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

The 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 steady-state emission spectra show that both systems exhibit a strong cation effect. Fluorescence decays and fluorescence anisotropy decays, recorded from the femtosecond to the nanosecond time-scale, 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.