Modulating Excited Charge-Transfer States of G‑Quartet Self-Assemblies by Earth Alkaline Cations and Hydration

Branislav Milovanović, Ivana M. Stanković, Milena Petković, Mihajlo Etinski

Guanine self-assemblies are promising supramolecular platforms for optoelectronic applications. The study J. Phys. Chem. C 2012 116 14682-14689 reported that alkaline cations cannot modulate the electronic absorption spectrum of G-quadruplexes, although a cation effect is observable during electronic relaxation due to different mobility of Na+ and K+ cations. In this work, we theoretically examined whether divalent Mg2+ and Ca2+ cations and hydration might shift excited charge transfer states of a cation templated stacked G-quartets to the absorption red tail. Our results showed that earth alkaline cations blue-shifted nπ* states and stabilized charge transfer ππ* states relative to those of complexes with alkaline cations, although the number of charge separation states was not significantly modified. Earth alkaline cations were not able to considerably increase the amount of charge transfer states below the Lb excitonic states. Hydration shifted charge transfer states of Na+ coordinated G-octet to the absorption red tail, although this part of the spectrum was still dominated by monomer-like excitations. We found G-octet electron detachment states at low excitation energies in aqueous solution. These states were distributed over a broad range of excitation energies and could be responsible for oxidative damage observed upon UV irradiation of biological G-quadruplexes.