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

An investigation of electron transfer in DNA at low temperatures in an aque ous 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 electro n spin resonance spectroscopic observation of the buildup in the intercalat or electron adduct electron spin resonance (ESR) signal and the loss of the DNA anion signal with time at 77 K. The intercalators investigated, mitoxa ntrone, ethidium bromide. 1,10-phenanthroline, and 5-nitro-1,10-phenanthrol ine, test the effect of charge and electron affinity. The time flame of the experiment, minutes to weeks, allowed the use of large intercalator spacin gs (low loadings) at which random intercalation is most likely. The fractio n of the electron captured by the intercalator was found to increase with l n(t) as expected for a single-step tunneling process. Fits of results to ex pressions for electron capture by intercalators based on a random distribut ion 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 el ectron-transfer range from 4 base pairs (ethidium) to 10 base pairs (mitoxa ntrene) after 1 min at 77 K. The low temperatures employed allow for the ob servation of single-step tunneling free from competing mechanisms such as h opping. 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 par ticularly facile route for transfer of excess electrons through DNA. We fin d that the transfer distances and rates correlate with intercalator electro n affinities calculated by density functional theory.