Developing a Self-Healing Supramolecular Nucleoside Hydrogel Based on Guanosine and Isoguanosine

Large-Scale Synthesis of High-Purity Isoguanosine and Resolution of its Crystal Structure by Microcrystal Electron Diffraction

Isoguanosine (isoG) is a natural structural isomer of guanosine (G) with significant potential for applications in ionophores, genetics, gel formation, and cancer therapy. However, the cost of commercially available isoG on a gram scale is relatively high. To date, a detailed method for the large-scale preparation of high-purity isoG has not been reported. This study presented a simple and convenient approach for the large-scale synthesis of isoG through the diazotization of 2,6-diaminopurine riboside with sodium nitrite and acetic acid at room temperature. Further, this method could synthesize isoG derivatives (2'-fluoro-isoguanosine (1) and 2'-deoxy-isoguanosine (2)) from 2,6-diaminopurine nucleoside derivatives using diazotization. The structural information of natural and modified nucleosides is crucial for the modification and substitution of DNA/RNA. This study obtained the single-crystal structure of isoG for the first time and analyzed it in detail using microcrystal electron diffraction. The three-dimensional supramolecular structure of isoG adopted similarly base-pair motifs from π-π stacking interaction of diverse layers, intramolecular hydrogen bonding, and distinct hydrogen bonding interactions from sugar residues. This study has contributed to further isoG modification and its applications in medicinal chemistry and materials.

Super-Rapid In Situ Formation of a Silver Ion-Induced Supramolecular Hydrogel with Efficient Antibacterial Activity for Root Canal Disinfection

Supramolecular hydrogels prepared using low-molecular-weight gelators have attracted considerable attention for biomedical applications. However, in situ supramolecular hydrogels are limited in terms of their prolonged gelation time and/or unstable nature at high temperatures. In this study, we constructed a stable supramolecular Ag-isoG hydrogel through super-rapid in situ formation, wherein hydrogelation process occurred instantaneously upon mixing isoG and Ag⁺ within 1 s under ambient conditions. Interestingly, unlike most nucleoside-based supramolecular hydrogels, this Ag-isoG hydrogel remains stable even at a high temperature (100 °C). Moreover, the as-designed hydrogel demonstrated significant antibacterial activity against Staphylococcus aureus and the oral bacterium Streptococcus mutans owing to the strong chelating ability of Ag ions, and the hydrogel exhibited relatively low cytotoxicity in root canal and an easy removal feature by saline. The hydrogel was then applied to a root canal infection model, which demonstrated strong antibacterial activity against Enterococcus faecalis, with performance even better than that of the regular calcium hydroxide paste. This feature makes the Ag-isoG hydrogel a prospective alternative material as intracanal medicaments for root canal treatment.

Optimization of guanosine-based hydrogels with boric acid derivatives for enhanced long-term stability and cell survival

Tissue defects can lead to serious health problems and often require grafts or transplants to repair damaged soft tissues. However, these procedures can be complex and may not always be feasible due to a lack of available tissue. Hydrogels have shown potential as a replacement for tissue grafts due to their ability to support cell survival and encapsulate biomolecules such as growth factors. In particular, guanosine-based hydrogels have been explored as a potential solution, but they often exhibit limited stability which hampers their use in the biofabrication of complex grafts. To address this issue, we explored the use of borate ester chemistry and more complex boric acid derivatives to improve the stability and properties of guanosine-based hydrogels. We hypothesized that the aromatic rings in these derivatives would enhance the stability and printability of the hydrogels through added π-π stack interactions. After optimization, 13 compositions containing either 2-naphthylboronic acid or boric acid were selected. Morphology studies shows a well-defined nanofibrilar structure with good printable properties (thixotropic behaviour, print fidelity and printability). Moreover, the pH of all tested hydrogels was within the range suitable for cell viability (7.4–8.3). Nevertheless, only the boric acid-based formulations were stable for at least 7 days. Thus, our results clearly demonstrated that the presence of additional aromatic rings did actually impair the hydrogel properties. We speculate that this is due to steric hindrance caused by adjacent groups, which disrupt the correct orientation of the aromatic groups required for effective π-π stack interactions of the guanosine building block. Despite this drawback, the developed guanosine-boric acid hydrogel exhibited good thixotropic properties and was able to support cell survival, proliferation, and migration. For instance, SaOS-2 cells planted on these printed structures readily migrated into the hydrogel and showed nearly 100% cell viability after 7 days. In conclusion, our findings highlight the potential of guanosine-boric acid hydrogels as tissue engineering scaffolds that can be readily enhanced with living cells and bioactive molecules. Thus, our work represents a significant advancement towards the development of functionalized guanosine-based hydrogels.

