Heterocyclic compounds, aromatic purines

The bases that occur in nucleic acids are aromatic heterocyclic compounds derived from either pyrimidine or purine. Five of these bases are the main components of nucleic acids in all living creatures. The purine bases adenine (abbreviation Ade, not A ) and guanine (Gua) and the pyrimidine base cytosine (Cyt) are present in both RNA and DNA. In contrast, uracil (Ura) is only found in RNA. In DNA, uracil is replaced by thymine (Thy), the 5-methyl derivative of uracil. 5-methylcyto-sine also occurs in small amounts in the DNA of the higher animals. A large number of other modified bases occur in tRNA (see p. 82) and in other types of RNA. 

The bases occurring in nucleic acids are derivatives of the aromatic heterocyclic compounds purine and pyrimidine (see p. 80). The biosynthesis of these molecules is complex, but is vital for almost all cells. The synthesis of the nucleobases is illustrated here schematically. Complete reaction schemes are given on pp. 417 and 418. 

Pyrimidine is a six-membered aromatic heterocyclic compound that contains two nitrogen atoms, separated by a carbon atom, in the ring. Nucleic acids, DNA and RNA, contain substituted purines and pyrimidines. Cytosine, uracil, thymine and alloxan are just a few of the biologically significant modified pyrimidine compounds, the first three being the components of the nucleic acids. 

The monofunctional complexes [PtCl(dien)]+, [PtCl(NH3)3]+, and the active d.v-compounds cA-[PtCl(NH3)2 (Am)]+, where Am is an heterocyclic or aromatic amine ligand like pyridine, pyrimidine, purine, or aniline, only form stable monoadducts with DNA [50][51]. However, when Am = A-methyl-2,7-diazapyrenium (a strong intercalator), the monoadduct is stable only on single-stranded DNA. On double-stranded DNA it is hydrolyzed with release of c T-[Pt(NH3)2(Am)(H20)]3+ or of Am generating the aqua monoadduct of cisplatin [52],... 

Purine is a heterocyclic compound with four nitrogen atoms. Each N has a pair of electrons on it in the Lewis structure. Explain which of these pairs are part of the pi system and which are not. Explain whether purine is aromatic or not. 

Purine is one of many fused heterocyclic compounds whose rings share two atoms and the bond between them. For example, the following compounds all contain fused heterocyclic aromatic rings ... 

There is, for example, no end-of-text chapter entitled Heterocyclic Compounds. Rather, heteroatoms are defined in Chapter 1 and nonaromatic heterocyclic compounds introduced in Chapter 3 heterocyclic aromatic compounds are included in Chapter 11, and their electrophilic and nucleophilic aromatic substitution reactions described in Chapters 12 and 23, respectively. Heterocyclic compounds appear in numerous ways throughout the text and the biological role of two classes of them—the purines and pyrimidines—features prominently in the discussion of nucleic acids in Chapter 27. 

Acetylenecarboxylic acids, reactions with heterocyclic compounds, 1, 125 Aminochromes, 5, 205 Anthranils, 8, 277 Aromatic quinolirines, 5, 291 Aza analogs, of pyrimidine and purine bases, 1, 189... 

Quinoline, indole, imidazole, purine, and pyrimidine are other examples of heterocyclic aromatic compounds. The heterocyclic compounds discussed in this section are examined in greater detail in Chapter 21. 

Xanthina oxidase, xanthine dehydrogenase, Schardtnger enzyme an enzyme of aerobic purine degradation, which catalyses the oxidation of hypoxan-thine to xanthine, and xanthine to uric acid Hypox-anthine + HjO + 62 -> Xanthine -h H2O2 Xanthine + H2O -H O2 -> Uric acid + H2O2. It is a dimeric enzyme, M, 275,000, pH-optimum 4.7, pi 5.35, containing 2 FAD, 2 Mo and 8 Fe (data for the enzyme from milk). The substrate specificity is low it catalyses the oxidation of other purines (e. g. adenine), aU-phatic and aromatic aldehydes, pyrimidines, pteri-dines and other heterocyclic compounds. 

It should be emphasized that not even all aromatic compounds are hydroxylated by the aerobic type of reaction. Heterocyclic compounds such as purines, pteridines (Forrest et al., 1956) and nicotine (Hoch-stein and Rittenberg, 1959) are hydroxylated by a hydration reaction where the mechanism is essentially similar to that involved in the conversion of succinate to malate. One may speculate that the presence of a nitrogen atom in the ring of a compound such as nicotine allows sufficient polarization of bonds to permit the hydration type of mechanism to take place. 

Nucleic acids are acidic substances present m the nuclei of cells and were known long before anyone suspected they were the primary substances involved m the storage transmission and processing of genetic information There are two kinds of nucleic acids ribonucleic acid (RNA) and deoxyribonucleic acid (DNA) Both are complicated biopolymers based on three structural units a carbohydrate a phosphate ester linkage between carbohydrates and a heterocyclic aromatic compound The heterocyclic aro matic compounds are referred to as purine and pyrimidine bases We 11 begin with them and follow the structural thread... 

Two nitrogen containing heterocyclic aromatic compounds—pyrimidine and purine— are the parents of the bases that constitute a key structural unit of nucleic acids... 

Section 28 1 Many biologically important compounds are related to the heterocyclic aromatic compounds pyrimidine and purine... 

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