Self‐Assembled Ionophores from Isoguanosine: Diffusion NMR Spectroscopy Clarifies Cation's and Anion's Influence on Supramolecular Structure

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.

Co‐existence of Distinct Supramolecular Assemblies in Solution and in the Solid State

“…A pulsed field gradient (PFG) NMR diffusion‐ordered spectroscopy (DOSY) analysis of NH peaks showed measurably different diffusion constants for K⋅Pic and Sr⋅(Pic) 2 complexes (see Table 1 and Figure 3 c and d) 14e, 20a, 21. Changes in the measured diffusion coefficients ( D ) also reveal a variation in the binding affinity of picrate ions, that is, picrate binds weakly to GaceC10⋅K + complexes and strongly to the GaceC10⋅Sr 2+ complex.…”

Applications of NMR diffusion methods with emphasis on ion pairing in inorganic chemistry: a mini‐review

“…This idea of using equivalent diffusion coefficients from two very different species also finds applications in the areas of detecting and monitoring encapsulation phenomena, and one example is shown in Fig. .…”

Differential stabilization of adenine quartets by anions and cations

“…Anion binding to nucleobases, a feature quite common in positively charged model nucleobases (charge brought about by metal coordination or protonation), is even observed in real DNA and RNA structures [23]. There are also scattered reports on association patterns between supramolecular metal–nucleobase assemblies and anions, with anions interacting with the periphery however [24, 25]. Recent calculations on the stability of a NaCl ion pair bonded to a stack composed of G 4 (with its known affinity for Na + ) and A 4 of A 4 –N7 further corroborate such a possibility [26].…”