Purines and pyrimidines are two of the building blocks of nucleic acids. Only two purines and three pyrimidines occur widely in nucleic acids. Here are their structures in organic chemists' line notation:

The Purines
Adenine		Guanine
The Pyrimidines
Cytosine	Thymine	Uracil

Crucial property: formation of hydrogen-bonded pairs composed of one purine and one pyrimidine, such as guanine-cytosine (left, below) and adenine-thymine (right).

Guanine-Cytosine	Adenine-Thymine

• Red dashes = the strong hydrogen bonds holding the dimers together

• "Squiggly lines = position of attachment to the nucleic acid chains of DNA or RNA.

• The dimers have exactly the same long dimension, enabling them to fit equally well into the three dimensional structures derived from them.
  • This steric "fit" is crucial to maintaining the helical structure of DNA

• Tubular structures = ab initio MO calculated structures for the dimers.

• Hydrogen bonding only possible in correct tautomers.

• Remember: tautomers are isomers related by changing the positions of attachment of a single hydrogen and a single double bond, in a three-atom system, such as the keto- and enol tautomers of a ketone.

• For example, the tautomers of adenine:
  

• Tautomers A and C both have an aromatic 6-ring

• Structures like C, which must be considered for adenine itself, are ruled out in the nucleic acids by the attachment to the sugar.

• Thus A is the favored tautomer for reasons of both stability and incorporation into nucleic acids

• The tautomers of guanine include the structure that hydrogen bonds well to a pyrimidine (A), and an aromatic one (B)

• Several that must be considered for guanine itself, but not for guanine incorporated into a nucleic acid, also can be drawn
  

• Here are the tautomers of cytosine, thymine, and uracil:

  Cytosine Tautomers
  Thymine Tautomers
  Uracil Tautomers

Again, these equilibria involving the "abnormal" tautomers have relevance only for the individual pyrimidine in aqueous solution. The inter-base hydrogen bonds in the dimers strongly stabilize the structure indicated in the base pairs.