Polydopamine Incorporation Enhances Cell Differentiation and Antibacterial Properties of 3D-Printed Guanosine-Borate Hydrogels for Functional Tissue Regeneration

Tissue engineering focuses on the development of materials as biosubstitutes that can be used to regenerate, repair, or replace damaged tissues. Alongside this, 3D printing has emerged as a promising technique for producing implants tailored to specific defects, which in turn increased the demand for new inks and bioinks. Especially supramolecular hydrogels based on nucleosides such as guanosine have gained increasing attention due to their biocompatibility, good mechanical characteristics, tunable and reversible properties, and intrinsic self-healing capabilities. However, most existing formulations exhibit insufficient stability, biological activity, or printability. To address these limitations, we incorporated polydopamine (PDA) into guanosine-borate (GB) hydrogels and developed a PGB hydrogel with maximal PDA incorporation and good thixotropic and printability qualities. The resulting PGB hydrogels exhibited a well-defined nanofibrillar network, and we found that PDA incorporation increased the hydrogel's osteogenic activity while having no negative effect on mammalian cell survival or migration. In contrast, antimicrobial activity was observed against the Gram-positive bacteria Staphylococcus aureus and Staphylococcus epidermidis. Thus, our findings suggest that our PGB hydrogel represents a significantly improved candidate as a 3D-printed scaffold capable of sustaining living cells, which may be further functionalized by incorporating other bioactive molecules for enhanced tissue integration.

Injectable catechin-based supramolecular hydrogel for highly efficient application in HPV-associated OSCC

Catechins are a group of natural polyphenols extracted from green tea. Notably, they have been proven to have excellent anti-HPV and anti-tumour properties and to be effective against some HPV-related diseases, showing great potential in the treatment of HPV-associated oral squamous cell carcinoma (HPV⁺ OSCC). However, the poor bioavailability, short half-lives, and stability issues of catechins hamper their clinical application. To overcome these shortcomings of catechins, we innovatively synthesised an injectable supramolecular hydrogel, namely catechin-phenylenebisboronic acid-isoguanosine (CPBisoG), with catechin (one of the simplest catechins) and isoguanosine (isoG), another natural product with self-assembly ability, via dynamic phenylborate diester bonds. The biodegradation and sustained-release time of the CPBisoG hydrogel in mice lasted up to 72 h. This supramolecular hydrogel not only functioned as a good local drug delivery platform with good stability, injectability, self-healing properties, biocompatibility, biodegradability, but also exhibited therapeutic effects toward HPV⁺ OSCC in vitro and in vivo. And interestingly, it also showed selective inhibition against HPV⁺ OSCC cells. In all, these results demonstrate that this catechin-based hydrogel could sustainedly and highly effectively treat HPV⁺ OSCC topically, which could also provide a promising strategy for the management of other HPV-associated diseases in the future.

Nucleoside-Based Supramolecular Hydrogels: From Synthesis and Structural Properties to Biomedical and Tissue Engineering Applications

Supramolecular hydrogels are of great interest in tissue scaffolding, diagnostics, and drug delivery due to their biocompatibility and stimuli-responsive properties. In particular, nucleosides are promising candidates as building blocks due to their manifold noncovalent interactions and ease of chemical modification. Significant progress in the field has been made over recent years to allow the use of nucleoside-based supramolecular hydrogels in the biomedical field, namely drug delivery and 3D bioprinting. For example, their long-term stability, printability, functionality, and bioactivity have been greatly improved by employing more than one gelator, incorporating different cations, including silver for antibacterial activity, or using additives such as boric acid or even biomolecules. This now permits their use as bioinks for 3D printing to produce cell-laden scaffolds with specified geometries and pore sizes as well as a homogeneous distribution of living cells and bioactive molecules. We have summarized the latest advances in nucleoside-based supramolecular hydrogels. Additionally, we discuss their synthesis, structural properties, and potential applications in tissue engineering and provide an outlook and future perspective on ongoing developments in the field.

