Function-preserving photooxidation of AS1411 aptamer by a G-quadruplex-guided photosensitizer

AS1411 is a G-quadruplex (G4) aptamer that binds nucleolin (NCL) and is widely used to promote cancer-associated delivery of photoactive cargos. Yet the fate of AS1411 after photosensitizer loading and light activation remains unclear as reactive oxygen species (ROS) generation could disrupt its guanine-rich fold and abolish NCL recognition, or remain locally confined while preserving targeting function. Here, we address this question using DBI-POE, a heavy-atom-free dibenzothioxanthene imide photosensitizer combining G4 recognition, fluorescence activation, and ROS generation. In this system, AS1411 converts weakly emissive, radical-prone DBI-POE aggregates into a fluorescent G4-bound complex with singlet oxygen (¹O₂)-biased reactivity and attenuated radical-probe responses. ¹H NMR spectroscopy, quantum-mechanical (QM) calculations, molecular dynamics (MD) simulations, and primer-extension mapping support preferential DBI-POE association with the 3'-terminal tetrad region and are consistent with a partially intercalated binding model. Upon irradiation, this complex undergoes site-localized guanine oxidation. This damage is suppressed under non-G4-forming conditions and in G4-disrupted controls. Remarkably, oxidation preserves the BG4-recognized G4 epitope and full-length NCL binding, defining a DBI-POE-specific, function-preserving mode of aptamer-guided photosensitization.