A Lead-Filled G-Quadruplex:  Insight into the G-Quartet's Selectivity for Pb²⁺ over K⁺

The lipophilic nucleoside, G 1, extracts Pb²⁺ picrate from water into organic solvents to give structures based on the hydrogen-bonded G-quartet. Crystal structures indicate important differences between (G 1)₈-Pb²⁺ and (G 1)₈-K⁺. The divalent Pb²⁺ templates a smaller G₈ cage than does K⁺, as judged by the M−O6 bond length, O6−O6 diagonal distance, and inter-tetramer separation. The more compact Pb²⁺ octamer correlates with NMR data indicating that N2−N7 hydrogen bonds in (G 1)₈-Pb²⁺ are kinetically more stable than in (G 1)₈-K⁺.