Many other organic nitrogen
bases have nitrogen atoms incorporated within an aromatic ring.
A nitrogen atom easily can replace a carbon in an aromatic ring,
with a nitrogen lone electron pair taking the place of the fourth
C-H bond. This lone pair then is the binding point for a proton
when the molecule functions as a base. Four such organic bases are
shown on the right. Purine and pyrimidine are the parent compounds
of a family of bases of great importance in biochemistry, especially
in DNA and other nucleic acids.
Clockwise from top left: