Electron spin resonance study of electron transfer rates in DNA:: Determination of the tunneling constant β for single-step excess electron transfer

An investigation of electron transfer in DNA at low temperatures in an aqueous grassy 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 flame 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 (mitoxantrene) 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 beta found, 0.8-1.2 Angstrom(-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.