Guanine—continued

The components of nucleic acids have been the subject of continuous DFT stud-ies61 S5,67 69. Jasien and Fitzgerald calculated dipole moments and polarizabilities for a series of molecules of biological interest including nucleic acid bases (adenine, thymine, cytosine, and guanine) and their pairs (adenine-thymine and cytosine-guanine)61. A good correlation between DFT(HL), experimental, and MP2 results was obtained for dipole moments and polarizabilities. More detailed analyses of DFT(SVWN) and DFT(B88/P86) results, which included vibrational frequencies, were reported for isolated bases and their... 

In DNA, base pairing occurs between guanine and cytosine in its most stable form [87], as in [92]. However, cytosine in its imino form [91] is able to form a hydrogen bond to adenine as in [93], and such a base pairing could lead to the formation of mutations if it continued in DNA replication. The kinetics of the tautomerisation of purine and pyrimidine bases has been... 

Herschlag s group continued its study of structure-function relationships in the hammerhead ribozyme using a base-rescue biochemical method. This method substitutes other atoms or molecules for bases at critical catalytic or structural positions and tests whether catalytic activity is lost. If so, the RNA bases (U, A, G, C) or a modified base (for instance, deazaguanine or 2-aminopurine substituted for guanine) is added to the solution to ascertain... 

There has been continued interest in the radiation chemistry of the purines since early reports on oriented DNA by Graslund et al. [35] which suggest that the main trapping site of one-electron oxidation in DNA is the guanine base. It is remarkable that in aqueous solution, the electron adducts of the purine nucleosides and nucleotides undergo irreversible protonation at carbon with a rate constant 2 orders of magnitude higher than that for carbon protonation of the electron adduct in thymidine [36]. It is therefore important to know the properties of the various purine reduction products and to ask why they have not been observed in irradiated DNA. 

Steenken et al. have concluded that in double-stranded DNA direct hydrogen atom abstraction from 2 -deoxyribose by G(-H) radical is very unlikely due to steric hindrance effects and a small thermodynamic driving force [94]. The EPR studies performed in neutral aqueous solutions at room temperature have shown that, in the absence of specific reactive molecules, the lifetime of the G(-H) radical in double-stranded DNA is as long as -5 s [80]. Therefore, the fates of G(-H) radicals are mostly determined by the presence of other reactive species and radicals. Thus, the G(-H) radical can be a key precursor of diverse guanine lesions in DNA. In the next section we begin from a discussion of the site-selective generation of the G(-H) radical in DNA, and then continue with a discussion of the reaction pathways of this guanine radical. 

The DNA undergoes replication. This process involves separation of a segment of the double helix into separate single strands, which then replicate such that guanine is opposite cytosine (and vice versa) and adenine is opposite thymine (and vice versa). This process continues until a complete copy of the DNA molecule has been produced. 

The nature of the interaction between nucleic acid bases has been a continual source of fascination ever since the nature of the genetic code was unraveled. However, the numbers of atoms and electrons in these systems erected a high barrier to the application of accurate quantum mechanical methods for many years. For example, the guanine-cytosine (GC) pair contains 29 atoms and 136 electrons. 

Pur" means adenosine or guanine, while "Pyr" means thymine or cytosine. Continuing with the slructute of p53, amino acids 320-3fiO are used for maintaining the adhesion of the four p53 proteins to each other (Prives, 1994). This domain is also called the "oligomeriiiation domain."... 

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