The atoms of purine bases are typically numbered using a system called the "IUPAC numbering" or "Hantzsch-Widman system." This system assigns numbers to the atoms in the purine structure to provide a consistent and standardized way of referring to specific atoms within the molecule.
In the case of purine, the numbering scheme starts with the nitrogen atom in position 1 of the pyrimidine ring. The atoms in the purine ring system are numbered sequentially in a clockwise or counterclockwise direction, with the nitrogen atoms receiving priority for numbering.
Here is a representation of the IUPAC numbering scheme for the atoms in a purine base, specifically adenine:
lua