Self-Assembled Ionophores from Isoguanosine: Diffusion NMR Spectroscopy Clarifies Cation's and Anion's Influence on Supramolecular Structure
✍ Scribed by Tamar Evan-Salem; Limor Frish; Fijs W. B. van Leeuwen; David N. Reinhoudt; Willem Verboom; Mark S. Kaucher; Jeffery T. Davis; Yoram Cohen
- Publisher
- John Wiley and Sons
- Year
- 2007
- Tongue
- English
- Weight
- 475 KB
- Volume
- 13
- Category
- Article
- ISSN
- 0947-6539
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✦ Synopsis
Abstract
Cation‐templated self‐assembly of the lipophilic isoguanosine (isoG 1) with different monovalent cations (M^+^=Li^+^, Na^+^, K^+^, NH~4~^+^, 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)~5~⋅M^+^ and decamers (isoG 1)~10~⋅M^+^ in the presence of alkali‐metal cations. The present diffusion NMR studies demonstrate, however, that isoG 1 does not form (isoG 1)~5~⋅M^+^ pentamers. In fact, the diffusion NMR data indicates that both doubly charged decamers of formula (isoG 1)~10~⋅2 M^+^ and singly charged decamers, (isoG 1)~10~⋅M^+^, are formed with lithium, sodium, potassium, and ammonium tetraphenylborate salts (LiB(Ph)~4~, KB(Ph)~4~, NaB(Ph)~4~ and NH~4~B(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)~10~⋅Cs^+^. 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)~10~⋅2 M^+^ than to the singly charged decamers (isoG 1)~10~⋅M^+^. The (isoG 1)~10~⋅2 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)~10~⋅2 M^+^ also prevail in 1:1 CD~3~CN:CDCl~3~, 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.