Electron spin resonance study of the temperature dependence of electron transfer in DNA: Competitive processes of tunneling, protonation at carbon, and hopping

In this work, we employ electron spin resonance (ESR) spectroscopy to inves tigate the effects of temperature on excess electron and hole transfer thro ugh DNA. The competitive processes of tunneling, protonation at carbon, and hopping are investigated in hydrated DNA solids (hydrated to 14 waters/nuc leotide) and frozen glassy aqueous (D2O) solutions of DNA intercalated with mitoxantrone (MX) at temperatures from 4 to 195 K. Monitoring the changes in the ESR signals of MX radicals, one-electron oxidized guanines (G(.+)), one-electron reduced cytosines [C(N3)D-. (CD.)], thymine anion radicals (T. -), and irreversibly deuterated thymine radicals [T(C6)D-. (TD.)] with time at different temperatures allows for distinguishing the thermal barriers o f each process. The tunneling of electrons from DNA radicals to MX is found to be the dominant process at temperatures less than or equal to 77 K. The value of the average tunneling distance decay constant, beta (avg), is fou nd to be the same at 4 and 77 K. Working with hydrated DNA allows the disti nction between electron adducts to cytosine and those to thymine, a distinc tion not possible in glassy systems. In the solid hydrated DNA, we find tha t CD. does not undergo significant electron loss in the time of our experim ents below 170K and that electron tunneling in DNA is mainly from T.-. Irre versible deuteration of T.- at carbon position 6, which results in TD., beg ins at 130 K and increases in relative fractions of the radicals as tempera ture increases. Hole and electron hopping resulting in the recombination of G(.+) and CD. are not substantial until temperatures near 195 K are reache d. Above 130 K, the tunneling processes are not competitive with deuteratio n of T.-, and above 170 K, they are not competitive with recombination, whi ch presumably results via activated excess electron hopping.