What fraction of DNA double-strand breaks produced by the direct effect is accounted for by radical pairs?

The purpose of this investigation was to determine what fraction of double strand breaks (dsb's), generated by the direct effect of ionizing radiation on DNA, can be accounted for by radical pairs. A radical pair is defined as two radicals trapped within a separation distance of <3 nm. Q-band EPR was used to measure the yield of radical pairs in calf thymus DNA films X-irradiated at 4 K. The EPR spectrum of DNA showed no evidence of radical pairs. To determine the relative sensitivity for radical pair detection via Q-band EPR, we measured the yield of radical pairs in single crystals of thymine, G(rp-Thy). Under the same conditions employed for DNA, G(rp-Thy) was approximately 8 nmol/J. The value of G(rp-Thy), in conjunction with the measured signal-to-noise, was used to calculate an upper limit for the yield of radical pairs in DNA, G(max)(rp-DNA) < 0.7-1.4 nmol/J. The upper limit, G(max)(rp-DNA), was compared with the yield of dsb's, G(total)(dsb) = 10 nmol/J, previously measured in pUC18 DNA films by Purkayastha, S.; Milligan, J. R.; Bernhard, W. A. Radiat. Res. 2007, 168, 357. We found that G(total)(dsb) > 2 x G(max)(rp-DNA), implying that a significant fraction of dsb's were not derived from a pair of trappable radicals. At least one of the two precursors needed to form a dsb was a diamagnetic (molecular) product. The hypothesis is that EPR silent lesions are formed through a molecular pathway. For example, a two-electron oxidation of deoxyribose would result in a deoxyribose carbocation intermediate that ultimately leads to a strand break.