Postirradiation Electron Transfer vs Differential Radical Decay in X-Irradiated DNA and Its Mixtures with Additives. Electron Spin Resonance Spectroscopy in LiBr Glass at Low Temperatures

Abstract

Free radical formation in DNA and in colyophilized mixtures of DNA with the additives mitoxantrone and riboflavin was monitored after X-ray irradiation in frozen aqueous glasses (7 M LiBr/D2O) at 77 K by electron spin resonance (ESR) spectroscopy. Specifically, the postirradiation time course at 77 K of the respective free radical intensity residing on DNA or on the additive was probed in order to test the hypothesis of electron transfer from DNA, e.g., to mitoxantrone after irradiation under these conditions (e.g., Messer, A.; Carpenter, K.; Forzley, K.; Buchanan, J.; Yang, S.; Razskazovskii, Y.; Cai, Z.; Sevilla, M. D. J. Phys. Chem. B 2000, 104, 1128). For both additives, different additive loadings and irradiation doses were employed. The observed relative change in contributions of DNA and of additive radical components to the experimental spectra with time could be ascribed, for both additives, unequivocally to independent, differential fading of component radicals. Transfer from DNA to the additive, e.g., by electron tunneling as proposed before could be ruled out to occur by a detailed, quantitative analysis of the experimental spectra using reconstruction techniques. Additional studies were performed with the nucleotides TMP and dCMP and its mixtures with mitoxantrone in order to describe the time course in systems which are expected to behave independently; the results supported the conclusions arrived at from the analysis of the DNA/additive system. A model was proposed to describe the postirradiation radical fading mechanisms which involve liberation of radiation-induced matrix-trapped defects with time. It was assumed that these defects are ESR-mute and react with radicals by net radical destruction. Some experimental observations are presented concerning influence of temperature and of the matrix on the fading processes. These seem to argue in favor of such a model although a detailed, quantitative description is still not possible.

Comment on Electron Transfer vs Differential Decay in Irradiated DNA

“…For variations in the loading from 1MX /200 bp to 1MX /20bp, we found only a small change in our values of β which are slightly (ca. 0.1-0.2) lower at 1/200 than at 1/50, 1,5 and we find no change in the electron-transfer distance reported in bps at 1 min, which are consistent at loadings from 1/200 to 1/20. 1,5 The β values from higher loadings are considered the most reliable and yield consistent values of β for loadings of 1/20 to1/50.…”

“…A recent report by Pal and Hu ¨ttermann 1 proposes that radical fading explains the results we find in our series of papers on excess electron transfer in DNA. [2][3][4][5][6][7][8] In this comment, we point out that the substantial signal loss with time (fading) observed by Pal and Hu ¨ttermann did not occur in our work and therefore cannot explain our results.…”

“…Pal and Hu ¨ttermann reported their spectra were taken 30 min after the completion of irradiation. 1 Figure 1 shows no substantial loss in total radical signal intensity even out to long times as we reported. 2 We note that Pal and Hu ¨ttermann report a 50% to 60% drop in total radical intensity over this time period.…”

“…A recent report by Pal and Hüttermann proposes that radical fading explains the results we find in our series of papers on excess electron transfer in DNA. − In this comment, we point out that the substantial signal loss with time (fading) observed by Pal and Hüttermann did not occur in our work and therefore cannot explain our results. − We show in Figure the total double integrated total radical intensity vs time series for two samples reported in our first publication . These samples contain DNA intercalated with mitoxantrone (MX) at ca.…”

Characterization of condensed plasmid DNA models for studying the direct effect of ionizing radiation

“…It is possible that these reactions may not be well modeled by experimental systems that attenuate the indirect effect by dehydration and/or with cryogenic temperatures [21, 22, 50-52]. We argue that they are more likely to be faithfully reproduced by DNA condensed with a cationic ligand in dilute aqueous solution.…”