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This chapter discusses DNA photoionization in aqueous solution resulting from direct absorption of ultraviolet (UV) radiation. While DNA photoionization at wavelengths shorter than 200 nm was reported in the 1990s, recent studies showed that it also takes place at much longer wavelengths, with efficiencies depending strongly on the secondary structure. The quantum yield of one-photon ionization determined for duplex genomic DNA at 266 nm is 2 × 10−3 and significantly higher, reaching 10−2, for guanine quadruplexes. The transient species issued from low-energy photoionization were studied by nanosecond flash photolysis from ∼30 nanoseconds to 300 milliseconds. At this time window, the ejected electrons are hydrated and the radicals are located on guanines or adenines. The quasi entire population of radical cations in genomic DNA undergoes deprotonation within 2 µs. Deprotonation in guanine quadruplexes is a highly anisotropic process, taking place from 30 ns to over 50 µs.

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