Molecular Rotor-Based Fluorescent Probe for Selective Recognition of Hybrid G-Quadruplex and as a K+ Sensor

This work demonstrates the significant fluorescence enhancement of thioflavin T (ThT) when binding to G-quadruplexes possessing hybrid structures by using UV–vis absorption spectra, fluorescence spectra, and Tm experiments to confirm the binding events. ThT binding does not disturb native G-quadruplex structures preformed in Na+ and K+ solutions. The fluorescence enhancement is caused by the rotation restriction of benzothiazole (BZT) and dimethylaminobenzene (DMAB) rings in the ThT excited state upon its G-quadruplex binding. This molecular rotor mechanism as a means of fluorescence enhancement is confirmed using a nonrotor analogue of ThT. Hydroxylation and electrolyte experiments demonstrate that ThT stacks on the tetrad of the hybrid G-quadruplexes, whereas electrostatic forces contribute more to ThT binding for other G-quadruplex structures. By stacking on the tetrad, the ThT binding favors selective identification of DNA hybrid G-quadruplex structures with enhanced fluorescence and can serve as a conformation probe to monitor G-quadruplex structure conversion between hybrid and other structures. Using these properties, we developed a selective and label-free fluorescent K+ sensor with a detection limit of 1 mM for K+ in the presence of 100 mM Na+. The coexistence of other metal ions produces a fluorescence response comparable to K+ alone. We believe that ThT can potentially provide structure identification of hybrid G-quadruplexes and aid in the construction of G-quadruplex-based sensors.