Label-Free Fluorescent Probing of G-Quadruplex Formation and Real-Time Monitoring of DNA Folding by a Quaternized Tetraphenylethene Salt with Aggregation-Induced Emission Characteristics

Biosensing processes such as molecular beacons require non‐trivial effort to covalently label or mark biomolecules. We report here a label‐free DNA assay system with a simple dye with aggregation‐induced emission (AIE) characteristics as the fluorescent bioprobe. 1,1,2,2‐Tetrakis[4‐(2‐bromoethoxy)phenyl]ethene is nonemissive in solution but becomes highly emissive when aggregated. This AIE effect is caused by restriction of intramolecular rotation, as verified by a large increase in the emission intensity by increasing viscosity and decreasing temperature of the aqueous buffer solution of 1,1,2,2‐tetrakis[4‐(2‐triethylammonioethoxy)phenyl]ethene tetrabromide (TTAPE). When TTAPE is bound to a guanine‐rich DNA strand (G1) via electrostatic attraction, its intramolecular rotation is restricted and its emission is turned on. When a competitive cation is added to the G1 solution, TTAPE is detached and its emission is turned off. TTAPE works as a sensitive poststaining agent for poly(acrylamide) gel electrophoresis (PAGE) visualization of G1. The dye is highly affinitive to a secondary structure of G1 called the G‐quadruplex. The bathochromic shift involved in the G1 folding process allows spectral discrimination of the G‐quadruplex from other DNA structures. The strong affinity of TTAPE dye to the G‐quadruplex structure is associated with a geometric fit aided by the electrostatic attraction. The distinct AIE feature of TTAPE enables real‐time monitoring of folding process of G1 in the absence of any pre‐attached fluorogenic labels on the DNA strand. TTAPE can be used as a K+ ion biosensor because of its specificity to K+‐induced and ‐stabilized quadruplex structure. Lighting up: Nonemissive TTAPE dye becomes emissive when bound to a guanine‐rich single‐stranded (ss) DNA molecule. The emission red‐shifts when the ssDNA folds into a G‐quadruplex structure. This folding process can be monitored by time‐dependent fluorescence measurement owing to the aggregation‐induced emission characteristics of the probe (see picture; PL=photoluminescence).