Self-assembly hydrogels of therapeutic agents for local drug delivery

Advanced drug delivery systems are of vital importance to enhance therapeutic efficacy. Among various recently developed formulations, self-assembling hydrogels composed of therapeutic agents have shown promising potential for local drug delivery owing to their excellent biocompatibility, high drug-loading efficiency, low systemic toxicity, and sustained drug release behavior. In particular, therapeutic agents self-assembling hydrogels with well-defined nanostructures are beneficial for direct delivery to the target site via injection, not only improving drug availability, but also extending their retention time and promoting cellular uptake. In brief, the self-assembly approach offers better opportunities to improve the precision of pharmaceutical treatment and achieve superior treatment efficacies. In this review, we intend to cover the recent developments in therapeutic agent self-assembling hydrogels. First, the molecular structures, self-assembly mechanisms, and application of self-assembling hydrogels are systematically outlined. Then, we summarize the various self-assembly strategies, including the single therapeutic agent, metal-coordination, enzyme-instruction, and co-assembly of multiple therapeutic agents. Finally, the potential challenges and future perspectives are discussed. We hope that this review will provide useful insights into the design and preparation of therapeutic agent self-assembling hydrogels.

Protocatechuic Acid-Based Supramolecular Hydrogel Targets SerpinB9 to Achieve Local Chemotherapy for OSCC

Protocatechuic acid (PCA) is a natural phenolic acid present in daily vegetables and fruits. Notably, PCA was demonstrated to inhibit the biological function of SerpinB9 (Sb9) and exhibit an excellent antitumor effect, showing great potential in cancer treatment. However, the short half-life time limits PCA's wide application against cancers. To overcome this shortage of PCA, we integrated PCA and another natural product with strong self-assembling properties, isoguanosine (isoG), to develop a novel multifunctional supramolecular hydrogel with good biocompatibility and injectability, which remarkably lengthens the releasing time of PCA and exerts considerable anticancer effects in vitro and in vivo. Besides, we surprisingly found that PCA could not only target Sb9 but also restrain cancer development through activating the JNK/P38 pathway, decreasing the ROS level, and repairing cancer stemness. In all, our results demonstrate that this PCA-based hydrogel could act as a multifunctional hydrogel system equipped with considerable anticancer effects, providing potential local administration integrating with targeted therapy and chemotherapy in one simple modality.

A guanosine-based hydrogel integrating photothermal effect of PDAAuNPs through dynamic borate bond for photothermal therapy of cancer

Photothermal therapy (PTT) has drawn extensive attention owing to its noninvasive and great tissue penetration depth. However, the physical encapsulation of photothermal agents may lead to their rapid release. Dual-functional hydrogel systems that integrate functions and carriers can potentially solve this problem. In this work, we successfully developed a dual-functional guanosine(G)-based hydrogel integrating the photothermal effect and localized delivery by introducing dynamic borate ester utilizing the photothermal property of PDA-AuNPs and the self-assembly ability of G. Both in vitro and in vivo results confirmed that the GBPA hydrogel not only exhibited excellent photothermal toxicity, stability, injectability, and biocompatibility, but also possessed high photothermal antitumor activity. These results suggested that the GBPA hydrogel could be used as a dual-functional hydrogel integrating photothermal effect and localized delivery in one system, which would possibly provide a new opportunity for the design of new dual-functional hydrogels for highly efficient cancer therapy.

A multifunctional supramolecular hydrogel for infected wound healing

Bacterial infection poses a significant threat to wound healing, and the preparation of novel wound dressings is very important. However, currently reported dressings serve as traditional physical barriers or functional ones with limited effects, such as antibacterial effect or adhesion. There is growing demand for developing wound dressing materials with antibacterial effect, good adhesion, proper degradation within the wound recovery time, and simple synthesis. In this study, based on a natural plant extract - tannic acid (TA) and natural guanosine (G), a supramolecular soft hydrogel (G-TA hydrogel) was successfully synthesized based on dynamic borate esters in a one-pot reaction. The hydrogel showed excellent antibacterial and adhesive properties and could be degraded within three days in vivo. In addition, the G-TA hydrogel also showed remarkable antioxidant capability, excellent injectability, a long in vitro lifespan, and good cytocompatibility on L929 cells. Furthermore, the hydrogel could accelerate the healing of full-thickness wounds on the back skin of mice, indicating its promising applications in wound repair.

Isoguanine (2-Hydroxyadenine) and 2-Aminoadenine Nucleosides with an 8-Aza-7-deazapurine Skeleton: Synthesis, Functionalization with Fluorescent and Clickable Side Chains, and Impact of 7-Substituents on Physical Properties

