An investigation of electron transfer in DNA at low temperatures in an aqueous glassy medium is reported for a system in which electrons are generated by radiation and trapped on DNA. The transfer of the electron from the DNA anion radical to randomly interspaced intercalators is followed by electron spin resonance spectroscopic observation of the buildup in the intercalator electron adduct electron spin resonance (ESR) signal and the loss of the DNA anion signal with time at 77 K. The intercalators investigated, mitoxantrone, ethidium bromide, 1,10-phenanthroline, and 5-nitro-1,10-phenanthroline, test the effect of charge and electron affinity. The time frame of the experiment, minutes to weeks, allowed the use of large intercalator spacings (low loadings) at which random intercalation is most likely. The fraction of the electron captured by the intercalator was found to increase with ln(t) as expected for a single-step tunneling process. Fits of results to expressions for electron capture by intercalators based on a random distribution suggest that the random model is appropriate up to loadings of about 1 per 10−20 DNA base pairs depending on the intercalator. The distances of electron-transfer range from 4 base pairs (ethidium) to 10 base pairs (mitoxantrone) after 1 min at 77 K. The low temperatures employed allow for the observation of single-step tunneling free from competing mechanisms such as hopping. The values of the tunneling constant β found, 0.8−1.2 Å-1, do not suggest that tunneling through the DNA base stack provides a particularly facile route for transfer of excess electrons through DNA. We find that the transfer distances and rates correlate with intercalator electron affinities calculated by density functional theory.

The free radicals produced from the irradiation of hydrated DNA with a heavy-ion beam have been investigated by ESR spectroscopy. The dominant free radical species formed after 60 MeV/nucleon (16)O(8+) ion-beam irradiations at low temperatures (77 K) are the same as those previously identified from studies using low-LET radiation, pyrimidine electron-gain radicals and purine electron-loss radicals; however, greater relative amounts of neutral carbon-centered radicals are found with the higher-LET radiation, and a new phosphorus-centered radical is identified. The fraction of neutral carbon radicals is also found to increase along the ion-beam track with the highest amounts found in the Bragg peak. The neutral carbon-centered radicals likely arise in part from the sugar moiety. The G values found for total trapped radicals at 77 K are significantly smaller for the (16)O(8+) ion beam than those found for low-LET radiation. The new phosphorus-centered radical species is identified by its large 31P parallel hyperfine coupling of about 780 G as a phosphoryl radical. This species is produced linearly with dose and is not found in significant amounts in DNA irradiated with low-LET radiation. The phosphoryl radical must be produced by the fragmentation of a P-O bond and suggests the possibility of a direct strand break. The yield of phosphoryl species is small (about 0.1% of all radicals); however, it clearly indicates that new mechanisms of damage which are not significant for low-LET radiation must be considered for high-LET radiation.

ESR spectroscopy at low temperatures is employed to investigate electron transfer within DNA doped with randomly spaced electron traps. The traps were introduced by careful bromination of DNA in ice-cooled aqueous solution. The procedure is shown by NMR and GC/MS techniques to modify thymine, cytosine, and guanine 2‘-deoxyribosides, transforming them into 5-bromo-6-hydroxy-5,6-dihydrothymine, T(OH)Br, 5-bromocytosine, CBr, and 8-bromoguanine, GBr, derivatives. The bromination products formed in molar ratio close to T(OH)Br/CBr/GBr = 0.2:1:0.23 and serve as internal electron scavengers on γ-irradiation. Paramagnetic products that result from electron scavenging in DNA by T(OH)Br and CBr units at 77 K have been identified by ESR as the 6-hydroxy-5,6-dihydrothymin-5-yl (TOH•) radical and the 5-bromocytosine σ* radical anion, CBr•-. Our quantitative estimates show that electron scavenging by T(OH)Br in bromine-doped DNA is over an order of magnitude more efficient than the more abundant CBr traps. This indicates that there is a high probability the electron survives encounters with the planar CBr traps through either transmission or reflection. The yields of electron scavenging by T(OH)Br moieties have been treated quantitatively considering the scavenging process as a competition between diffusion of electrons to T(OH)Br traps and their fixation on cytosines in the form of protonated radical anions. A mean displacement of the electron from its entry point evaluated using this model is about 11 bases at 77 K. After trapping at 77 K no further migration takes place until annealing to temperatures near 150 K and above. At these temperatures electron migration is activated and migration distances are found to increase with temperature likely through a hopping mechanism.