Structural dependence of oligonucleotide photooxidation.

Oxidative photosensitization was used to characterize the conformational-dependent reactivity of various structures formed by oligonucleotides 14-15 nucleotides in length. The rate and product composition from a single hit process was analyzed using quantitative ion exchange chromatography under native and denaturing conditions. The primary damage incurred under aerobic acetone sensitization was base oxidation that, in turn, would induce strand scission upon a secondary treatment with piperidine. The reactive intermediates of this process were not consistent with diffusible radical species or singlet oxygen, as indicated by isotope and quenching studies. Derivatization was most likely initiated through a type I photoprocess with a direct interaction between DNA bases and excited state acetone preceding an irreversible oxidation step. This dominant reaction demonstrated no obvious sequence or site specificity for initial modification; the relative reactivity among the oligonucleotides did not correspond to any simple trend of base composition or near neighbor analysis. Likewise, the steric requirements of base modification allowed for similar rates of oxidation for single-strand, helical, and aberrant forms of DNA. Hybridization of the most reactive oligonucleotides, however, did suppress their relative single-strand vs double-strand reactivity by as much as fourfold.