EPR measurements were made at Q-band microwave frequencies on powder samples pressed into pellets and X-irradiated at 4 K. Measurements were made at 4 K after no anneal, then after a 77 K anneal, and then after a 300 K anneal. In poly(dA):poly(T) the free radical distribution is approximately a simple sum of the distributions in the separate homopolymers. In poly(dA-T) the free radical distribution differs from that of poly(dA):poly(T). The clearest difference is that in poly-(dA):poly(T) the concentration of one-electron-reduced thymine$({\rm T}_{{\rm re}}^{\cdot})$ is reduced relative to the total radical concentration. On warming the thymine-containing samples from 77 K to room temperature, the${\rm T}_{{\rm ox}}^{\cdot}$ radical disappears and the$({\rm T}-{\rm H}5^{\prime})^{\cdot}$ radical appears. Also, the${\rm T}_{{\rm re}}^{\cdot}$ radical disappears and the$({\rm T}+{\rm H}6)^{\cdot}$ radical appears. There are three main conclusions. First, little or no transfer of free radicals between strands is needed to explain the data. Second, when A and T are interstacked, either the${\rm T}_{{\rm re}}^{\cdot}$ radical is less stable against recombination than other radical products or radical transfer occurs to an adjacent adenine. Third, in the poly- and oligonucleotides, the${\rm T}_{{\rm ox}}^{\cdot}$ radical is a likely precursor to the$({\rm T}-{\rm H}5^{\prime})^{\cdot}$ radical.

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