DNA in glasses at 77 K: high energy ionizing radiation versus UV electron ejection

Epr Studies on Radiation-Damaged Dna –the Contribution of Martyn Symons

This review presents a summary of Professor Martyn Symons’ role in radiation studies on the key biopolymer DNA, a contribution that cannot be overestimated. Some of his achievements are revealed, along with his influence on other research performed all over the world, illustrated by our own work on the effects of transition metal ions (iron and copper) and of thiols on radiation-induced damage to DNA.

Infrared characterization of the guanine radical cation: finger printing DNA damage

Oxidation of DNA represents a major pathway of genetic mutation. We have applied infrared spectroscopy in 77 K glass with supporting density functional theory (DFT) calculations (EDF1/6-31+G*) to provide an IR signature of the guanine radical cation G(+*), formed as a result of 193 nm photoionization of DNA. Deprotonation of this species to produce the neutral radical G(-H)(*) does not occur in 77 K glass. DFT calculations indicate that the formation of G(+*) within the double helix does not significantly perturb the geometry of the G/C pair, even though there is a significant movement of the N(1) proton away from G toward C. However, this is in stark contrast to drastic changes that are expected if full deprotonation of G/C occurs, producing the G(-H)(*)/C pair. These results are discussed in light of solution-phase time-resolved IR spectroscopic studies and demonstrate the power of IR to follow dynamics of DNA damage in natural environments.

Monitoring the direct and indirect damage of DNA bases and polynucleotides by using time-resolved infrared spectroscopy

The nucleotide 5'-dGMP and polynucleotide poly(dGdC).poly(dGdC) have been irradiated by using a 200-fs, 200-nm laser pulses and spectrally characterized by using time-resolved infrared spectroscopy. Under the experimental conditions, 200-nm excitation generates both electronic excited states and radical cations through photoionization; the former decay rapidly to vibrationally hot ground state. By using infrared signatures we have been able to follow these processes, and at time scales of >1 ns we observe an infrared marker band at 1,702 cm(-1) within both 5'-dGMP and the polynucleotide assigned to a photoionized product of guanine. This transient has also been reproduced through indirect chemistry through the reaction with photogenerated carbonate radical with 5'-dGMP. The ability to use time-resolved infrared spectroscopy in this way paves the way for developing solution-phase studies to investigate both direct and indirect radiation chemistry of DNA.

Origin of the heterogeneous distribution of the yield of guanyl radical in UV laser photolyzed DNA

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 5'-end radioactively labeled double-stranded and single-stranded 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 pi-stack, it can be assessed that the distance range of migration is <6-8 bp.

193 nm light induces single strand breakage of DNA predominantly at guanine

Irradiation of DNA with 193 nm light results in monophotonic photoionization, with the formation of a base radical cation and a hydrated electron (phi PI = 0.048-0.065). Although > 50% of the photoionization events initially occur at guanine in DNA, migration of the "hole" from the other bases to guanine occurs to yield predominantly its radical cation or its deprotonated form. From sequence analysis, the data reveal that 193 nm light induces single strand breaks (ssb) in double-stranded DNA preferential 3' to a guanine residue. However, it has previously been reported that 193 nm light yields very low yields of ssb (< 2% of the yield of e-aq). The distribution of these ssb at guanine is nonrandom, showing a dependence on the neighboring base moiety. The efficiency of ssb formation at nonguanine sites is estimated to be at least one order of magnitude lower. The preferred cleavage at guanine is consistent with migration and localization of the electron loss center at guanine. It is argued that singlet oxygen and the photoionized phosphate group of the sugar moiety are not major precursors to ssb. At present, the mechanisms of strand breakage are not known although a guanine radical or one of its products remain potential precursors.