Phosphoric ester bridge

The intemucleotide bond which is predominant in the ribonucleic acids is the phosphoric ester bridge between C-3 and C-5 of two adjacent nucleotides. 

In Scheme 19, it is shown that the radical-cation 117 can either be attacked by the nucleophile water (reactions 182 and 183), or by the nucleophile phosphate (reactions 184 and 185) of the phosphoric ester that is still attached to the radical-cation. Because the formation of the radical-cations is fast, the phosphate-bridged radicals 120 and 121, formed in reactions 184 and 185, may be in equilibrium with 117 and with one another. They are probably unstable, and rapidly revert to 117 before they can undergo radical-radical reactions. 

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