Radicals Generated in Tetramolecular Guanine Quadruplexes by Photoionization: Spectral and Dynamical Features

G-quadruplexes are four-stranded DNA structures playing a key role in many biological functions and are promising for applications in the field of nanoelectronics. Characterizing the generation and fate of radical cations (electron holes) within these systems is important in relation to the DNA oxidative damage and/or conductivity issues. This study focuses on guanine radicals in G-quadruplexes formed by association of four TGGGGT strands in the presence of Na⁺ cations, (TG4T)₄/Na⁺. Using nanosecond transient spectroscopy with 266 nm excitation, we quantitatively characterize hydrated ejected electrons and three types of guanine radicals. We show that, at an energy lower by 2.7 eV than the guanine ionization potential, one-photon ionization occurs with quantum yield of (3.5 ± 0.5) × 10^-3. Deprotonation of the radical cations is completed within 20 μs, leading to the formation of (G-H2)^• radicals, following a strongly nonexponential decay pattern. Within 10 ms, the latter undergoes tautomerization to deprotonated (G-H1)^• radicals. The dynamics of the various radicals determined for (TG4T)₄/Na⁺, in connection to those reported previously for telomeric G-quadruplexes TEL21/Na⁺, is correlated with energetic factors computed by quantum chemical methods. The faster deprotonation of radical cations in (TG4T)₄/Na⁺ compared to TEL21/Na⁺ explains that irradiation of the former does not generate 8-oxodGuo, which is readily detected by high-performance liquid chromatography/mass spectrometry in the case of TEL21/Na⁺.