Guanine (G) is one of the four main nucleobases found in nucleic acids DNA and RNA, the others are adenine, cytosine, and thymine (uracil in RNA). In DNA, guanine pairs with cytosine.

With the formula C5H5N5O, guanine is a derivative of purine, consisting of a pyrimidine ring system fused with imidazole-conjugated double bonds. Being unsaturated, the bicyclic molecule is flat. The guanine nucleoside is called guanosine.

Guanine, adenine and cytosine, is present in DNA and RNA, thymine, while generally seen only in DNA and uracil only in RNA. Guanine has two tautomeric forms, most keto form and the rare enol form. It binds to cytosine through three hydrogen bonds.

In cytosine, the amino group acts as hydrogen donor and the carbonyl C-2 and the amine N-3 as hydrogen bond acceptors. Guanine has a group in C-6, which acts as a hydrogen acceptor, while the N-1 and the amino group at C-2 act as hydrogen donors.

Guanine can be hydrolyzed by strong acid to glycine, ammonia, carbon dioxide and carbon monoxide. Guanine is deaminated to xanthine first. Guanine oxidizes more readily than adenine, another purine derivatives of DNA bases.

Its high melting point of 350 ° C reflects the intermolecular hydrogen bonds between the amino and oxo groups in the molecules in the crystal. Because of this intermolecular bonding, guanine is relatively insoluble in water but is soluble in dilute acids and alkalis.

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