Structural and functional characterization of a complex of Zn(II)-substituted pheophorbide and all-parallel G-quadruplex DNA.

Zn(II)-substituted pheophorbide (ZnPhed ), a chlorophyll derivative, exhibited strong binding affinity with all-parallel G-quadruplex DNA (d[(TTAGGG)], 6mer). Fluorescence spectroscopy, H nuclear magnetic resonance (NMR) spectroscopy, and computational analysis were applied to characterize the resulting ZnPhed -6mer complex. Fluorescence titration experiments revealed that the apparent binding constant () of the ZnPhed -6mer complex was (1.8 ± 0.39) × 10 M, approximately 60 times greater than that of a complex of metal-free pheophorbide (Phed ) with 6mer (∼0.3 × 10 M). H NMR spectroscopy demonstrated selective binding of ZnPhed to the 3'-terminal G-quartet of G-quadruplex DNA, forming a stable complex. On the basis of a computational analysis, the high binding affinity was attributed to the localized positive charge of the Zn(II) center, which enhanced the electrostatic interactions with the electron-rich G-quartet. Upon red light (680 nm) irradiation of the ZnPhed -6mer complex under both aerobic and anaerobic conditions, the G-quadruplex DNA was degraded, indicating the ability of ZnPhed to induce DNA damage through both oxygen-dependent and independent mechanisms. This dual photoreactivity suggested that ZnPhed can serve as an effective photosensitizer for photodynamic therapy (PDT), particularly under hypoxic conditions that are unsuitable for traditional reactive oxygen species (ROS)-mediated techniques. These findings provide valuable insight for the design of next-generation photosensitizers targeting G-quadruplex structures.