Nucleobases components

Figure 18. Schematic representation of feasible acid-base equilibria of nucleobase components.

Some other research results concern extraction and transport of nucleotides through the single salt binding of a cationic host with a phosphate moiety. Simple (poly)amines and more complicated aliphatic and heterocyclic nitrogen compounds were employed [170,181—183]. Obviously, they lack selectivity over different nucleobase components. However, they may discriminate among mono-, di-, and triphosphates. And, most important,... 

A number of factors have been implicated to be important for RNA catalysis, including the involvement of functional groups of the nucleobases or RNA backbone, divalent metal ions or other solvent components that might provide electrostatic... 

Virustatic antimetabolites are false DNA building blocks (B) or nucleosides. A nucleoside (e.g., thymidine) consists of a nucleobase (e.g., thymine) and the sugar deoxyribose. In antimetabolites, one of the components is defective. In the body, the abnormal nucleosides undergo bioactivation by attachment of three phosphate residues (p.287). 

Nucleic acids are broken down into their components by nucleases from the pancreas and small intestine (ribonucleases and deoxyribonucleases). Further breakdown yields the nucleobases (purine and pyrimidine derivatives), pentoses (ribose and deoxyribose). 

Guanine is a preferential DNA target to several oxidants it shows the lowest ionization potential among the different purine and pyrimidine nucleobases and it is the only nucleic acid component that exhibits significant reactivity toward singlet oxygen ( O2) at neutral pH. ... 

Several conductive CT solids with nucleobase skeletons have been developed in the TTF systems having uracil moieties (crt = 10 -2 S cm ) [123-127]. Also several attempts have been undertaken to investigate the CT complexes in a variety of biochemical systems, especially using nucleobases (Scheme 9) [18, 104]. Estimation of 7p of the nucleobases, as potential components in CT complexes, indicate that they are reasonably effective Tt-donors particularly in the case of guanine (G) 7d = 7.64—7.85 eV vs adenine (A, 7.80-8.26 eV), cytosine (C, 8.45-8.74 eV), and thymine (T, 8.74-8.87 eV) [128-131]. 

In contrast to the installation of the nucleobase via nucleophilic substimtion of a suitable leaving group on the isoxazolidinyl cycloadduct, Colacino et al. (51) and Sindona and co-workers (52,53) prepared isoxazolidinyl nucleosides using vinyl nucleobases as the dipolarophile (Scheme 1.5). In Sindona s work, while a three-component reaction of hydroxylamine, formaldehyde, and 20 afforded a complex mixture of cycloadducts and byproducts, the known dipole 21 reacted with N-9-vinyladenine (20) in benzene at reflux to afford a racemic mixture of adduct 22 and its enantiomer (45%). The ester function was then used to effect a resolution by pig... 

An alternative to the terrestrial synthesis of the nucleobases is to invoke interstellar chemistry. Martins has shown, using an analysis of the isotopic abundance of 13C, that a sample of the 4.6 billion year old Murchison meteorite which fell in Australia in 1969 contains traces of uracil and a pyrimidine derivative, xanthine. Samples of soil that surrounded the meteor when it was retrieved were also analyzed. They gave completely different results for uracil, consistent with its expected terrestrial origin, and xanthine was undetectable [48], The isotopic distributions of carbon clearly ruled out terrestrial contamination as a source of the organic compounds present in the meteorite. At 0°C and neutral pH cytosine slowly decomposes to uracil and guanine decomposes to xanthine so both compounds could be the decomposition products of DNA or RNA nucleobases. They must have either travelled with the meteorite from its extraterrestrial origin or been formed from components present in the meteorite and others encountered on its journey to Earth. Either way, delivery of nucleobases to a prebiotic Earth could plausibly have been undertaken by meteors. The conditions that formed the bases need not have been those of an early Earth at all but of a far more hostile environment elsewhere in the Solar System. That environment may have been conducive to the production of individual bases but they may never have been able to form stable DNA or RNA polymers this development may have required the less extreme conditions prevalent on Earth. 

As noted above, the nucleophilicity of water allows it to enter into reactions that cause the degradation of biological macromolecules, including DNA and proteins. Analogous problems are associated with the assembly of biopolymers. In water, the assembly of nucleosides from component sugars and nucleobases, the assembly of nucleotides from nucleosides and phosphate, and the assembly of oligonucleotides from nucleotides are all thermodynamically uphill in water. 

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