One-Photon Ionization of DNA: from electron ejection to oxidative damage – OPHID

Combined experimental and theoretical approach based on (i) steady-state and time-resolved absorption spectroscopy, (ii) chromatographic quantification of final products and (iii) quantum chemistry methods. OPHID involves work with single and double strands. As guanine runs are known to have the lowest ionization potential among the DNA bases and are encountered in DNA telomeric sequences, the study focuses also on guanine quadruplexes

Our study showed clearly that absorption of low energy UV photons directly by DNA may generate base radicals with a probability 10-3. Such a probability is comparable to that corresponding to formation of other well-known lesions induced by the same energy UV radiation. Base radicals disappear on the millisecond timescale and their lifetime is longer in double helices compared to single strands. Oxidation markers have been detected following both UVC and UVB irradiation of genomic DNA.

R. Improta, OPHID partner, obtained a «Chaire d'Alemebert (Idex)« at ’Université Paris Saclay. He will be hosted at LIDYL (2017-2018); research project: “Towards the study of the interaction between Nucleic Acids and protein by Time Resolved optical methods: insights from Quantum Mechanical calculations”

A COST project entitled: «EU-DNA network : understanding damage repair and developing novel DNA sensitizers«, to which all three OPHID partners participate, has been submitted.

The results obtained in the frame of OPHID have given 17 articles published in international journals and presented in 12 international conferences (11 invited lectures). A special effort was made to communicate the outcomes of our research to photobiologists, in particular thought publications and conferences specific to this community.

The loss of an electron from DNA is one of the primary events leading to damage of the genetic code. It is known to occur through photosensitized processes involving other cellular components as mediators or by the direct effect of ionizing radiation. Yet, electron ejection can also take place following absorption of a single UV photon, provided that its energy is sufficiently high. In the case of DNA bases, this process has been extensively studied for excitation up to ca. 200 nm. However, a few studies have shown that electron ejection from DNA can be triggered by photons corresponding to the lowest absorption band peaking around 260 nm with a weak intensity tail till 400 nm. Thus, the UVA and UVB components of the solar light reaching the surface of the Earth could behave as direct oxidants, in addition to their ability to produce free radicals in cells, and to induce dimerisation of pyrimidine bases in DNA. Consequently, understanding the detailed mechanism(s) of the UVA and UVB action on DNA is an important public health issue and requires ground-breaking fundamental research.
The objective of the present project is to study the factors that govern one-photon ionization of DNA in aqueous solution and describe successive events leading to the formation of final products. To this end, we will use a combined experimental and theoretical approach based on (i) steady-state and time-resolved absorption spectroscopy, (ii) chromatographic quantification of final products and (iii) quantum chemistry methods. We will work with single and double strands. As guanine runs are known to have the lowest ionization potential among the DNA bases and are encountered in DNA telomeric sequences, we will also focus on guanine quadruplexes.
The three participating teams have gained an international recognition for their recent contribution to DNA photophysics and photochemistry. The group of D. Markovitsi (team 1) was the first one to demonstrate the collective behaviour of DNA bases within helices and realize a series of pioneering time-resolved experiments from the femtosecond to the millisecond time scales. The group of T. Douki (team 2) has developed state-of-art analytical tools for the characterization of DNA lesions. R. Improta (team 3, international collaborator) has achieved full quantum mechanical description of the excited state relaxation of DNA components. Team 1 has long-standing and fruitful collaborations in the field of DNA with both team 2 and team 3.
Our strategy will consist of a three-step approach. First we will monitor the hydrated electron in order to determine the quantum yields for its formation after irradiation in the UVA, UVB and UVC spectral domains. These experimental results, obtained for various base sequences, will be correlated with the corresponding calculated ionization potentials. In a second step, we will quantify a wide array of well-known oxidative lesions and determine the quantum yield of formation of these photoproducts as a function of the irradiation wavelength. Finally, knowing the UV absorption spectra of the relevant photoproducts, we will search for their fingerprints in the time-resolved signals and, thus, establish the reaction dynamics. In parallel, quantum chemical calculations will provide a description of the reaction paths on the atomic level.