Bioinspired Amino Acid Based Materials in Bionanotechnology: From Minimalistic Building Blocks and Assembly Mechanism to Applications

Inspired by natural hierarchical self-assembly of proteins and peptides, amino acids, as the basic building units, have been shown to self-assemble to form highly ordered structures through supramolecular interactions. The fabrication of functional biomaterials comprised of extremely simple biomolecules has gained increasing interest due to the advantages of biocompatibility, easy functionalization, and structural modularity. In particular, amino acid based assemblies have shown attractive physical characteristics for various bionanotechnology applications. Herein, we propose a review paper to summarize the design strategies as well as research advances of amino acid based supramolecular assemblies as smart functional materials. We first briefly introduce bioinspired reductionist design strategies and assembly mechanism for amino acid based molecular assembly materials through noncovalent interactions in condensed states, including self-assembly, metal ion mediated coordination assembly, and coassembly. In the following part, we provide an overview of the properties and functions of amino acid based materials toward applications in nanotechnology and biomedicine. Finally, we give an overview of the remaining challenges and future perspectives on the fabrication of amino acid based supramolecular biomaterials with desired properties. We believe that this review will promote the prosperous development of innovative bioinspired functional materials formed by minimalistic building blocks.

Manipulating supramolecular gels with surfactants: Interfacial and non-interfacial mechanisms

Gel is a class of self-supporting soft materials with applications in many fields. Fast, controllable gelation, micro/nano structure and suitable rheological properties are essential considerations for the design of gels for specific applications. Many methods can be used to control these parameters, among which the additive approach is convenient as it is a simple physical mixing process with significant advantages, such as avoidance of pH change and external energy fields (ultrasound, UV light and others). Although surfactants are widely used to control the formation of many materials, particularly nanomaterials, their effects on gelation are less known. This review summarizes the studies that utilized different surfactants to control the formation, structure, and properties of molecular and silk fibroin gels. The mechanisms of surfactants, which are interfacial and non-interfacial effects, are classified and discussed. Knowledge and technical gaps are identified, and perspectives for further research are outlined. This review is expected to inspire increasing research interest in using surfactants for designing/fabricating gels with desirable formation kinetics, structure, properties and functionalities.

New Fmoc-Amino Acids/Peptides-Based Supramolecular Gels Obtained through Co-Assembly Process: Preparation and Characterization

One of the methods of obtaining supramolecular gels consists of the possibility of self-assembly of low molecular weight gelators (LMWGs). However, LMWG-based gels are often difficult to handle, easy to destroy and have poor rheological performance. In order to improve the gels’ properties, the LMWGs molecules are co-assembled, which induces more cross-links with more stable structures. Starting from these aspects, the present study refers to the preparation of a bionic hydrogel stabilized with a physiologically occurring, bifunctional biomolecule, L-lysine, co-assembled with other amino acids or peptides (such as a modified amino acid (Fmoc-serine or Fmoc-glutamic acid) or a tripeptide (Fmoc-Gly-Gly-Gly)) with the potential to support the repair of injuries or the age-related impaired structures or functions of living tissues. The introduction of a copartner aims to improve hydrogel characteristics from a morphological, rheological and structural point of view. On the other hand, the process will allow the understanding of the phenomenon of specific self-association and molecular recognition. Various characterization techniques were used to assess the ability to co-assemble: DLS, FT-IR, SEM and fluorescence microscopy, rheology and thermal analysis. Studies have confirmed that the supramolecular structure occurs through the formation of inter- and intramolecular physical bonds that ensure the formation of fibrils organized into 3D networks. The rheological data, namely the G′ > G″ and tan δ approximately 0.1−0.2 gel-like behavior observed for all studied samples, demonstrate and sustain the appearance of the co-assembly processes and the ability of the samples to act as LMWG. From the studied systems, the Fmoc−Lys−Fmoc_ Fmoc−Glu sample presented the best rheological characteristics that are consistent with the observations that resulted from the dichroism, fluorescence and SEM investigations.

