Kr-86 Ion-Beam Irradiation of Hydrated DNA: Free Radical and Unaltered Base Yields

Abstract

This work reports an ESR and product analysis investigation of Kr-86 ion-beam irradiation of hydrated DNA at 77 K. The irradiation results in the formation and trapping of both base radicals and sugar phosphate radicals (DNA backbone radicals). The absolute yields (G, μmol/J) of the base radicals are smaller than the yields found in similarly prepared γ-irradiated DNA samples, and the relative yields of backbone radicals relative to base radicals are much higher than that found in γ-irradiated samples. From these results, we have elaborated our radiation chemical model of the track structure for ion-beam irradiated DNA as it applies to krypton ion-beams. The base radicals, which are trapped as ion radicals or reversibly protonated or deprotonated ion radicals, are formed almost entirely in the track penumbra, a region in which radiation chemical effects are similar to those found in γ-irradiated samples. By comparing the yields of base radicals in ion-beam samples to the yields of the same radicals in γ-irradiated samples, the partition of energy between the low-LET region (penumbra) and the core is experimentally determined. The neutral sugar and other backbone radicals, which are not as susceptible to recombination as are ion radicals, are formed largely in the track core. The backbone radicals show a linear dose response up to very high doses. Unaltered base release yields in Kr-86 irradiated hydrated DNA are equal to sugar radical yields within experimental error limits, consistent with radiation-chemical processes in which all base release originates with sugar radicals. Two phosphorus-centered radicals from fragmentation of the DNA backbone are found in low yields.

Gamma and ion-beam irradiation of DNA: Free radical mechanisms, electron effects, and radiation chemical track structure

“…ten times that found in γ-irradiated samples irradiated to the same dose (Becker et al, 2003, 2012; Adhikary et al, 2014a). This suggests that the LEE reactivity is largely a core process in the ion-beam irradiated samples.…”

“…In Kr-86 irradiated DNA, the G-values for of sugar radical formation are the same at 0.026±.001 μmol/J as the LET changes from 1080 keV/μm to 2000 keV/μm (Becker et al, 2012). At 4000 keV/μm, it drops to 0.0069 μmol/J, indicating that DNA• + (on the sugar backbone) is undergoing significant recombination with a proximate electron or DNA• − before deprotonation of DNA• + can occur, at this higher LET.…”

“…For both pathways, a frank single strand break occurs as a result, in agreement with the experiments that show that LEE causes strand breaks in plasmid DNA. In experiments with both Ar and Kr beams, the approximate ratio of the C3′• dephos to ROPO 2 • − formed is approximately 20:1 (Becker et al, 2003, 2012; Adhikary et al, 2014a). …”

“…Table 1 shows selected G-values for Argon and Krypton ion-beams as a function of LET and hydration (Becker et al 2003, 2012). From the yields of radicals, it has been possible to develop a model of the radiation chemical track structure in irradiated DNA that includes a description of the spatial arrangement of the radicals in the track and that allows for determination of the energy partition between the core and penumbra.…”

“…Figure 5 shows the ESR spectra of two radicals formed in DNA irradiated with Argon and with Krypton ion-beams (Becker et al 2003, 2012). Because of the hyperfine couplings from four protons, the C3′• dephos is characterized by a wide ESR spectrum of over 140 G, as shown in the Figure.…”