钾离子浓度依赖的铅离子稳定G-四链体构型转化

已有研究普遍认为铅离子(Pb²⁺)诱导富G适体链形成的G-四链体(Pb²⁺-G4)比钾离子(K⁺)诱导富G适体链形成的G-四链体(K⁺-G4)更为稳定，因而Pb²⁺可以置换K⁺-G4中的K⁺，而且K⁺的存在不影响Pb²⁺-G4的稳定性。有趣的是本研究发现K⁺ (20 μmol?L^?1–1 mmol?L^?1)不仅可以诱导10 µmol?L^?1 Pb²⁺稳定的T2TT(Pb²⁺-T2TT，杂合G4结构)发生构型转换，甚至还可取代Pb²⁺-T2TT中的Pb²⁺，形成K⁺稳定的T2TT (K⁺-T2TT，平行G4结构)，最终转化形成的K⁺-G4结构与单独K⁺诱导富G适体链形成K⁺-G4的构型基本一致。随后，进一步考察了另外7条富G适体链，发现这一转化过程具有一定的普适性。该研究结果为理解G4构型转化以及内嵌离子交换提供了新的视角，也为拓展G4在生化分析和生物领域的应用提供了新的理论基础。

K+ Concentration-Dependent Conformational Change of Pb2+-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 Pb²⁺-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, Pb²⁺-stabilized G-quadruplex was more stable than K⁺-stabilized G-quadruplex, and Pb²⁺ 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 Pb²⁺, and these methods show excellent selectivity for Pb²⁺ over K⁺. Although G-quadruplex-based Pb²⁺ sensors were developed, their applications in real samples containing K⁺ were greatly limited. Thus, how K⁺ and Pb²⁺ compete for binding to G-quadruplex and how K⁺ concentrations affect the stability of Pb²⁺-stabilized G-quadruplex remain elusive. In this study, eight G-rich DNA sequences were selected to investigate the effect of K⁺ concentration on Pb²⁺-stabilized G-quadruplex. Previous studies have established that the presence of K⁺ cannot alter the stability of Pb²⁺-stabilized G-quadruplex. In contrast to this, our results indicated that K⁺ could induce a conformational switch in Pb²⁺-stabilized T2TT (G-rich DNA sequence, forming G-quadruplex in the presence of Pb²⁺), and further replace Pb²⁺ in Pb²⁺-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.