Cytosine Cyt

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. 

Cytosine (Cyt) and 2 -deoxycytine (dCyd) hydroxyhydroperoxides generated by either OH radical or one-electron oxidation have been shown to be highly unstable, so far preventing their isolation and characterization by spectroscopic measurements such as NMR and MS. However, information is available on the chemical transformation of the 5- and 6-hydroperoxides as inferred from the isolation and identification of the main radical oxidation products of Cyt and dCyd. 

An electron trapping reaction analogous to that in thymine might be expected for cytosine Cyt + Cyt(C6 + H) (see Fig. le for structures). But this reaction is not... 

Fig. 1. Chemical structures of the DNA backbone (2-deoxyribose phosphate unit) and of the nucleobases thymine (Thy), cytosine (Cyt), guanine (Gua), and adenine (Ade).

B = adenine (Ade), cytosine (Cyt), guanine (Gua), thymine (Thy), uracil (Ura), or other natural or unnatural nucleobase p = nucleobase protecting group, e.g. benzoyl for Ade and Cyt or isobutyryl for Gua, Thy and Ura are normally left unprotected R = 5 -protecting group, e.g. 4,4 -dimethoxytrityl (DMTr)... 

Transport Direction of Uracil (Ura), Cytosine (Cyt), Adenine (Ade), Guanine (Gua), and K+ Ion in the Transport against the Concentration Gradient of Them through the q-Chito Membrane... 

Zero-point corrected barrier heights (Kcal-mof ) for the proton transfer for cytosine (CYT(A) species see Figure 8),... 

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