A Nitroxide-Tagged Platinum(II) Complex Enables the Identification of DNA G-Quadruplex Binding Mode.

We reported a novel strategy for investigating small molecule binding to G-quadruplexes (GQs). A newly synthesized dinuclear platinum(II) complex (PtL) containing a nitroxide radical was shown to selectively bind a GQ-forming sequence derived from human telomere (hTel). Using the nitroxide moiety as a spin label, electron paramagnetic resonance (EPR) spectroscopy was carried out to investigate binding between PtL and hTel GQ. Measurements indicated that two molecules of PtL bind with one molecule of hTel GQ. The inter-spin distance measured between the two bound PtL, together with molecular docking analyses, revealed that PtL predominately binds to the neighboring narrow and wide grooves of the G-tetrads as hTel adopts the antiparallel conformation. The design and synthesis of nitroxide tagged GQ binders, and the use of spin-labeling/EPR to investigate their interactions with GQs, will aid the development of small molecules for manipulating GQs involved in crucial biological processes.