Electron Spin Resonance Study of Electron Transfer in DNA: Inter-Double-Strand Tunneling Processes

Abstract

In this work, we employ frozen glassy aqueous (D2O) solutions of DNA at various concentrations in order to test for inter-DNA-double-strand electron transfer, i.e., transfer from one DNA double strand to another. Electrons generated by radiation are trapped on DNA and transfer to a randomly interspersed 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 radicals with time at 77 K allows for the direct observation of the rate of electron transfer (ET). The fraction of MX radicals and the apparent ET distances 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 β and the ET distances found are in good agreement with our previous results for dilute aqueous glassy media. We find that extensive tunneling of electrons and holes in frozen D2O aqueous solutions (ices) and solid DNA (hydrated 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 results, suggesting that the DNA strands in ices are as closely packed as those in the hydrated solid DNA samples. Our results suggest that previous reports of extensive electron-transfer distances for DNA in icy media are found to be better explained by substantial inter-double-strand electron transfer. After correction for the inter-double-strand electron/hole transfer, we find similar values of β and ET distances along one DNA ds (10 ± 1 bp at 1 min) in each medium (glass, ice, or hydrated DNA solid). Another simple tunneling model that assumes no difference in the transfer rates along the DNA helix, across it, or through the solution is found to give reasonable results for the ET distances, suggesting that at 77 K DNA is not an especially effective conduit for the transfer of excess electrons.