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+.