Origin of the Heterogeneous Distribution of the Yield of Guanyl Radical in UV Laser Photolyzed DNA

Abstract

Oxidative guanine lesions were analyzed, at the nucleotide level, within DNA exposed to nanosecond ultraviolet (266 nm) laser pulses of variable intensity (0.002-0.1 J/cm 2). Experiments were carried out, at room temperature, in TE buffer (20 mM Tris-HCl, pH 7.5; 1 mM EDTA) containing 35 mM NaCl, on 59-end radioactively labeled double-stranded and singlestranded oligomer DNA at a size of 33-37 nucleobases. Lesions were analyzed on polyacrylamide gel electrophoresis by taking advantage of the specific removal of 8-oxodG from DNA by the formamidopyrimidine DNA glycosylase (Fpg protein) and of the differential sensitivity of 8-oxodG and oxazolone to piperidine. The quantum yields of lesions at individual sites, determined from the normalized intensities of bands, were plotted against the irradiation energy levels. Simplified model fitting of the experimental data enabled to evaluate the spectroscopic parameters characterizing excitation and photoionization processes. Results show that the distribution of guanine residues, excited to the lowest triplet state or photoionized, is heterogeneous and depends on the primary and secondary DNA structure. These findings are generalized in terms of excitation energy and charge-migration mediated biphotonic ionization. On the basis of the changes in the yield of the guanyl radical resulting from local helical perturbations in the DNA p-stack, it can be assessed that the distance range of migration is ,6-8 bp.

Guanine Holes Are Prominent Targets for Mutation in Cancer and Inherited Disease

“…Guanines with the lowest ionization potentials, as determined by neighboring bases, are the strongest hole-attracting sites. The resulting radical cations (G •+ ) are then expected to undergo a number of chemical modifications, leading to a variety of stably modified bases, including 8-oxoG, a key toxicological lesion, oxazolone, imidazolone and others, some of which can result in base changes during DNA replication if left unrepaired [22], [65]–[67]. Guanine-protein crosslinks may also lead to SBSs [68].…”

Binding of NF-κB to Nucleosomes: Effect of Translational Positioning, Nucleosome Remodeling and Linker Histone H1

“…Additionally, in contrast to conventional light sources, high intensity laser irradiation induces specific biphotonic oxidative lesions in DNA (in addition to monophotonic pyrimidine dimers) [74]. These lesions are extremely sensitive to local DNA structure and can be easily mapped by enzymatic DNA strand cleavage followed by electrophoresis under denaturing conditions [75], [76]. In our study we have mapped the UV laser specific biphotonic lesions 8-oxoG by Fpg glycosylase and the monophotonic cyclobutane pyrimidine dimers (CPDs) by T4 Endonuclease V cleavage, both generated in the NF-κB cognate sequence upon UV laser irradiation.…”

Biphotonic Ionization of DNA: From Model Studies to Cell

“…In contrast, competitive hydration and formation of 8‐oxodGuo are favored in well‐stacked regions, including repeated G runs. The factor responsible for the observed selective biphotonic generation and its heterogeneous distribution under ns laser photolysis of DNA has been tentatively attributed to the occurrence of S 1 and/or T 1 excited state energy transfer followed by hole migration and subsequent trapping by guanine bases . It is noteworthy that these nonradiative phenomena are strongly sequence‐ and structure‐dependent.…”

“…Indeed, the occurrence of excited state exciton formation and nonradiative energy migration as well as long‐range hole migration with localization at guanine residues was suggested to explain the heterogeneity of excitation/charge distribution within DNA; however, several aspects still remain matters of debate and controversy. The sequence/secondary structure‐dependent heterogeneity of distribution upon laser biphotonic photolysis of DNA was rationalized on the basis of kinetic equations analysis, bringing into play electronic excitation nonradiative energy transfer‐mediated biphotonic excitation and hole migration toward guanines . Briefly, the following working model for nanosecond photolysis of DNA has been proposed .…”

“…The sequence/secondary structure‐dependent heterogeneity of distribution upon laser biphotonic photolysis of DNA was rationalized on the basis of kinetic equations analysis, bringing into play electronic excitation nonradiative energy transfer‐mediated biphotonic excitation and hole migration toward guanines . Briefly, the following working model for nanosecond photolysis of DNA has been proposed . In the initial step, an arbitrary base A, T, C or G is excited by one‐photon absorption to S 1 that subsequently is converted to T 1 by intersystem crossing.…”