Toward artificial ion channels: A lipophilic G-quadruplex

Single crystals of a lipophilic G-quadruplex formed by 5'-tert-butyl-dimeth ylsilyl-2',3',-di-O-isopropylidene G 2 were obtained from a CH3CN solution containing potassium picrate and cesium picrate. The X-ray structure showed that 16 units of G 2 and 4 equiv of alkali picrate form the lipophilic G-q uadruplex. The quadruplex has a filled cation channel, with three K+ ions a nd one Cs+ ion located along its central axis. The quadruplex is formed by a pair of head-to-tail (G 2)(8)-K+ octamers. Both octamers use eight carbon yl oxygens to coordinate K+. The two (G 2)(8)-K+ octamers are of opposite p olarity, being coaxially stacked in a head-to-head orientation. A Cs+ catio n, with an unusual coordination geometry, caps the cation channel. The Csis coordinated to four acetonitrile solvent molecules in an eta(2)-fashion. Within an octamer the two tetramers are stacked so that they are 3.3 Angst rom apart and twisted by 30 degrees. A second stacking interaction is defin ed by the bend-to-head arrangement between the two (G 2)(8)-K+ octamers. Th is stacking, with a 90 degrees twist, positions the exocyclic amines of the central two quartets so that both exocyclic NH2(B) protons can hydrogen bo nd to the picrate anions that rim the quadruplex equator. The four picrates form an anionic belt that wraps around the cation channel. The sugars are well ordered in the structure. Circular dichroism spectra indicate that the G-quadruplex retains its helical structure in chlorinated solvents.