7-Functionalized 8-aza-7-deaza-2'-deoxyisoguanine and 8-aza-7-deaza-2-aminoadenine 2'-deoxyribonucleosides decorated with fluorescent pyrene or benzofuran sensor tags or clickable side chains with terminal triple bonds were synthesized. 8-Aza-7-deaza-7-iodo-2-amino-2'-deoxyadenosine was used as the central intermediate and was accessible by an improved two-step glycosylation/amination protocol. Functionalization of position-7 was performed either on 8-aza-7-deaza-7-iodo-2-amino-2'-deoxyadenosine followed by selective deamination of the 2-amino group or on 7-iodinated 8-aza-7-deaza-2'-deoxyisoguanosine. Sonogashira and Suzuki-Miyaura cross-coupling reactions were employed for this purpose. Octadiynyl side chains were selected as linkers for click reactions with azido pyrenes. K_Taut values calculated from H₂O/dioxane mixtures revealed that side chains have a significant influence on the tautomeric equilibrium. Photophysical properties (fluorescence, solvatochromism, and quantum yields) of the new 8-aza-7-deazapurine nucleosides with fluorescent side chains were determined. Remarkably, a strong excimer fluorescence in H₂O was observed for pyrene dye conjugates of 8-aza-7-deazaisoguanine and 2-aminoadenine nucleosides with a long linker. In other solvents including methanol, excimer fluorescence was negligible. The 2-aminoadenine and isoguanine nucleosides with the 8-aza-7-deazapurine skeleton expand the class of nucleosides applicable to fluorescence detection with respect to diagnostic and therapeutic purposes.

Chirality from D-guanosine to L-guanosine shapes a stable gel for three-dimensional cell culture

It is proved that L-guanosine (L-G) as an enantiomer of D-guanosine (D-G) forms more stable gels than D-G, suggesting that alteration of chirality may be a new strategy for improving the lifetime stability of supramolecular hydrogels. Experiments for three-dimensional cell culture reveal that the L-G gel is a candidate for the extracellular matrix.

Self-Healing Hydrogels: Preparation, Mechanism and Advancement in Biomedical Applications

Polymeric hydrogels are widely explored materials for biomedical applications. However, they have inherent limitations like poor resistance to stimuli and low mechanical strength. This drawback of hydrogels gave rise to ''smart self-healing hydrogels'' which autonomously repair themselves when ruptured or traumatized. It is superior in terms of durability and stability due to its capacity to reform its shape, injectability, and stretchability thereby regaining back the original mechanical property. This review focuses on various self-healing mechanisms (covalent and non-covalent interactions) of these hydrogels, methods used to evaluate their self-healing properties, and their applications in wound healing, drug delivery, cell encapsulation, and tissue engineering systems. Furthermore, composite materials are used to enhance the hydrogel's mechanical properties. Hence, findings of research with various composite materials are briefly discussed in order to emphasize the healing capacity of such hydrogels. Additionally, various methods to evaluate the self-healing properties of hydrogels and their recent advancements towards 3D bioprinting are also reviewed. The review is concluded by proposing several pertinent challenges encountered at present as well as some prominent future perspectives.

Advanced Methods for the Characterization of Supramolecular Hydrogels

With the increased research on supramolecular hydrogels, many spectroscopic, diffraction, microscopic, and rheological techniques have been employed to better understand and characterize the material properties of these hydrogels. Specifically, spectroscopic methods are used to characterize the structure of supramolecular hydrogels on the atomic and molecular scales. Diffraction techniques rely on measurements of crystallinity and help in analyzing the structure of supramolecular hydrogels, whereas microscopy allows researchers to inspect these hydrogels at high resolution and acquire a deeper understanding of the morphology and structure of the materials. Furthermore, mechanical characterization is also important for the application of supramolecular hydrogels in different fields. This can be achieved through atomic force microscopy measurements where a probe interacts with the surface of the material. Additionally, rheological characterization can investigate the stiffness as well as the shear-thinning and self-healing properties of the hydrogels. Further, mechanical and surface characterization can be performed by micro-rheology, dynamic light scattering, and tribology methods, among others. In this review, we highlight state-of-the-art techniques for these different characterization methods, focusing on examples where they have been applied to supramolecular hydrogels, and we also provide future directions for research on the various strategies used to analyze this promising type of material.

Anion mediated, tunable isoguanosine self-assemblies: decoding the conformation influence and solvent effects

Systematic investigations were performed with various substituted groups at C8 purine and ribose. A series of isoG analogs, C8-phenyl substituted isoG were synthesized and applied for Cs+ coordination. The structural proximity between purine and ribose limited pentaplex formation for C8-phenyl substituted isoG derivatives. Based on this observation, deoxy isoG derivative with modification on ribose (tert-butyldimethylsilyl ether) was applied to assemble with the Cs+ cation. Critical solvent (CDCl3 and CD3CN) and anion (BPh4 -, BARF-, and PF6 -) effects were revealed, leading to the controllable formation of various stable isoG pentaplexes, including singly charged decamer, doubly charged decamer, and 15-mer, etc. Finally, the X-ray crystal structure of [isoG20Cs3]3+(BARF-)3 was successfully obtained, which is the first example of multiple-layer deoxy isoG binding with the Cs+ cation, providing solid evidence of this new isoG ionophore beyond two-layer sandwich self-assembly.

