Electron Spin Resonance Study of Electron Transfer Rates in DNA:  Determination of the Tunneling Constant β for Single-Step Excess Electron Transfer

Abstract

An investigation of electron transfer in DNA at low temperatures in an aqueous glassy 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 electron spin resonance spectroscopic observation of the buildup in the intercalator electron adduct electron spin resonance (ESR) signal and the loss of the DNA anion signal with time at 77 K. The intercalators investigated, mitoxantrone, ethidium bromide, 1,10-phenanthroline, and 5-nitro-1,10-phenanthroline, test the effect of charge and electron affinity. The time frame of the experiment, minutes to weeks, allowed the use of large intercalator spacings (low loadings) at which random intercalation is most likely. The fraction of the electron captured by the intercalator was found to increase with ln(t) as expected for a single-step tunneling process. Fits of results to expressions for electron capture by intercalators based on a random distribution 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 electron-transfer range from 4 base pairs (ethidium) to 10 base pairs (mitoxantrone) after 1 min at 77 K. The low temperatures employed allow for the observation of single-step tunneling free from competing mechanisms such as hopping. The values of the tunneling constant β found, 0.8−1.2 Å-1, do not suggest that tunneling through the DNA base stack provides a particularly facile route for transfer of excess electrons through DNA. We find that the transfer distances and rates correlate with intercalator electron affinities calculated by density functional theory.

Quantum mechanical description of the interactions between DNA and water

“…Much of the experimental research showing DNA conductivity has been done while the DNA molecule was in a vacuum supporting the proposition that DNA is, indeed, a conducting molecule in vacuo [23,20,21]. Interestingly, at the same time, DNA has also been observed as a conductor in solvent [24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40]22]. Unfortunately, much of the research on solvated systems includes an intercalater that is beyond the scope of this paper [24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40].…”

“…Long-range charge transfer has been observed in single DNA molecules [19], in ropes consisting of a few DNA molecules [20], in thin films of DNA molecules [21], in DNA in aqueous glasses [22], in crystalline DNA [23], in intercalated solvated DNA [24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40] and in solvated DNA with modified bases. The idea of long-range electron transfers or conductance is still a matter of debate in DNA, but much of the experimental [35,41] and theoretical [16] evidence seems to support that it does indeed occur.…”

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

“…Methods of analysis were similar to those used in our previous work. 37 The ESR spectra of DNA and MX radicals in 7 M LiBr, D 2 O aqueous solutions, and hydrated solids at 77 K were produced as benchmark spectra ( Figure 1). The similarity of the ESR spectrum of MX radicals in the 7 M LiBr glass (that consists of chiefly electron adducts) with the ESR spectra of MX radicals in ice and hydrated solids (that have both electron adducts and holes) is considered good evidence that MX species with both one electron gain and one electron loss have similar ESR spectra in these systems.…”

Excess‐Electron Injection and Transfer in Terthiophene‐Modified DNA: Terthiophene as a Photosensitizing Electron Donor for Thymine, Cytosine, and Adenine

“…[4] However, the number of the studies on EET in DNA is scarce when compared to those regarding HT. [5][6][7][8][9] Several research groups have revealed that the excess electron can migrate through DNA by a hopping mechanism. [5][6][7][8] However, there remains only limited information on the rate constant for the hopping of excess electrons in DNA.…”

“…[5][6][7][8][9] Several research groups have revealed that the excess electron can migrate through DNA by a hopping mechanism. [5][6][7][8] However, there remains only limited information on the rate constant for the hopping of excess electrons in DNA. In our previous paper, we showed that the excess electron can migrate through a consecutive sequence of thymine (T) with time constants of the order of 10 10 s…”