Guanine—continued reactions

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. 

Polynucleotide phosphorylase is the only known enzyme that forms high polymers of nucleotides, and therefore it is believed to have an important function in the synthesis of ribonucleic acids. Several points remain to be determined about RNA synthesis, however. The relatively poor reaction with the guanine nucleotide is hard to reconcile with the large amounts of guanine found in nucleic acids. Another question relates to the mechanism of control of the synthesis to form specific nucleic acids. While it is possible to devise hypothetical control mechanisms, such as the participation of protein as a template, continued investigation is required to answer the question. 

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