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.

Guanine Radicals Generated in Telomeric G-Quadruplexes by Direct Absorption of Low-Energy UV Photons: Effect of Potassium Ions

“…Another process, (G-H2) • → (G-H1) • tautomerization, was observed on the millisecond time scale for both TEL21/Na + [8] and (TG 4 T) 4 /Na + [9]. In contrast, we found that tautomerization is completely inhibited in the case of (TG 4 T) 4 /K + , showing that K + ions modify the reactivity of guanine radicals [11,12]. This is an important issue, because, in principle, each type of radical should lead to different final lesions, although those resulting from (G-H2) • have not been identified so far.…”

“…The φ1 values, gathered in Table 1, indicate small variations with the number of strands composing the structure or the ending groups, but quite significant increase when Na + ions are replaced by K + . This cation effect was observed for tetramolecular quadruplexes formed by the same sequence and exhibiting the same topology [9,11,12]. Therefore, it was important to explore whether a similar φ1 enhancement also occurs for other types of G-quadruplexes.…”

“…Thus, replacing Na + by K + ions enhanced by a factor 2 the propensity of telomeric G-quadruplexes to photo-eject an electron, as in the case of tetramolecular systems ( Table 1). The mechanism proposed to explain the φ 1 enhancement is related to a non-vertical electron photo-detachment and is described in reference [11]. Briefly, charge transfer (CT) states are formed during the excited state relaxation [20][21][22].…”

“…The term 'low-energy' is used in comparison to the ionization potential of the various components of nucleic acids, which preclude vertical electron photo-detachment upon irradiation around the absorption maximum (~260 nm). Yet, one-photon ionization quantum yields (φ 1 ) ranging from 3.5 × 10 −3 to 8.1 × 10 −3 were determined at 266 nm for four different G-quadruplex structures [8][9][10][11][12]. These structures were formed by folding of the telomeric sequences GGG(TTAGGG) 3 and TAGGG(TTAGGG) 3 TT in the presence of Na + ions (TEL21/Na + and A large number of articles reports oxidative damage and/or charge transport in G-quadruplexes, in which G radical generation is mediated by other molecules.…”

DNA Nucleobase under Ionizing Radiation: Unexpected Proton Transfer by Thymine Cation in Water Nanodroplets

“…The loss of ap roton from the guanidinium radicalc ation, that is, an intermediate in guanine oxidative lesion production, is indeed recognized as avery fast and ubiquitous process, [42][43][44] and has recently been related to the formation of guanineamine cross-links, as evidenced by AIMD. [45] Proton transfer is also observed, at low temperature, in cationic adenine-thymine stacks,i nitiating important competition to catch the protonb y different electron lone pairs in the surrounding environment.…”