Electron spin resonance study of electron transfer in DNA: Inter-double-strand tunneling processes

In this work, we employ frozen glassy aqueous (D2O) solutions of DNA at var ious concentrations in order to test for inter-DNA-double-strand electron t ransfer, i.e., transfer from one DNA double strand to another. Electrons ge nerated by radiation are trapped on DNA and transfer to a randomly interspe rsed intercalator, mitoxantrone (MX). The monitoring of the buildup of the ESR signal of the MX radicals and the loss of the ESR signal of the DNA rad icals with time at 77 K allows for the direct observation of the rate of el ectron transfer (ET). The fraction of MX radicals and the apparent ET dista nces after irradiation are found to increase with the concentration of DNA as well as with time; A model that assumes transfer both along and between DNA double strands (ds's) is proposed and found to fit experimental results for the concentration dependence of apparent ET distances. Values for beta and the ET distances found are in good agreement with our previous results for dilute aqueous glassy media. We find that extensive tunneling of elect rons and holes in frozen D2O aqueous solutions (ices) and solid DNA (hydrat ed to 21 waters per nucleotide) can also be explained by inter-double-helix transfer. DNA in ices and DNA in hydrated solids give nearly identical res ults, suggesting that the DNA strands in ices are as closely packed as thos e in the hydrated solid DNA samples. Our results suggest that previous repo rts of extensive electron-transfer distances for DNA in icy media are found to be better explained by substantial inter-double-strand electron transfe r. After correction for the inter-double-strand electron/hole transfer, we find similar values of beta and ET distances along one DNA ds (10 +/- 1 bp at 1 min) in each medium (glass, ice, or hydrated DNA solid). Another simpl e tunneling model that assumes no difference in the transfer rates along th e DNA helix, across it, or through the solution is round to give reasonable results for the ET distances, suggesting that at 77 K DNA is not an especi ally effective conduit For the transfer of excess electrons.