K<sup>+</sup> Concentration-Dependent Conformational Change of Pb<sup>2+</sup>-Stabilized G-quadruplex

DNA can adopt a diverse range of structural conformations, including duplexes, triplexes, and quadruplexes.Among these structures, G-quadruplexes have attracted much more attention of researchers.For G-rich DNA sequences, they can fold into multiple G-quadruplex conformations, such as parallel, antiparallel, or hybrid, and the exact conformation is influenced by G-rich DNA sequence, strand concentration, and binding cations.Among the factors influencing the G-quadruplex conformation and stability, cations played a really important role.Numerous studies have reported cation-dependent stability and topological changes of G-quadruplexes; however, most of studies have focused on the effect of individual cation (such as charge, radii, or hydration, etc.), and only few have assessed the effect of competition between cations at different concentrations.Actually, most biological and aqueous systems contained multiple cations and each of the cations had very different concentrations.Thus, investigation of the competitions between different cations (at different concentrations) for binding with G-quadruplexes and their effects on polymorphism of G-quadruplex is critical, which would improve our understanding of the roles of G-quadruplexes and assist us in further exploring their potential applications in biochemical, biomedical, and environmental systems.Under this situation, we focused on K + -and Pb 2+ -stabilized G-quadruplex, two major cations that are usually used to stabilize G-quadruplex.It has been shown that for a given G-quadruplex forming DNA sequence, Pb 2+ -stabilized G-quadruplex was more stable than K + -stabilized G-quadruplex, and Pb 2+ could substitute K + in K + -stabilized G-quadruplex.However, the concentrations of K + that allow such a substitution are not completely studied.Previous studies have used G-quadruplex-based fluorescent, colorimetric, and electrochemical sensors for detecting Pb 2+ , and these methods show excellent selectivity for Pb 2+ over K + .Although G-quadruplex-based Pb 2+ sensors were developed, their applications in real samples containing K + were greatly limited.Thus, how K + and Pb 2+ compete for binding to G-quadruplex and how K + concentrations affect the stability of Pb 2+ -stabilized G-quadruplex remain elusive.In this study, eight G-rich DNA sequences were selected to investigate the effect of K + concentration on Pb 2+ -stabilized G-quadruplex.Previous studies have established that the presence of K + cannot alter the stability of Pb 2+ -stabilized G-quadruplex.In contrast to this, our results indicated that K + could induce a conformational switch in Pb 2+ -stabilized T2TT (G-rich DNA sequence, forming G-quadruplex in the presence of Pb 2+ ), and further replace Pb 2+ in Pb 2+ -stabilized T2TT and transform it into 2K + -stabilized T2TT, which is strictly K + concentration-dependent. Importantly, such a conformational switch could be observed for other seven selected G-rich sequences as well.Therefore, our findings provide a new insight into the exchange and competition of cations in G-quadruplex.