Chapter 28: Nucleosides, Nucleotides and Nucleic Acids

Pyrimidines and Purines

• These are the "nitrogen bases" that are a key structural unit within nucleic acids.
• If you manipulate the 3D-JSMOL images, you'll see that they are planar structures (this is important for later).
• These are weak bases, similar to pyridine and are not very reactive to electrophilic aromatic substitution.
• Note that it is derivatives of pyrimidine and purine that are important in the nulceic acids.

Pyrimidine Purine

Uracil (U) Thymine (T) Cytosine (C) Adenine (A) Guanine (G)

• The bases with the carbonyl groups (uracil, thymine, cytosine and guanine) are not aromatic as drawn.
• However, if we consider the important resonance contributor from the amide systems, then we can see their aromatic character as shown for cytosine below:

Study Tip: pyrimidine sounds most like pyridine, the structure it resembles most closely... just an extra N. and  pyrimidine         pyridine

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© Dr. Ian Hunt, Department of Chemistry