Telomeres, which are involved in cell division, carcinogenesis, and aging and constitute important therapeutic targets, are prone to oxidative damage. This propensity has been correlated with the presence of guanine-rich sequences, capable of forming four-stranded DNA structures (G-quadruplexes). Here, we present the first study on oxidative damage of human telomere G-quadruplexes without mediation of external molecules. Our investigation has been performed for G-quadruplexes formed by folding of GGG(TTAGGG)3 single strands in buffered solutions containing Na+ cations (TEL21/Na+). Associating nanosecond time-resolved spectroscopy and quantum mechanical calculations (TD-DFT), it focuses on the primary species, ejected electrons and guanine radicals, generated upon absorption of UV radiation directly by TEL21/Nat We show that, at 266 nm, corresponding to an energy significantly lower than the guanine ionization potential, the one-photon ionization quantum yield is 4.5 x 10(-3). This value is comparable to that of cyclobutane thymine dimers (the major UV-induced lesions) in genomic DNA; the quantum yield of these dimers in TEL21/Na+ is found to be (1.1 +/- 0.1) x 10(-3). The fate of guanine radicals, generated in equivalent concentration with that of ejected electrons, is followed over 5 orders of magnitude of time. Such a quantitative approach reveals that an important part of radical cation population survives up to a few milliseconds, whereas radical cations produced by chemical oxidants in various DNA systems are known to deprotonate, at most, within a few microseconds. Under the same experimental conditions, neither one-photon ionization nor long-lived radical cations are detected for the telomere repeat TTAGGG in single-stranded configuration, showing that secondary structure plays a key role in these processes. Finally, two types of deprotonated radicals are identified: on the one hand, (G-1-12)* radicals, stable at early times, and on the other hand, (G-H-1)(center dot) radicals, appearing within a few milliseconds and decaying with a time constant of similar to 50 ms.