Self-assembled guanosine-hydrogels for drug-delivery application: Structural and mechanical characterization, methylene blue loading and controlled release

It is known that guanosine derivatives (G) self-assemble in water forming long, flexible, and interacting aggregates (the so-called G-quadruplexes): by modulating the quadruplex charges, e.g. simply using a mixture of guanosine 5'-monophosphate (GMP) and guanosine (Gua), multi-responsive, self-healing hydrogels can be obtained. In this paper, the potential application of G-hydrogels as drug delivery systems has been assessed. Hydrogels were prepared at different Gua:GMP molar ratios. The photosensitizer Methylene Blue and the pro-apoptotic protein cytochrome C were used as cargo molecules. Small angle x-ray scattering and atomic force microscopy experiments confirmed the presence of G-quadruplexes disposed in swollen matrices with different mesh-sizes. Rheology measurements showed that the Gua:GMP molar ratio leads to specific drug release mechanisms, as the gel strength is finely tuned by electrostatic repulsion and van der Waals attraction between G-quadruplexes. Noteworthy, the gel cohesion and the drug release were pH responsive. Swelling, self-healing and cell viability features were also investigated: the results qualify the Gua:GMP hydrogel as an excellent biomaterial that can entrap and deliver key biomolecules in a sustained and responsive release manner.

Reversed Cation Selectivity of G8 -Octamer and G16 -Hexadecamer towards Monovalent and Divalent Cations

A reverse-binding-selectivity between monovalent and divalent cations was observed for two different self-assembly G16 -hexadecamer and G8 -octamer systems. The dissociation constant between G4 -quadruplex and monomer was calculated via VT-1 H NMR experiments. Quantitative energy profiles revealed entropy as the key factor for the weaker binding toward Ba2+ compared with K+ in the G8 -octamer system despite stronger ion-dipole interactions. This study is the first direct comparison of the G4 -quartet binding affinity between mono and divalent cations and will benefit future applications of G-quadruplex-related research. Further competition experiments between the G8 -octamer and 18-crown-6 with K+ demonstrated the potential of this G8 system as a new potassium receptor.

The development of isoguanosine: from discovery, synthesis, and modification to supramolecular structures and potential applications

First systematical review of isoguanosine, an unnatural base, as an isomer of guanosine shows significant differences in diverse properties.

Dual-functional guanosine-based hydrogel integrating localized delivery and anticancer activities for cancer therapy

Supramolecular hydrogel delivery systems have attracted widely attention owing to incorporating various therapeutic agents in carriers to decrease unpredictable toxicities, improve curative efficacy, and protect drug bioactivity. Nonetheless, the dual-functional supramolecular hydrogel integrating localized delivery and antineoplastic activities in one system have rarely observed. In this study, we successfully developed a novel supramolecular hydrogel, isoguanosine-borate-guanosine (isoGBG), with reversibly and dynamic borate ester bonds formed via boric acids and diols derived from nature products guanosine and isoguanosine in one pot by following a simple procedure. Both in vivo and in vitro results demonstrated that the isoGBG hydrogel not only displays excellent stability, self-healing properties and biocompatibility, but also has highly anti-tumor activities through inducing tumor cell apoptosis and excellent inhibition effect of tumor recurrence. These findings suggested that isoGBG hydrogel can serve as a dual-function hydrogel system integrating drug carrier and anti-cancer compound in one system, which provided a promising strategy for the design of functional supramolecular hydrogel in the local management of cancer in the future.

The Development and Lifetime Stability Improvement of Guanosine-Based Supramolecular Hydrogels through Optimized Structure

Guanosine is an important building block for supramolecular gels owing to the unique self-assembly property that results from the unique hydrogen bond acceptors and donor groups. Guanosine-derived supramolecular hydrogels have promise in the fields of drug delivery, targeted release, tissue engineering applications, etc. However, the property of poor longevity and the need for excess cations hinder the widespread applications of guanosine hydrogels. Although guanosine-derived supramolecular hydrogels have been reviewed previously by Dash et al., the structural framework of this review is different, as the modification of guanosine is described at the molecular level. In this review, we summarize the development and lifetime stability improvement of guanosine-based supramolecular hydrogels through optimized structure and elaborate on three aspects: sugar modification, base modification, and binary gels. Additionally, we introduce the concept and recent research progress of self-healing gels, providing inspiration for the development of guanosine-derived supramolecular hydrogels with longer lifespans, unique physicochemical properties, and biological activities.