Potassium Ions Enhance Guanine Radical Generation upon Absorption of Low-Energy Photons by G-Quadruplexes and Modify Their Reactivity

Abstract : G-Quadruplexes are formed by guanine rich DNA/RNA sequences in the presence of metal ions, which occupy the central cavity of these four-stranded structures. We show that these metal ions have a significant effect on the photogeneration and the reactivity of guanine radicals. Transient absorption experiments on G-quadruplexes formed by association of four TGGGGT strands in the presence of K$^+$ reveal that the quantum yield of one-photon ionization at 266 nm (8.1 × 10$^{–3}$) is twice as high as that determined in the presence of Na$^+$. Replacement of Na$^+$ with K$^+$ also suppresses one reaction path involving deprotonated radicals, (G-H2)$^•$ → (G-H1)$^•$ tautomerization. Such behavior shows that the underlying mechanisms are governed by dynamical processes, controlled by the mobility of metal ions, which is higher for Na$^+$ than for K$^+$. These findings may contribute to our understanding of the ultraviolet-induced DNA damage and optimize optoelectronic devices based on four-stranded structures, beyond DNA.