Convention and Significance of Pyrimidine Ring Atom Numbering in Nucleosides

Numbering of C atoms of pyrimidine rings in nucleosides

The numbering of carbon atoms in pyrimidine rings in nucleosides follows a specific convention. Pyrimidines are a class of organic compounds that have a heterocyclic ring structure consisting of two nitrogen atoms and four carbon atoms. In nucleosides, these pyrimidine rings are linked to a sugar molecule.

The carbon atoms in the pyrimidine ring are conventionally labeled from 1 to 6, with the nitrogen atoms numbered as N1 and N3. The atom numbering starts from the nitrogen atom labeled as N1 and proceeds in a clockwise direction until reaching the carbon atom labeled as C5. The carbon atom numbered as 1 is typically located adjacent to N1, and the carbon atom numbered as 3 is generally adjacent to N3

The numbering scheme is as follows:

N1
/
C5 – C6 – C4 – C2 – N3

C1

In this scheme, the carbon atom labeled as C2 is located opposite to the carbon atom labeled as C4, and the carbon atom labeled as C6 is opposite to the nitrogen atom labeled as N1 in the pyrimidine ring

The specific numbering of carbon atoms allows for a standard and consistent way to describe the structure and properties of nucleosides and nucleotides, which are critical components of nucleic acids such as DNA and RNA. It also aids in understanding the chemical reactions and interactions involving these compounds in various biological processes

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