Abstract
Single crystals of a lipophilic G-quadruplex formed by 5‘-tert-butyl-dimethylsilyl-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-quadruplex. The quadruplex has a filled cation channel, with three K+ ions and 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 carbonyl oxygens to coordinate K+. The two (G 2)8-K+ octamers are of opposite polarity, being coaxially stacked in a head-to-head orientation. A Cs+ cation, with an unusual coordination geometry, caps the cation channel. The Cs+ is coordinated to four acetonitrile solvent molecules in an η2-fashion. Within an octamer the two tetramers are stacked so that they are 3.3 A apart and twisted by 30°. A second stacking interaction is defined by the head-to-head arrangement between the two (G 2)8-K+ octame...
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