A Lead-Filled G-Quadruplex:  Insight into the G-Quartet's Selectivity for Pb<sup>2+</sup> over K<sup>+</sup>

The lipophilic nucleoside, G <b>1</b>, extracts Pb<sup>2+</sup> picrate from water into organic solvents to give structures based on the hydrogen-bonded G-quartet. Crystal structures indicate important differences between (G <b>1</b>)<sub>8</sub>-Pb<sup>2+</sup> and (G <b>1</b>)<sub>8</sub>-K<sup>+</sup>. The divalent Pb<sup>2+</sup> templates a smaller G<sub>8</sub> cage than does K<sup>+</sup>, as judged by the M−O6 bond length, O6−O6 diagonal distance, and inter-tetramer separation. The more compact Pb<sup>2+</sup> octamer correlates with NMR data indicating that N2−N7 hydrogen bonds in (G <b>1</b>)<sub>8</sub>-Pb<sup>2+</sup> are kinetically more stable than in (G <b>1</b>)<sub>8</sub>-K<sup>+</sup>.