UVA-induced cyclobutane pyrimidine dimers in DNA: a direct photochemical mechanism?

International audienceThe carcinogenic action of UVA radiation is commonly attributed to DNA oxidation mediated by endogenous photosensitisers. Yet, it was recently shown that cyclobutane pyrimidine dimers (CPD), well known for their involvement in UVB genotoxicity, are produced in larger yield than oxidative lesions in UVA-irradiated cells and skin. In the present work, we gathered mechanistic information on this photoreaction by comparing formation of all possible bipyrimidine photoproducts upon UVA irradiation of cells, purified genomic DNA and dA20:dT20 oligonucleotide duplex. We observed that the distribution of photoproducts, characterized by the sole formation of CPD and the absence of (6-4) photoproducts was similar in the three types of samples. The CPD involving two thymines represented 90% of the amount of photoproducts. Moreover, the yields of formation of the DNA lesions were similar in cells and isolated DNA. In addition, the effect of the wavelength of the incident photons was found to be the same in isolated DNA and cells. This set of data shows that UVA-induced cyclobutane pyrimidine dimers are formed via a direct photochemical mechanism, without mediation of a cellular photosensitiser. This is possible because the double-stranded structure increases the capacity of DNA bases to absorb UVA photons, as evidenced in the case of the oligomer dA20:dT20. These results emphasize the need to consider UVA in the carcinogenic effects of sunlight. An efficient photoprotection is needed that can only be complete by completely blocking incident photons, rather than by systemic approaches such as antioxidant supplementation

Guanine Radicals Induced in DNA by Low-Energy Photoionization

International audienceCONSPECTUS. Guanine (G) radicals are precursors to DNA oxidative damage, correlated with carcinogenesis and aging. During the past few years, we demonstrated clearly an intriguing effect: G radicals can be generated upon direct absorption of UV radiation with energy significantly lower than the G ionization potential. Using nanosecond transient absorption spectroscopy, we studied the primary species, ejected electrons and guanine radicals, which result from photoionization of various DNA systems in aqueous solution. The DNA propensity to undergo electron detachment at low photon energies greatly depends on its secondary structure. Non-detected for monomers or unstacked oligomers, it may be one order of magnitude higher for G-quadruplexes than for duplexes. The experimental results point toward non-vertical processes, associated with the relaxation of electronic excited states. Theoretical studies are required to validate the mechanism and determine the factors that enter in play. In any case, such a mechanism, which may be operative over a broad excitation wavelength range, explains the occurrence of oxidative damage observed upon UVB and UVA irradiation. Quantification of G radical populations and their time evolution questioned some widespread views. It appeared that G radicals may be generated with the same probability as pyrimidine dimers, which are considered to be the major lesions induced upon absorption of low-energy UV radiation by DNA. As the most important part of the initially created radical cations undergoes deprotonation, the vast majority of the final reaction products is expected to stem from long-lived deprotonated radicals. Consequently, the widely used oxidation marker 8-oxodG is not representative of the extent of oxidative damage when this is triggered by generation of radical cations. Beyond the biological consequences, photo-generation of electron holes in G-quadruplexes may inspire applications in nano-electronics; although four-stranded structures are currently studied as molecular wires, their behavior as photoconductors has not been explored so far.In the present Account, after highlighting some key experimental issues, we first describe the photo-ionization process and, then, we focus on radicals. We use as show-cases new results obtained for genomic DNA and Oxytricha G-quadruplexes. Generation and reaction dynamics of G radicals in these systems provide a representative picture of the phenomena reported previously for duplexes and G-quadruplexes, respectively

UV-Induced Adenine Radicals Induced in DNA A-Tracts: Spectral and Dynamical Characterization

International audienceAdenyl radicals generated in DNA single and double strands, (dA)20 and (dA)20·(dT)20, by one- and two-photon ionization by 266 nm laser pulses decay at 600 nm with half-times of 1.0 ± 0.1 and 4 ± 1 ms, respectively. Though ionization initially forms the cation radical, the radicals detected for (dA)20 are quantitatively identified as N6-deprotonated adenyl radicals by their absorption spectrum, which is computed quantum mechanically employing TD-DFT. Theoretical calculations show that deprotonation of the cation radical induces only weak spectral changes, in line with the spectra of the adenyl radical cation and the deprotonated radical trapped in low temperature glasses