Electron Spin Resonance Study of the Temperature Dependence of Electron Transfer in DNA:  Competitive Processes of Tunneling, Protonation at Carbon, and Hopping

Abstract

In this work, we employ electron spin resonance (ESR) spectroscopy to investigate the effects of temperature on excess electron and hole transfer through DNA. The competitive processes of tunneling, protonation at carbon, and hopping are investigated in hydrated DNA solids (hydrated to 14 waters/nucleotide) and frozen glassy aqueous (D 2 O) 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 •+ ), oneelectron 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 of 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, β avg , is found to be the same at 4 and 77 K. Working with hydrated DNA allows the distinction between electron adducts to cytosine and those to thymine, a distinction not possible in glassy systems. In the solid hydrated DNA, we find that CD • does not undergo significant electron loss in the time of our experiments below 170K and that electron tunneling in DNA is mainly from T •-. Irreversible deuteration of T •at carbon position 6, which results in TD • , begins at 130 K and increases in relative fractions of the radicals as temperature increases. Hole and electron hopping resulting in the recombination of G •+ and CD • are not substantial until temperatures near 195 K are reached. Above 130 K, the tunneling processes are not competitive with deuteration of T •-, and above 170 K, they are not competitive with recombination, which presumably results via activated excess electron hopping.

Synthesis of 5-(Pyridinyl and Pyridiniumyl)-2′-Deoxyuridine Nucleosides: Reversible Electron Traps for Dna

“…In addition to the syntheses of four pyridinyl-dU nucleosides, this paper also reports the synthesis of four pyridiniumyl-dU nucleoside traps. [59][60][61][62][63][64] Thus other factors remaining the same, excess electron hopping from a reduced uridine should proceed more rapidly than from a reduced cytidine. N,N 0 -Dimethyl-4,4 0 -bipyridinediium (MV 2þ ), N,N 0 -polymethylene-2,2 0 -bipyridinediium (2,2 0 -BpyEt 2þ and 2,2 0 -BpyPr 2þ ), and N-methylpyridinium (MP þ ) salts were thus candidates for attachment to dU (see Fig.…”

Charge transfer dynamics in DNA revealed by time-resolved spectroscopy

“…Sevilla and colleagues further examined the effect of temperature on EET using DNA with MX as the intercalator and D 2 O as the solvent, which was employed to distinguish T radical anion and protonated reduced C. 29 They found that the excess electron tunnelling from the nucleobase to MX was the dominant process at a temperature of <77 K. The estimated β values of DNA at 4 and 77 K were confirmed to be the same. This lack of temperature dependence can be attributed to the absence of the contribution of higher vibronic states in this temperature range.…”

Analysis of the Contribution of Charge Transport in Iodine-125-Induced DNA Damage

“…Upon reaction with a radical anion, BrdU undergoes one electron reduction that subsequently leads to DNA strand cleavage (27, 28, 30). At low temperatures (below 80K), EET by the tunneling mechanism has been observed whereas EET by the hopping mechanism has been observed at higher temperatures (31-33). Due to their lower reduction potentials, cytosine and thymine bases are expected to be most easily reduced (13).…”