Figure 1.

G-quartet and G-quadruplex structures and topologies. (a) G-quartet structure, showing Hoogsteen-to-Watson-Crick face hydrogen bonds and the central dehydrated monovalent ion integral to formation and stabilization. Unimolecular (b) parallel and (c) antiparallel G-quadruplex topologies. Adapted from (1,6,10,11). Dark lines follow the nucleic acid strand, arrowheads denote strand directionality, and gray boxes denote quadruplexes. The examples drawn here are for sequences having three quartets.