Domination of H-Bond Interactions in the Solvent-Triggering Gelation Process

Gels prepared with the solvent-triggering method are attractive for their easy and fast preparation; however, the role of solvents in this process remains unclear, which hinders the efficient and accurate control of desired gel properties. In this study, the role of solvents in the solvent-triggering gelation process is studied using 9-fluorenylmethoxycarbonyl (Fmoc)-protected diphenylalanine (Fmoc-FF) as the gelator. Density functional theory (DFT)-based calculations and corresponding wavefunction analyses are conducted to identify the H-bonding interaction sites between the molecules. The calculation results clearly annotate the activating role of DMF and the triggering role of H₂O in the gelation process. The solvation of Fmoc-FF by DMF can activate the H-bonding sites on the peptide chain, showing a conformation reversal and higher electrostatic potentials. Then, the H-bonding between Fmoc-FF and H₂O is facilitated to trigger gelation. The physical Fmoc-FF/DMF/H₂O gels show easily tuned mechanical strengths (G' of 10²-10⁵ Pa), injectable potentials (general yield strain < 100%), and stable recoverability (80-98% within 100 s). The regulation of these properties depends on not only the gelator concentration but also the H-bonding interactions with solvent molecules, which have seldom been studied in detail before. By understanding the effect of solvents, low-molecular-weight gelator-based gels can be designed, prepared, and tuned efficiently for potential applications.

Controlled aggregation properties of single amino acids modified with protecting groups

The self-assembling properties of single amino acids modified with protecting groups under controlled conditions of temperature and concentration are illustrated.

Supramolecular gel formation regulated by water content in organic solvents: self-assembly mechanism and biomedical applications

As one of the most important and fruitful methods, supramolecular self-assembly has a significant advantage in designing and fabricating functional soft materials with various nanostructures. In this research, a low-molecular-weight gelator, N,N'-di(pyridin-4-yl)-pyridine-3,5-dicarboxamide (PDA-N4), was synthesized and used to construct self-assembled gels via a solvent-mediated strategy. It was found that PDA-N4 could form supramolecular gels in mixed solvents of water and DMSO (or DMF) at high water fraction (greater than or equal to 50%). By decreasing the water fraction from 50% to 30%, the gel, suspension and solution phases appeared successively, indicating that self-assembled aggregates could be efficiently modulated via water content in organic solvents. Moreover, the as-prepared PDA-N4 supramolecular gels not only displayed solid-like behavior, and pH- and thermo-reversible characteristics, but also showed a solution-gel-crystal transition with the extension of aging time. Further analyses suggested that both the crystal and gel had similar assembled structures. The intermolecular hydrogen bonding between amide groups and the π-π stacking interactions between pyridine groups played key roles in gel formation. Additionally, the release behavior of vitamin B12 (VB12) from PDA-N4 gel (H2O/DMSO, v/v = 90/10) was evaluated, and the drug controlled release process was consistent with a first-order release mechanism. The human umbilical venous endothelial cell culture results showed that the PDA-N4 xerogel has good cytocompatibility, which implied that the gels have potential biological application in tissue engineering and controlled drug release.

Supramolecular fibrous gels with helical pitch tunable by polarity of alcohol solvents

A sugar-based, low-molecular-weight gelator 16AG, can gelatinize primary alcohols by forming supramolecular fibers. We obtained non-helical, tape-like fibers in methanol and ethanol but helical fibers in alcohols with at least three carbons. The pitch of the helical fibers became shorter with increasing carbon number.

