General structural characteristics of G4s. (A) The cyclic arrangement of guanines in a canonical G-quartet or G-tetrad. G-quadruplexes (G4s) arise from stacking of two or more G-quartets. The glycosidic bond conformations anti and syn, as well as a cation occupying the central pore are noted. Black and yellow dashed lines denote hydrogen bond and metal ion coordination, respectively. (B) As in other nucleic acid structures, the pentose pucker can be C2′-endo or C3′-endo. In these two examples, the glycosidic bond angles are syn. (C) Various connectivities of G4 structures, which include all-parallel, anti-parallel, and mixed. Diamonds represent guanine bases. Four colors (red, orange, blue, green) denote the stacked guanines of three successive G-tracts or stacks. The loops connecting the all-parallel guanine stacks are of the propeller type. Lateral and diagonal loops are noted in the anti-parallel G4. Outline arrowheads in the backbone denote 5′-to-3′ chain direction. (D) Different local strand polarities (directions) of guanines in G-tracts. Three guanines of a G-tract are highlighted in cyan. In the left panel, the 5′ to 3′ direction of the riboses of the three guanines is the same. In the right panel, the direction of one of the guanines is opposite.
