The Influence of Hydration on the Absolute Yields of Primary Free Radicals in Gamma-Irradiated DNA at 77 K. II. Individual Radical Yields

Abstract

In this report we present an electron spin resonance (ESR) investigation of the yields of the individual free radicals formed in gamma-irradiated frozen DNA as a function of hydration and dose at 77 K. Analysis of the ESR spectra taken at low hydration shows that the ion radical composition remains nearly constant with dose and that few secondary radicals are formed even at high doses (above about 50 kGy). For fully hydrated samples, the radical composition changes dramatically with dose. Thymine anion radical (T-.) is found in abundance at low doses but nearly disappears at higher doses, with a corresponding increase in the N-3 deuterated cytosine anion radical (CD.). Guanine cation radical (G+.) decreases at high doses, with a concomitant increase in secondary radical species (S.). Analysis of the dose-response data for G values (the yields in microM/J), k values (the destruction constants) and k' values (a new constant that characterizes the change in G with dose) was performed for each of the DNA base free radicals present at 77 K. The G value for each of the base radicals increases with the hydration level. The k values for CD. and G+. increase slightly with hydration; however, that of T-. increases substantially. Destruction constants for neutral radicals such as TH. and CD. are found to be substantially smaller than those for ion radicals and provide an indication of the radical charge state. A negative k' value for T-. and a positive k' value for CD. are explained in terms of radiation effects that result in the formation of a deuterated cytosine base, i.e., C(N3)D+, which greatly increases cytosine's electron affinity. The ratio of anion radical to cation radical concentrations is found to be about 1.6 and is invariant with hydration. A speculation on the imbalance based on hole-hole combinations in spurs is presented.

The Influence of Packing on Free Radical Yields in Solid-State DNA: Film Compared to Lyophilized Frozen Solution

“…In both film and lyophilized DNA, damage transfer occurs from the tightly bound hydration waters to the DNA molecule (6,7,11,12,16,32). The more extensive hydration of lyophilized DNA, at relative humidities greater than 75% (discussed above), suggests that the hydration waters are more widely (i.e.…”

“…[1][2][3][4][5]. Several variables influence the extent of direct-type damage experienced by DNA, including DNA hydration (6)(7)(8)(9)(10)(11)(12)(13)(14), conformation (6,8,9,11,14) and packing (10)(11)(12)15,16). The latter, DNA packing, reflects the aggregation of DNA molecules with each other, as well as the molecular order of the surrounding DNA hydration waters.…”

Theoretical Modeling of Radiation‐Induced DNA Damage

“…80,23,132 However, it is these carbon-centered sugar radicals that lead to the most important types of DNA damage (e.g., C 1 . sugar radicals result in strand break formation 24,[133][134][135][136] There are two principal mechanisms for the sugar radical formation: (i) the abstraction of a hydrogen atom from the sugar moiety by the hydroxyl radical (OH . )…”

“…As expected, experiments in aqueous phases using electron spin resonance (ESR) spectroscopy show that stable anion base radicals are found for all DNA bases. 80,81 The electron affinities of isolated DNA bases change on base-pair formation. In this context, Bowen and co-workers 85 studied the radical anions of AT and 9-methyl-A:1-methyl-T (MAMT) using photoelectron spectroscopy (PES) and B3LYP/6-31þG…”

“…Interestingly, the experimentally observed dipole bound states of all the pyrimidine (T, C, and U) radical anions in gas-phase transform to valence bound states on addition of a single water molecule and, as mentioned above, in an aqueous medium T, C, and U are predicted to have substantial positive valence electron affinities [77][78][79] as found experimentally. 80,81 The effect of bulk solvent on molecular ion radicals can be modeled by several methods including: (i) treating the solvent by the polarized continuum model (PCM) and (ii) placing a number of discrete water molecules around the molecule often followed by the PCM model for completeness. Though the former approach is simpler to study it lacks the hydrogen bonding between solute and solvent while the second approach takes into account the hydrogen bonding between solute and solvent (waters).…”