Nonpurine

Nonpurine is a term used in chemistry and biochemistry to denote any molecule that is not a purine. In the context of nucleic acids, purines refer to the two-ring nitrogenous bases adenine and guanine. The term nonpurine is often used to describe the complementary set of bases, increasingly called pyrimidines, which include cytosine, thymine, and uracil, along with their chemical derivatives. Structurally, purines consist of a fused bicyclic ring system, whereas pyrimidines are single six-membered rings. In DNA, RNA, and some other nucleic acids, bases are categorized as purines or pyrimidines; the standard human genome uses two purines (A and G) and two pyrimidines (C and T in DNA, C and U in RNA). Therefore, nonpurine bases are typically the pyrimidines.

The term does not designate a distinct functional class of compounds beyond this categorization. Outside nucleic acids, "nonpurine" may describe any chemical species that does not possess a purine skeleton, including a wide range of heterocycles and other aromatic systems. In pharmacology and medicinal chemistry, the phrase may appear when contrasting purine-based drugs or nucleoside analogs with nonpurine alternatives.

In summary, nonpurine is a relative term primarily used to distinguish non-purine nitrogenous bases, most commonly the pyrimidines, from purine bases within nucleic acid chemistry.