G4-Quartet·M⁺ Borate Hydrogels

Abstract

The ability to modulate the physical properties of a supramolecular hydrogel may be beneficial for biomaterial and biomedical applications. We find that guanosine (G 1), when combined with 0.5 equiv of potassium borate, forms a strong, self-supporting hydrogel with elastic moduli >10 kPa. The countercation in the borate salt (MB(OH)4) significantly alters the physical properties of the hydrogel. The gelator combination of G 1 and KB(OH)4 formed the strongest hydrogel, while the weakest system was obtained with LiB(OH)4, as judged by (1)H NMR and rheology. Data from powder XRD, (1)H double-quantum solid-state magic-angle spinning (MAS) NMR and small-angle neutron scattering (SANS) were consistent with a structural model that involves formation of borate dimers and G4·K(+) quartets by G 1 and KB(OH)4. Stacking of these G4·M(+) quartets into G4-nanowires gives a hydrogel. We found that the M(+) cation helps stabilize the anionic guanosine-borate (GB) diesters, as well as the G4-quartets. Supplementing the standard gelator mixture of G 1 and 0.5 equiv of KB(OH)4 with additional KCl or KNO3 increased the strength of the hydrogel. We found that thioflavin T fluoresces in the presence of G4·M(+) precursor structures. This fluorescence response for thioflavin T was the greatest for the K(+) GB system, presumably due to the enhanced interaction of the dye with the more stable G4·K(+) quartets. The fluorescence of thioflavin T increased as a function of gelator concentration with an increase that correlated with the system's gel point, as measured by solution viscosity.

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

“…Specifically, 1 H NMR chemical shifts are markedly sensitive to intermolecular NH⋅⋅⋅N and NH⋅⋅⋅O hydrogen bonding interactions which interconnect Hoogsteen faces, such that structurally different assemblies (herein quartet and ribbon‐like) exhibit distinct 1 H DQ‐SQ spectral patterns, facilitating a direct identification of quartet and ribbon‐like assemblies 9c, 23. For example, 1 H DQ‐SQ correlation NMR spectra have been presented for guanosine dihydrate (G⋅2H 2 O),23b isopropylidene‐guanosine (Gace)23c and 3′, 5′‐dipropanoyl deoxyguanosine dG(C3) 2 ,23a in which there are crystal structures exhibiting different types of ribbon‐like self‐assembly.…”

“…A two‐dimensional 1 H DQ‐SQ correlation NMR spectrum of the GaceC10⋅K⋅Pic 8:1 complex is presented in Figure 4 a. The 1 H DQ peaks at δ DQ equal to 22.6 ppm (14.8+7.8) and 21.2 ppm (11.9+9.3) are assigned to intramolecular H−H proximities of NH1 and one of the NH2 protons in ribbon‐like and quartet assemblies, respectively 9c, 23. Moreover, as noted above,9c the NH1−NH1 autocorrelation peak at 23.8 (11.9+11.9) ppm is indicative of inter‐quartet stacking.…”

“…The 1 H DQ peaks at δ DQ equal to 22.6 ppm (14.8+7.8) and 21.2 ppm (11.9+9.3) are assigned to intramolecular H−H proximities of NH1 and one of the NH2 protons in ribbon‐like and quartet assemblies, respectively 9c, 23. Moreover, as noted above,9c the NH1−NH1 autocorrelation peak at 23.8 (11.9+11.9) ppm is indicative of inter‐quartet stacking. A 1 H NOESY‐like spin‐diffusion NMR spectrum of the GaceC10⋅K⋅ Pic 8:1 complex shown in Figure 4 b clearly reveals the co‐existence of both quartet and ribbon‐like arrangements.…”

Structure and Distribution of Cross-Links in Boron-Modified Phenol–Formaldehyde Resins Designed for Soft Magnetic Composites: A Multiple-Quantum 11B–11B MAS NMR Correlation Spectroscopy Study

“…In the molecular models representing thermally converted PF resins the methylene (−O−CH 2 −) carbons are further substituted by amino groups. As reference compounds boric acid, borate anion (B(OH) 3 and B(OH) 4 − ) and the recently reported cyclic five-member guanosine−borate monoesters and guanosine−borate diesters 35,36 were employed in the DFT calculations (for optimized geometries of the reference compounds see Supporting Information). As graphically presented in Figure 6b, the 11 B NMR chemical shielding parameters calculated for all the reference compounds as well as for the proposed structural models nicely fit their experimental counterparts (i.e., the assigned 11 B NMR chemical shifts).…”

Hydrogels Based on Ag+‐Modulated Assembly of 5′‐Adenosine Monophosphate for Enriching Biomolecules

“…Metal–nucleobase interactions extensively exist in systems formed by various metal ions and four nucleobases (adenine, thymine, guanine, and cytosine) and their derivatives. For example, guanine and its derivatives have been extensively studied because of the ability to form G4‐quartets and further self‐assemble into highly ordered aggregates in the presence of metal ions . Additionally, nanoparticles formed by metal–nucleobase coordination of adenine derivatives and metal ions have also been reported .…”