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Differences Between Purines & Pyrimidines

Purines and pyrimidines are building blocks for many of the organic compounds found in nature and in our bodies. In fact, purines and pyrimidines are the basis for the genetic material stored in our DNA. They are mono and dicyclic organic compounds that contain nitrogen and carbon in their rings. Additionally, these molecules have varying substituent groups that allow them to have great diversity.

  1. Chemical Composition

    • The basic structure of a pyrimidine (C4H4N2) is a monocyclic 6 atom cyclic aromatic ring with nitrogen atoms at positions 1 and 3. Positions 2, 4, 5 and 6 are occupied by carbon atoms. A purine (C5H4N4) on the other hand is a dicyclic aromatic compound. Nitrogen atoms are located at positions 1, 3, 7 and 9. Carbon atoms are located at positions 2, 4, 5, 6 and 8. A purine molecule is actually a pyrimidine molecule with an imidazole (C3H4N2) ring bonded to carbons at the 4 and 5 positions. These are only the basic structure of pyrimidines and purines. Substituent groups can be added to vastly increase the variety of both of these types of molecules.

    Chemical Properties

    • Because purines are so much larger, heavier and have more intermolecular interactions, they have a much higher melting point and boiling point than do pyrimidines. Because of this volatility difference, these compounds can be separated through distillation relatively easily.

    Hydrogen Bonding in DNA

    • In the case of DNA we have four nucleotides: adenine, cytosine, guanine and thymine. Adenine binds with thymine, and cytosine binds with guanine. This has everything to do with the number of hydrogen bonds (weak bonds) that each type of molecule can donate or receive. Thymine and cytosine are both pyrimidines, but they can participate in a different number of hydrogen bonds. Adenine and guanine are both purines but also have a different number of hydrogen bonds. Adenine and thymine both can participate in two hydrogen bonds and therefore bind together. Cytosine and guanine can each participate in three hydrogen bonds and therefore bind together. This particular type of pairing determines our genetic makeup.

    Importance in Natural Organic Compounds

    • Although purines and pyrimidines are important in the synthesis and structure of DNA, one pyrimidine is important in the synthesis and structure of RNA. Thymine is replaced with a derivative called uracil.

      Pyrimidines are not nearly as common as precursor chemicals for naturally occurring compounds as are purines. Purines, for instance, are a precursor for the natural synthesis of the stimulants theobromine, theophylline and caffeine. Purines are also an important component of ATP, cAMP, AcetylCoA and NADH.

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