Figure 1.

Experimentally observed G-quadruplex structures, orientations, and conformations. (A) G4s are nucleic acid secondary structures consisting of four guanine bases organized in a square planar arrangement and connected through Hoogsteen hydrogen bonds. Within the central channel, dipole interactions between guanine O6 and a cation, usually K⁺, lend additional stability to the G4 structure. (B) There is great structural heterogeneity in the G4s, with these motifs capable of adopting parallel, antiparallel, or hybrid conformations in a unimolecular, bimolecular or tetramolecular manner.