Base , guanine

Xanthine oxidase, mol wt ca 275,000, present in milk, Hver, and intestinal mucosa (131), is required in the cataboHsm of nucleotides. The free bases guanine and hypoxanthine from the nucleotides are converted to uric acid and xanthine in the intermediate. Xanthine oxidase cataly2es oxidation of hypoxanthine to xanthine and xanthine to uric acid. In these processes and in the oxidations cataly2ed by aldehyde oxidase, molecular oxygen is reduced to H2O2 (133). Xanthine oxidase is also involved in iron metaboHsm. Release of iron from ferritin requires reduction of Fe " to Fe " and reduced xanthine oxidase participates in this conversion (133). 

Figure 35-1. A segment of one strand of a DNA molecule in which the purine and pyrimidine bases guanine (G), cytosine (C), thymine (T), and adenine (A) are held together by a phosphodiester backbone between 2 -de-oxyribosyl moieties attached to the nucleobases by an W-glycosidic bond. Note that the backbone has a polarity (ie,a direction). Convention dictates that a single-stranded DNA sequence is written in the 5 to 3 direction (ie, pGpCpTpA, where G, C,T, and A represent the four bases and p represents the interconnecting phosphates).

The structure of DNA resembles a ladder that has been twisted around itself. The rungs of the ladder are composed of bases (guanine, thymine, cytosine, and adenine) that form hydrogen bonds. 

Hydrolytic cleavage of the glycosidic bond holding the DNA bases to the sugar-phosphate backbone is typically a very slow process under physiological conditions (pH 7.4 37°C). Loss of the pyrimidine bases cytosine and thymine occurs with a rate constant of 1.5 X 10 s (ty2 = 14,700 years), while loss of the purine bases guanine and adenine proceeds slightly faster, with a rate constant of 3.0 X... 

Langer H, Doltsinis NL (2003) Excited state tautomerism of the DNA base guanine a restricted open-shell kohn-sham study. J Chem Phys 118 5400-5407... 

M The DNA bases guanine and cytosine can form three hydrogen bonds, whereas adenine and thymine only form two (gray carbon, white hydrogen, red oxygen, blue nitrogen). 

The purine base guanine is also formed in concentrated solutions of ammonium cyanide, i.e., the same substance which became known from Or6 s adenine synthesis. Or6, as well as Stanley Miller, was involved in a new series of experiments (Levi et al., 1999). The yield of guanine is, however, 10 10 times lower than that of adenine surprisingly, the synthesis is just as effective at 253 K as at 353 K. Low temperatures seem conceivable in certain parts of Earth as well as on the Jovian moon Europa (see Sect. 3.1.5) or in the Murchison meteorite. 

The genetic code is composed of four letters —two pyrimidine nitrogenous bases, thymine and cytosine, and two purine bases, guanine and adenine—which can be regarded functionally as arranged in codons (or triplets). Each codon consists of a combination of three letters therefore, 43 (64) different codons are possible. Sixty-one codons code for specific amino acids (three produce stop signals), and as only 20 different amino acids are used to make proteins, one amino acid can be specified by more than one codon. 

Phosphorylases remove the ribose sugars to yield the bases guanine or hypoxan-thine (from adenine or inosine nucleosides). 

The close correspondence of the DNA absorption spectrum with that of a mixture of mononucleotides of the same composition illustrates the weak nature of the interactions between neighboring purine and pyrimidine bases guanine (G), cytosine (C), adenine (A), and thymine (T) at an interplanar separation of 3.36 A in the unexcited double-helical configuration. On the other hand the structureless fluorescence band of (calf-thymus) DNA is red-shifted by 3500 cm-1 from the fluorescence spectral origin of the mononucleotides it closely resembles the fluorescence spectrum of the dinocleotide ApT (and of poly dAT) and is accordingly identified131 with the fluorescence... 

If the terminal pyrophosphate is removed from a molecule of ATP, the remainder is AMP, adenosine monophosphate, one of the four building blocks of the important biological macromolecules, the nucleic acids. There are two types of nucleic acids (26) ribonucleic acid (RNA), and deoxyribonucleic acid (DNA). RNA is a polymer of four different nucleotides, one of which is AMP, the ribose phosphate of adenine. The other three nucleotides are also ribose phosphates of heterocyclic bases, guanine, cytosine, and uracil. The structure of the four bases is shown in Figure 6. 

The de novo pathways for purine and pyrimidine biosynthesis appear to be nearly identical in all living organisms. Notably, the free bases guanine, adenine, thymine, cytidine, and uracil are not intermediates in these pathways that is, the bases are not synthesized and then attached to ribose, as might be expected. The purine ring structure is built up one or a few atoms at... 

Alkylating agents that can cause methylation and ethylation of bases. Guanine, for example, reacts with dimethylsulphate to form o-methylguanine. 

The reported strategies utilized in DNA sensing include (1) sequence-specific hybridization processes based on the oxidation signal of most electroactive DNA bases, guanine and adenine [13,24] or (2) quasi-specific detection of small molecules capable of binding by intercalation or complexation with DNA, such as metal coordination complexes, antibiotics, pesticides, pollutants, etc. [17,18] or in the presence of some metal tags such as gold, silver nanoparticles, etc. [23,50,51]. 

Figure 7.18 Alkylated (methylated) forms of the nitrogenous base guanine.

AZAGUANINE An analogue of the normal DNA and RNA purine base guanine selection for resistance to the toxic effects of 8-azaguanine is the basis of several mutation-detection systems. 

The electronic structure of hydrogen-bonded pairs of bases guanine-cytosine... 

Fig. 3.4. Differential pulse voltammetric determination of purine and pyrimidine bases guanine (2 x lO- M), adenine (3 x I0-5 M), thymine (3 x 0 4 M) and cytosine (3 x 10 4 M) in borate buffer (pH = 10.02) (a) with ultrasonic pretreatment (power intensity 72 W/cm2, horn tip-electrode separation 5 mm, (b) successive scan without ultrasonic pretreatment. DPV conditions scan rate 5 mV/s, amplitude 50 mV. (Reprinted from ref. [68] with...

Electron (T -EE), Strand Breaks, Solvation of DNA Bases, Guanine Radical Cation (G +), TD-DFT Study... 

Gas phase ionization potentials (IPs) of DNA bases, guanine, adenine, thymine and cytosine, have been calculated using a variety of levels of theory [33-40], In Table 20-1, we compare representative theoretical values with available... 

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