Deprotonation Dynamics of Guanine Radical Cations^†

This review is dedicated to guanine radical cations (G⁺)^· that are precursors to oxidatively generated damage to DNA. (G⁺)^· are unstable in neutral aqueous solution and tend to lose a proton. The deprotonation process has been studied by time-resolved absorption experiments in which (G⁺)^· radicals are produced either by an electron abstraction reaction, using an external oxidant, or by low-energy/low-intensity photoionization of DNA. Both the position of the released proton and the dynamics of the process depend on the secondary DNA structure. While deprotonation in duplex DNA leads to (G-H1)^· radicals, in guanine quadruplexes the (G-H2)^· analogs are observed. Deprotonation in monomeric guanosine proceeds with a time constant of ~60 ns; in genomic DNA, it is completed within 2 µs; and in guanine quadruplexes, it spans from at least 30 ns to over 50 µs. Such a deprotonation dynamics in four-stranded structures, extended over more than three decades of times, is correlated with the anisotropic structure of DNA and the mobility of its hydration shell. In this case, commonly used second-order reaction models are inappropriate for its description.