Nucleic acid bases , pyrimidine ring imidazole rings

Pyrimidine and imidazole rings are particularly important in biological chemistry. Pyrimidine, for instance, is the parent ring system in cytosine, thymine, and uracil, three of the five heterocyclic amine bases found in nucleic acids An aromatic imidazole ring is present in histidine, one of the twenty amino acids found in proteins. 

Purine contains a pyrimidine ring fused with an imidazole nucleus. Guanine and adenine are two purine bases that are found in nucleic acids, DNA and RNA. 

The purines are an important class of heterocycles in which an imidazole ring is fused to a pyrimidine ring. Uric acid (the main product of nitrogen metabolism in birds and reptiles), caffeine (present in coffee), and adenine and guanine (nitrogen bases present in the nucleic acids DNA and RNA) are examples of naturally occurring purines. 

The most important of the diazines is pyrimidine 10.2. Pyrimidine derivatives uracil 10.4, thymidine 10.5, and cytosine 10.6 are the monocyclic bases of nucleic acids. The bicyclic bases are the purines adenine 10.7 and guanine 10.8. The purine ring is essentially a fusion of the pyrimidine and imidazole rings. 

These principles apply in bicyclic and multicychc systems as well, and large numbers of stable, aromatic systems are known. One of the most important bicyclic ring systems is that of purine, which is present among the bases of nucleic acids (Chapter 3). Here, a pyrimidine ring and an imidazole ring are fused. The latter will exhibit its usual tautomeric behavior, but now the two forms are not equivalent, and in purine itself form 7.15 is known to predominate over 7.16. In the nucleoside component of the nucleic acids (and in other compounds), the bond to the sugar moiety is located at the NH of form 7.15 (incorrectly but historically labeled the 9-position). 

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