Comprehensive Study of Guanine Excited State Relaxation and Photoreactivity in G-quadruplexes

Abstract

G-quadruplexes (G4) are four-stranded DNA/RNA structures playing a key role in many biological functions and promising for nanotechnology applications. Here, combining theoretical calculations and multiscale time-resolved fluorescence, we describe, for the first time, an ensemble of photoactivated processes involving the guanines of the G4 core. We use as showcase the G4 formed by the human telomeric sequence GGG­(TTAGGG)3 in the presence of Na+ ions. According to quantum mechanical/molecular mechanics calculations, the hyperchromism at the red part of the absorption spectrum, typical of G4 structures, arises mainly from the inner Na+ ions. Various relaxation pathways, leading to excited states localized on individual bases, neutral excimers, and excited charge transfer states between two guanines or a guanine and a thymine in the loop, are mapped. Their fingerprints are detected in the fluorescence anisotropies and the fluorescence decays, spanning five decades of time. Finally, a reaction funnel leading to guanine dimerization is identified.

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

“…Not only the excitation intensity but also the presence of oxygen has no effect on the decays ( Figure S2b); this observation concerns both the radical decays and those of the reaction intermediate and final photoproduct. Regarding the latter, we remark that the steady-state differential absorption spectrum determined for TEL21/K + was similar to that reported for TEL21/Na + (Figure S3), which was tentatively attributed to guanine dimers [21]. However, in the case of TEL21/K + , the spectral intensity, normalized by the number of absorbed photons, was three times lower compared to that of TEL21/Na + , suggesting a lower quantum yield of the associated photo-reaction.…”

“…(iv) ( G -H2) ● → ( G -H1) ● tautomerization can be detected at 720 nm. Changes in the UV spectral range are not informative because, on the one hand, G -quadruplexes absorb at longer wavelengths compared to monomeric guanosine [ 8 ], and, on the other, photon absorption may induce formation of photo-dimers [ 8 , 21 ]; some of them absorb in the UVA region, and their dynamics intervenes on the millisecond time-scale [ 25 , 26 ].…”

“…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]. A small fraction of the CT states may undergo charge migration, followed by charge separation.…”

“…(iv) (G-H2) • → (G-H1) • tautomerization can be detected at 720 nm. Changes in the UV spectral range are not informative because, on the one hand, G-quadruplexes absorb at longer wavelengths compared to monomeric guanosine [8], and, on the other, photon absorption may induce formation of photodimers [8,21]; some of them absorb in the UVA region, and their dynamics intervenes on the millisecond time-scale [25,26]. In order to get an insight into the species responsible for the transient absorption in Figure 3, we considered the spectra of the three types of radicals reported in the literature for monomeric guanosines [23,24].…”

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

“…A recent computational study on TEL21/Na + identified two types of CT states, between two guanines of the Gquadruplex core on the one hand, and between a guanine and the thymine of the loop, on the other. 22 Experimentally, the formation of low energy CT states during the excited state relaxation in G-quadruplexes has been evidenced by time-resolved fluorescence anisotropy studies. 9, 22-25 A low energy band is also present in the steady-state fluorescence spectra of dinucleotides dGpT ( Figure SI-4).…”