Figure 1.

(A) Top and (B) side view of the crystal structure of the tetrameric G-quadruplex [d(TGGGGT)]₄ (PDB code 1s45). Nucleotides are shown as sticks colored by atom type and backbone as gray cartoon. K⁺ ions are depicted as purple spheres. Hydrogen bonds are represented as dashed black lines. (C) Chemical structure of compound 1 at physiological pH 7.4 (see ‘Materials and Methods’).