Reactions of oxide radical ion (O−) with pyrimidine nucleosides

The reaction of oxide radical ions (O−) with nucleosides 1-5 at pH 13.7 has been investigated by the pulse radiolysis technique. Rate constants for the reaction of the O− radical with the various nucleosides are in the range of (0.7-1.3)×109 M−1 s−1. The intermediate derived from uridine (1) disappeared by second-order kinetics, whereas the initial transient species derived from 2′-deoxyribonucleosides (2-5) undergo unimolecular transformations with rate constant in the range of (0.6-1.0)×104 s−1 followed by a second slower unimolecular process with rate constant in the range of 30-50 s−1. The observed differences suggest two distinct mechanistic paths. In the uridine case, hydrogen abstraction preferentially occurs at the C2′ position, whereas in the 2′-deoxynucleosides favored hydrogen abstraction occurs at the C5′ position. Both attacks lead to release of the base as shown by γ-radiolysis experiments associated with product studies.