be used to synthesise guanine by heating 2,4,5-triamino-1,6-dihydro-6-oxy pyrimidine with formic acid for long hours.

Biosynthesis of guanine is not de novo but is achieved by breakdown of guanosine by the enzyme guanosine phosphorylase.

Uses of Guanine

• Crystallised form of guanine gives a shimmering lustre and is hence added in eye shadows, nail paints, metallic paints, plastics and pearls.
• Amphibians like spiders and scorpions convert ammonia (product of protein metabolism in cells) to guanine for easy excretion with minimal water loss.
• Guanine is also found in specialised skin cells of fish as well as the eye deposits of deep sea fish and other reptiles that helps in vision and camouflaging purposes.

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