Self-assembled ionophores from isoguanosine: diffusion NMR spectroscopy clarifies cation's and anion's influence on supramolecular structure

Tamar Evan-Salem, Limor Frish, F.W.B. van Leeuwen, David Reinhoudt, Willem Verboom, Mark S. Kaucher, Jeffery T. Davis, J.T. Davis, Yoram Cohen

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Abstract

Cation-templated self-assembly of the lipophilic isoguanosine (isoG 1) with different monovalent cations (M+=Li+, Na+, K+, NH4+, and Cs+) was studied in solvents of different polarity by using diffusion NMR spectroscopy. Previous studies that did not use diffusion NMR techniques concluded that isoG 1 forms both pentamers (isoG 1)5M+ and decamers (isoG 1)10M+ in the presence of alkali-metal cations. The present diffusion NMR studies demonstrate, however, that isoG 1 does not form (isoG 1)5M+ pentamers. In fact, the diffusion NMR data indicates that both doubly charged decamers of formula (isoG 1)102 M+ and singly charged decamers, (isoG 1)10M+, are formed with lithium, sodium, potassium, and ammonium tetraphenylborate salts (LiB(Ph)4, KB(Ph)4, NaB(Ph)4 and NH4B(Ph)4), depending on the isoG 1:salt stoichiometry of the solution. In the presence of CsB(Ph)4, isoG 1 affords only the singly charged decamers (isoG 1)10Cs+. By monitoring the diffusion coefficient of the B(Ph)4- ion in the different mixtures of solvents, we also concluded that the anion is more strongly associated to the doubly charged decamers (isoG 1)102 M+ than to the singly charged decamers (isoG 1)10M+. The (isoG 1)102 M+ species can, however, exist in solution without the mediation of the anion. This last conclusion was supported by the finding that the doubly charged decamers (isoG 1)102 M+ also prevail in 1:1 CD3CN:CDCl3, a solvent mixture in which the B(Ph)4- ion does not interact significantly with the self-assembled complex. These diffusion measurements, which have provided new and improved structural information about these decameric isoG 1 assemblies, demonstrate the utility of combining diffusion NMR techniques with conventional NMR methods in seeking to characterize labile, multicomponent, supramolecular systems in solution, especially those with high symmetry.
Original languageUndefined
Pages (from-to)1969-1977
Number of pages9
JournalChemistry: a European journal
Volume13
Issue number7
DOIs
Publication statusPublished - 2007

Keywords

  • METIS-241412
  • Supramolecular Chemistry
  • Self-Assembly
  • Anions
  • IR-72333
  • isoguanosine
  • diffusion NMR spectroscopy
  • cations

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