Properties of Purine and Pyrimidine Bases

TABLE I. Electron Donor Properties of Purine and Pyrimidine Bases... 

Understand the properties of purine and pyrimidine bases and nucleosides, and nucleotides with varying amounts of phosphate. Recognize the structures of the various xanthines, cyclic nucleotides, uric acid, and bases found in nucleic acids. 

Properties of Amino Acids Structures of Common Amino Acids Properties of Purine and Pyrimidine Bases The Genetic Code Properties of Fatty Acids Carbohydrate Names and Symbols... 

The absorption of nucleic acids also arises from n n and n- n transitions of purine and pyrimidine bases. The spectra of nucleic acids and derivatives occur around 200-300nm (Table 7.3). The spectra of all four nucleosides are sensitive to pH. Protonation of C and G results in a large red shift. Deprotonation of U and T also results in a large red shift. In a typical nucleic acid, the spectral properties of the isolated chromophores merge into a smooth single band with = 260nm. The average molar extinction coefficient of... 

The electroactivity of purine and pyrimidine bases were found by Emil Palecek in 1958. While bases have electroactive properties and they are able to receive reduction and/or oxidation, other components of nucleic acids such as sugar and phosphate groups are electroinactive (reviewed in Refs. 3-6). In these reviews, oxidation parts of A and G [6] and reduction parts of A, C, and G [3-6] were shown besides the effect of secondary structure of DNA on A and C reduction signals at mercury electrode. 

The reactive -NHj, -OH and -NH groups of purine and pyrimidine bases are responsible for certain properties of N.a., e.g. formation of specific hydrogen bonds between purines and pyrimidines, leading to secondary structures. Thus complementary linear chains can form a double helix (see DNA), or a linear strand can fold on itself, forming alternate linear and helical regions (RNA). Other forces involved in the... 

During the discussion of tRNA, small amounts of odd bases in the molecule, especially methylated bases, have been emphasized. Our knowledge of the mechanism of methylation and of the biological significance of methylation of purine and pyrimidine bases is still at an early stage. But, in contrast to messenger RNA, both transfer and ribosomal RNA contain methylated bases. The transfer RNA is particularly rich in methylated bases, but, in spite of earlier reports, ribosomal RNA contains one-fourth as many methylated bases as tRNA. The proportion of methylated bases in ribosomal RNA varies with its sedimentation properties. 18 S RNA is 20% richer in methylated bases than 28 S RNA. This is, however, only in vivo. When 28 S RNA is used as a substrate for methylation in vitro, the degree of methylation that takes place is similar to what is observed with 16 S RNA [159, 160]. 

The understanding of the tautomeric properties of the purine and pyrimidine bases of the nucleic acids and the determination of the electronic properties of the principal tautomers are of fundamental importance in molecular biology, in particular in connection with the theory of mutations (for general references see, e.g. refs. 1-6.) B. Pullman and A. Pullman have presented recently in these Advances3 a detailed review of the problem as it concerns the purine bases. The present paper... 

As with polypeptides, the light absorption properties of polynucleotides reflect those of the individual components. The spectra of the purine and pyrimidine bases as ribonucleosides are shown in Fig. 5-5. The number of individual electronic transitions and their origins are not immediately obvious, but many measurements in solutions and in crystals, as well as theoretical computations,7 83 84 have been made. Cytosine has n-n transitions at -275, 230, 200, and... 

The chemical properties of the purine and pyrimidine bases include highly conjugated double bond systems within the ring structures. For this reason, nucleic acids have a very strong absorption maximum at about 260 nm, which is used for nucleic acid quantitation. Moreover, the bases can exist in two tautomeric forms, the keto and enol forms (Figure 10.2). In DNA and RNA, the keto forms are by far the more predominant, and this property makes it possible for the bases to form intermolecular hydrogen bonds (see Figure 10.18). 

Theoretically, the purine- and pyrimidine-based nucleic acid constituents and the barbiturates have the potential to occur in several tautomeric forms of the keto/ enol and amino/imino type where the aromatic character of the six-membered pyrimidine ring is fully or, as in the barbiturates, partially retained, as illustrated in Fig. 15.4. In these molecular species, which are all feasible on the basis of organic chemical considerations, the hydrogen-bonding donor/acceptor properties of the functional amino, imino, enol and keto groups vary considerably, being donor in one form and acceptor in the other. 

This amide of glutamic acid has properties similar to those of asparagine. The y-amido nitrogen, derived from ammonia, can be used in the synthesis of purine and pyrimidine nucleotides (Chapter 27), converted to urea in the liver (Chapter 17), or released as NH3 in the kidney tubular epithelial cells. The last reaction, catalyzed by the enzyme glutaminase, functions in acid-base regulation by neutralizing H+ ions in the urine (Chapter 39). 

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