High-Strength Albumin Hydrogels With Hybrid Cross-Linking

Natural protein-based hydrogels possess excellent biocompatibility; however, most of them are weak or brittle. In the present work, high strength hybrid dual-crosslinking BSA gels (BSA DC gels), which have both chemical cross-linking and physical cross-linking, were fabricated by a facile photoreaction-heating process. BSA DC gels showed high transparency (light transmittance of ~90%) and high strength. At optimal conditions, BSA DC gel exhibited high compressive strength (σc,f) of 37.81 ± 2.61 MPa and tensile strength (σt,f) of 0.62 ± 0.078 MPa, showing it to be much stronger than physically cross-linked BSA gel (BSA PC gel) and chemically cross-linked BSA gel (BSA CC gel). More importantly, BSA DC gel displayed non-swelling properties while maintaining high strength in DI water, pH = 3.0, and pH = 10.0. Moreover, BSA DC gel also demonstrated large hysteresis, rapid self-recovery, and excellent fatigue resistance properties. It is believed that our BSA DC gel can potentially be applied in biomedical fields.

Hydrogels Self-Assembled from an Azobenzene Building Block: Stability toward UV Irradiation in the Gel and Thin-Film States

Azobenzene and its derivatives are widely used as photoresponsive units for the fabrication of photoresponsive smart materials. In this work, two azobenzene derivatives were designed and investigated as hydrogelators, namely N-4-azodiphenyl-maleimic acid (ADPMA) and N-4-azodiphenyl-succinic acid (ADPSA) bearing azobenzene and carboxylic acid segments. In the process of deprotonation/protonation of the carboxylic acids by pH variation, the self-assembly of these two gelators was triggered. ADPMA could transform from solution to hydrogel while the solution of ADPSA formed a precipitate under the same conditions. The solution-gel/precipitate transformation can be repeated by changing the pH. In contrast to the conventional responsiveness to UV-light irradiation for azobenzene-based gels, the ADPMA hydrogel shows typical trans-cis isomerization of the azobenzene unit in solution, yet the hydrogel demonstrates remarkable stability to UV light irradiation in both the bulk gel and thin film states.

Highly Efficient Recovery of Oils in Water via Serine-Based Organogelators

We report here the gelation of a series of petroleum products by serine derivatives. Among Fmoc (9-fluorenylmethoxycarbonyl) amino acids modified by long-chain amines with different substituents, serine-based compounds exhibit excellent performance in gel formation. Further studies on the variation of serine-based gelators demonstrate a considerable structure-property relationship between oil gelation performance and the molecular structure. Oils could be separated and collected by acid and distillation. Gelators exhibit the potential to be applied in an effective treatment of oil-containing water produced from frequent marine oil spills.

Colorimetric chiral recognition of D/L-phenylalanine based on triangular silver nanoplates

A new colorimetric analysis approach for chiral recognition of D- and L-forms of phenylalanine (phe) was developed based on triangular silver nanoplates (TAg-NPs). The TAg-NPs could be used as chiral colorimetric probes for D- and L-forms of phe. Upon addition of D-phe to TAg-NPs solution, a color change from blue to purple to pink could be observed, while no obvious color change was found on addition of L-phe. L-phe could prevent the TAg-NPs from being etched to small size particles while the protective effect of D-phe was weak. Moreover, the enantiomeric excess of D-phe could be determined using the proposed chiral assay in the percentage of L-phe from 0 to 100% with a correlation coefficient of 0.9855. The phenomenon could be monitored by bare eyes and quantified analysis by UV-Vis spectrophotometry. The developed approach had several advantages, such as simplicity, visualization, short analysis time and low cost. This study presented a fast visualization analysis method of chiral D/L-phenylalanine and may lay the foundation for the development of visualization chiral recognition of other target analytes.

