Light-Controlled Folding and Chiroptical Sensing of G-Quadruplex DNA.

Combining reversible optical control with structural sensing of G-quadruplex (G4) DNA remains a significant challenge for nucleic acid-targeting photoswitches. Herein, we report the synthesis and photophysical characterization of a visible-light-responsive ortho-fluoroazobenzene derivative, exAzoPy2, designed to modulate G4 DNA structures. The interaction of exAzoPy2 with unfolded and folded G4 topologies was investigated using circular dichroism (CD) and fluorescence spectroscopy. The photoswitch exhibits isomer-dependent effects on antiparallel G4 structures: the cis-rich photostationary state promotes G4 folding, whereas photoinduced cis-to-trans isomerization favors the unfolded state. In addition to structural modulation, exAzoPy2 generates topology- and isomer-dependent induced circular dichroism (ICD) signals in the ligand absorption region. Distinct ICD responses depend on DNA conformation and G4 topology, enabling chiroptical discrimination of different G4 states. The combination of visible-light responsiveness, reversible structural modulation, and ICD-based spectroscopic readout highlights exAzoPy2 as a photoswitch capable of both controlling and sensing different G4 DNA architectures.