Kinetic control in noncovalent synthesis: regioselective ligand exchange into a hydrogen bonded assembly.

This paper illustrates the use of a kinetically controlled exchange reaction to effect regioselective modification of a hydrogen-bonded assembly. Both the bound anion and cation can control the exchange of ligand into the different layers of a synthetic G-quadruplex.

Self-assembly of Janus dendrimers into uniform dendrimersomes and other complex architectures.

Self-assembled nanostructures obtained from natural and synthetic amphiphiles serve as mimics of biological membranes and enable the delivery of drugs, proteins, genes, and imaging agents. Yet the precise molecular arrangements demanded by these functions are difficult to achieve. Libraries of amphiphilic Janus dendrimers, prepared by facile coupling of tailored hydrophilic and hydrophobic branched segments, have been screened by cryogenic transmission electron microscopy, revealing a rich palette of morphologies in water, including vesicles, denoted dendrimersomes, cubosomes, disks, tubular vesicles, and helical ribbons. Dendrimersomes marry the stability and mechanical strength obtainable from polymersomes with the biological function of stabilized phospholipid liposomes, plus superior uniformity of size, ease of formation, and chemical functionalization. This modular synthesis strategy provides access to systematic tuning of molecular structure and of self-assembled architecture.

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

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.

A unimolecular G-quadruplex that functions as a synthetic transmembrane Na+ transporter.

We describe the covalent post-modification of a hydrogen-bonded assembly with the subsequent formation of a potent transmembrane Na+ ion transporter. Olefin metathesis is used to cross-link all 16 guanosine subunits in a lipophilic G-quadruplex. The resulting unimolecular G-quadruplex folds in the environment of a phospholipid membrane and functions as a Na+ ion transporter as judged by fluorescence and 23Na NMR transport assays.

Cation exchange in lipophilic G-quadruplexes: not all ion binding sites are equal.

Lipophilic guanosine derivatives that form G-quadruplexes are promising building blocks for ionophores and ion channels. Herein, cation exchange between solvated cations (K+ and NH4+) and bound cations in the G-quadruplex [G1]16.4Na+.4DNP- was studied by electrospray ionization mass spectrometry and solution 1H, 15N NMR spectroscopy. The ESI-MS and 1H NMR data provided evidence for the formation of mixed-cationic Na+, K+ G-quadruplexes. The use of 15NH4+ cations in NMR titrations, along with 15N-filtered 1H NMR and selective NOE experiments, identified two mixed-cationic intermediates in the cation exchange pathway from [G1]16.4Na+.4DNP- to [G1]16.4NH4+.4DNP-. The central Na+, bound between the two symmetry-related G8-Na+ octamers, exchanges with either K+ or NH4+ before the two outer Na+ ions situated within the C4 symmetric G8 octamers. A structural rationale, based on differences in the cations' octahedral coordination geometries, is proposed to explain the differences in site exchange for these lipophilic G-quadruplexes. Large cations such as Cs+ can be exchanged into the central cation binding site that holds the two symmetry-related C4 symmetric G8 octamer units together. The potential relevance of these findings to both supramolecular chemistry and DNA G-quadruplex structure are discussed.

Using diffusion NMR to characterize guanosine self-association: insights into structure and mechanism.

This paper presents results from a series of pulsed field gradient (PFG) NMR studies on lipophilic guanosine nucleosides that undergo cation-templated assembly in organic solvents. The use of PFG-NMR to measure diffusion coefficients for the different aggregates allowed us to observe the influences of cation, solvent and anion on the self-assembly process. Three case studies are presented. In the first study, diffusion NMR confirmed formation of a hexadecameric G-quadruplex [G 1](16)4 K(+)4 pic(-) in CD(3)CN. Furthermore, hexadecamer formation from 5'-TBDMS-2',3'-isopropylidene G 1 and K(+) picrate was shown to be a cooperative process in CD(3)CN. In the second study, diffusion NMR studies on 5'-(3,5-bis(methoxy)benzoyl)-2',3'-isopropylidene G 4 showed that hierarchical self-association of G(8)-octamers is controlled by the K(+) cation. Evidence for formation of both discrete G(8)-octamers and G(16)-hexadecamers in CD(2)Cl(2) was obtained. The position of this octamer-hexadecamer equilibrium was shown to depend on the K(+) concentration. In the third case, diffusion NMR was used to determine the size of a guanosine self-assembly where NMR signal integration was ambiguous. Thus, both diffusion NMR and ESI-MS show that 5'-O-acetyl-2',3'-O-isopropylidene G 7 and Na(+) picrate form a doubly charged octamer [G 7](8)2 Na(+)2 pic(-) 9 in CD(2)Cl(2). The anion's role in stabilizing this particular complex is discussed. In all three cases the information gained from the diffusion NMR technique enabled us to better understand the self-assembly processes, especially regarding the roles of cation, anion and solvent.

Water-mediated association provides an ion pair receptor.

In this paper, we report on the formation and properties of a water-stabilized dimer comprising calix[4]arene-guanosine conjugate cG 2. The 1,3-alternate calixarene cG 2 was poorly soluble in dry CDCl(3) and gave an ill-resolved NMR spectrum, consistent with its nonspecific aggregation. The compound was much more soluble in water-saturated CDCl(3). Two sets of well-resolved (1)H NMR signals for the guanosine residues in cG 2, present in a 1:1 ratio, indicated that the compound's D(2) symmetry had been broken and provided the first hint that cG 2 dimerizes in water-saturated CDCl(3). The resulting dimer, (cG 2)(2).(H(2)O)(n)(), has a unique property: it extracts alkali halide salts from water into organic solution. This dimer is a rare example of a self-assembled ion pair receptor. The identity of the (cG 2)(2).NaCl.(H(2)O)(n)() dimer was confirmed by comparing its self-diffusion coefficient in CDCl(3), determined by pulsed-field gradient NMR, with that of control compound cA 3, an adenosine analogue. The dimer's stoichiometry was also confirmed by quantitative measurement of the cation and anion using ion chromatrography. Two-dimensional NMR and ion-induced NMR shifts indicated that the cation binding site is formed by an intermolecular G-quartet and the anion binding site is provided by the 5'-amide NH groups. Once bound, the salt increases the dimer's thermal stability. Both (1)H NMR and ion chromatography measurements indicated that the cG 2 dimer has a modest selectivity for extracting K(+) over Na(+) and Br(-) over Cl(-). The anion's identity also influences the association process: NaCl gives a soluble, discrete dimer whereas addition of NaBPh(4) to (cG 2)(2).(H(2)O)(n)() leads to extensive aggregation and precipitation. This study suggests a new direction for ion pair receptors, namely, the use of molecular self-assembly. The study also highlights water's ability to stabilize a functional noncovalent assembly.