Figure 14.9 Other important purines in biochemistry include uric acid and our friend, caffeine. Guanine was first isolated from guano (bird manure), and thymine from thymus tissue). (Some of the common names of the bases reflect the circumstances of their discovery. Note that there are no N-N bonds: the N atoms are always separated by at least one C atom. There are four N atoms and five C atoms in the purine ring system. The five-membered ring on the right is just like imidazole (the side chain of the amino acid histidine). The six-membered ring on the left is just like pyrimidine. The purine ring is a fused (joined together) bicyclic (two rings) heterocycle. There are two purine bases in RNA and DNA (Figure 14.8). Purine bases of RNA and DNA Figure 14.8 The purine bases. The form shown in the middle (amino + keto) is predominant. The -OH group undergoes keto/enol tautomerism, and the -NH 2 group undergoes amino/imino tautomerism, as shown. Figure 14.7 shows three forms of cytosine. As a result, there are a lot of opportunities for tautomeric equilibria. Nucleic acid bases are heterocycles “decorated” with -OH and -NH 2 groups. The -OH group undergoes keto/enol tautomerism, and the -NH 2 group undergoes amino/imino tautomerism. Tautomeric forms of cytosine Figure 14.7 Three forms of the cytosine base. When we draw a pair of resonance forms of a molecule, we are just indicating two different valence-bond representations of a single chemical species – no atoms are moved.) (Don’t confuse tautomers with resonance hybrids. The heavy atom “skeleton” of the compound remains unchanged. We can define tautomers as isomers which differ only in the positions of protons (and, as a consequence, double bonds). Tautomerism is a form of chemical equilibrium which we encounter repeatedly in biochemistry. Nucleotide bases may exist in two or more tautomeric forms depending on the pH. Thymine takes the place of uracil in DNA because thymine has greater resistance to photochemical mutation from things like UV radiation from the sun. Therefore, thymine is also called “5-methyluracil”. Thymine is just like uracil, except that it has a methyl group at position 5. One of them, cytosine (C) is found in both DNA and RNA, but the second pyrimidine is different in DNA from what is found in RNA: thymine (T) in DNA and uracil (U) in RNA. Pyrimidine bases of RNA and DNA Figure 14.6 The pyrimidine bases.īoth DNA and RNA contain two major pyrimidine bases (Figure 14.6). But we will only see N atoms in the rings of the nucleic acid bases). Heteroatoms are non-carbon atoms, such as N, O, and S. (What’s a heterocycle? It’s just like an aromatic ring, such as benzene, except that there are heteroatoms in the ring. ![]() The numbering convention used for pyrimidine and purine heterocycles are shown in the figure. The nitrogenous bases found in nucleic acids are derivatives of two parent compounds, pyrimidines and purines (Figure 14.5). The Bases Figure 14.5 the nitrogenous bases found in nucleic acids with their numbering conventions. Both ribose and deoxyribose occur in their β-furanose form. Deoxyribose, the DNA sugar, is so peculiar that its structure wasn’t figured out until about 1930, long after most other sugar structures were known. As we will see later, this difference causes DNA to have very different properties from RNA. Deoxyribose is a reduced sugar: one C is reduced from -CH(OH) to -CH 2 (methylene). That’s why it is called deoxyribose: an oxygen atom is missing. Instead, that carbon is -CH 2 (Figure 14.4). It’s an unusual sugar: one of the -OH groups – the one at C2 – is missing. In the nucleic acids, it is in the beta anomeric form.ĭNA is deoxyribonucleic acid and its sugar is D-2- deoxyribose. As shown in Figure 14.4, it forms a furanose ring. Sugars Figure 14.4 D-ribose (left) is found in RNA, while D-2-deoxyribose (right) is found in DNA.ĭ-Ribose is the sugar in RNA.
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