Cation Effect on the Electronic Excited States of Guanine Nanostructures Studied by Time-Resolved Fluorescence Spectroscopy

Abstract

International audienceThe effect of metal ions on the excited states of guanine nanostructures, short d(TG4T)4 quadruplexes and long G4-wires, are studied by fluorescence spectroscopy. The steadystate emission spectra show that both systems exhibit a strong cation effect. Fluorescence decays and fluorescence anisotropy decays, recorded from the femtosecond to the nanosecond timescale, reveal the following picture. In the presence of K+, emission arises mainly from delocalized ππ* states (excitons), whose decay spans several decades of times. In contrast, the fluorescence in the presence of Na+ is dominated by emission from charge transfer excited states decaying essentially on the subnanosecond time-scale. Such a difference is not due to the initially populated (Franck−Condon) states. The interproton distances derived from two-dimensional NMR measurements on the ground state of d(TG4T)4 show that the geometrical arrangement of guanines, governing the electronic coupling, is the same for both cations, in line with the UV absorption spectra. The observed cation effect is correlated with the excited state relaxation: the increased mobility of Na+ ions within the quadruplex favors trapping of ππ* excitons by charge transfer excited states, whereas such a process is hindered for the larger K+ ions. This is rationalized by quantum calculations on two stacked guanine tetrads

Quantum Mechanical Calculations Unveil the Structure and Properties of the Absorbing and Emitting Excited Electronic States of Guanine Quadruplex

“…Interestingly, we reproduce almost quantitatively the experimental hypochromic effect of the absorption maximum, which suggests that two stacked tetrads are a fairly reliable model also of longer species, especially when thinking that in TG 4 T edge effects and thermal fluctuations decrease the probability that a larger number of tetrads are strongly stacked. In agreement with experimental results, these features are similar for Na + and K + cations 33. The very good agreement with the experimental spectra indicates that M052X can model accurately the effect of the stacking and the hydrogen bonds on G excited states, and supports the reliability of our assignment.…”

“…25 On the other hand, a detailed atomistic description of the nature and properties of the absorbing excited states of G‐Quads, which are responsible for the peculiar optical and CD spectra,19–25 is still lacking. Analogously, experiments have shown that the minima responsible for the fluorescence spectra are very different from isolated G,32, 33 but their structures are completely unknown. Time‐resolved experiments show that the excited‐state lifetimes of G‐Quads are orders of magnitude greater than that of G,32, 33 but no description, even qualitative, of their decay paths is available, and the roles played by key factors such as the nature of the cation or the quadruplex topology have not yet been elucidated.…”

Steady-State and Time-Resolved Studies into the Origin of the Intrinsic Fluorescence of G-Quadruplexes

“…These characteristics were also observed in the detailed measurements of GQS emission by Markovitsi and co-workers, who studied structures formed from assembly of four DNA oligonucleotides 50,66,71 and from the human telomeric sequence Tel21 (dGGGTTAGGGTTAGGGTTAGGG). 57 The steadystate emission of Tel21 is compared to our default system dG 3 T in Figure 7.…”

Self-assembly of guanosine and deoxy-guanosine into hydrogels: monovalent cation guided modulation of gelation, morphology and self-healing properties

“…nanostructures and energy transfer in DNA quadruplexes, 71,72 to the best of our knowledge no report is available on the metal ion regulated blue uorescence of guanosine gels. The morphological insight of G and dG gels was characterized using transmission electron microscopy (TEM).…”