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.
- Supramolecular Chemistry
- diffusion NMR spectroscopy