High-Strength, Durable All-Silk Fibroin Hydrogels with Versatile Processability toward Multifunctional Applications

Hydrogels have been the focus of extensive research due to their potential use in fields including biomedical, pharmaceutical, biosensors, and cosmetics. However, the general weak mechanical properties of hydrogels limit their utility. Here, we generate pristine silk fibroin (SF) hydrogels with excellent mechanical properties via a binary solvent induced conformation transition (BSICT) strategy. In this method, the conformational transition of SF is regulated by moderate binary solvent diffusion and SF/solvent interactions. β-sheet formation serves as the physical crosslinks that connect disparate protein chains to form continuous 3D hydrogel networks, avoiding complex chemical and/or physical treatments. The Young's modulus of these new BSICT-silk fibroin hydrogels can reach up to 6.5±0.2 MPa, tens to hundreds of times higher than that of conventional hydrogels (0.01-0.1 MPa). These new materials filled the "empty soft materials space" in the elastic modulus/strain Ashby plot. More remarkably, the BSICT-SF hydrogels can be processed into different constructions through different polymer and/or metal based processing techniques, such as molding, laser cutting, and machining. Thus, these new hydrogel systems exhibit potential utility in many biomedical and engineering fields.

Supramolecular Gel Formation Based on Glycolipids Derived from Renewable Resources

The potential applications of self-assembled supramolecular gels based on natural molecules encouraged the researchers to develop a versatile synthetic method for their structural analogues. Herein, we report a facile synthesis of glycolipid from renewable resources, cashew nut shell liquid,d and d-glucose in good yield. Gelation behavior of these glycolipids were studied in a wide range of solvents and oils. To our delight, compound 5b formed a hydrogel with Critical gelator concentration (CGC) of 0.29% w/v. Morphological analysis of the hydrogel depicts the formation of twisted fibers with an entangled network. Formation of a twisted fibrous structure was further identified by CD spectral studies with respect to temperature. The molecular self-assembly assisted by hydrogen bonding, hydrophobic, and π⁻π stacking interactions were identified by X-ray diffraction (XRD) and FTIR studies. Rheological analysis depicted the mechanical strength and stability of the hydrogel, which is crucial in predicting the practical applications of supramolecular soft materials.

Discrimination of supramolecular chirality using a protein nanopore

Supramolecular chirality may emerge from self-assembly processes to yield architectures that differ only in the topological arrangement of their constituent parts. Since the properties of the resulting enantiomeric assemblies are identical, purification and characterisation can be challenging. Here, we have examined the hypothesis that the intrinsic chirality of a protein nanopore can be exploited to detect supramolecular chirality. Transient blockages in the ion current flowing through a single membrane-spanning α-haemolysin nanopore were shown to discriminate between M4L6 tetrahedral coordination cages of opposing chiralities. The single-molecule nature of the approach facilitated direct access to the rates of association and dissociation with the nanopore, which allowed the concentrations of the enantiomeric supramolecular assemblies to be determined in situ. Thus, we have established that a protein nanopore can be used to discriminate the chiral topologies of supramolecular assemblies, even when they are too large to fully enter the nanopore.

Aliphatic-Alcohol-Induced Opaque-to-Transparent Transformation and Application of Solubility Theory in a Bis-Dipeptide-Based Supramolecular Gel

A bis-dipeptide supramolecular gelator (DMPV) is prepared, based on l-valine moieties having a pyridinyl group and a long fatty diamine. It is found that the gelator can immobilize organic/water binary mixed solvents, and gel-to-gel transitions with unprecedented opaque-to-transparent transformations are observed upon using aliphatic alcohols such as methanol, ethanol, 1-propyl alcohol, and isopropanol as the organic components. Morphological investigations indicate that a reassembly process occurs, and microstructure evolutions from agglomerates to nanofibers are observed. Opaque and transparent assemblies can interconvert, and respond and restore under mechanical force and pH stimuli. Moreover, Hansen and Flory-Huggins parameters are used to investigate the effect of the solvent on the gelation performance of DMPV. This may facilitate the structure and solvent optimizations and aid the development of advanced gel systems.

Molecular Insights into Gelation of Di-Fmoc-l-Lysine in Organic Solvent-Water Mixtures

Despite significant interest in molecular gels due to their intriguing structure formation through self-assembly and their stimuli-responsive behavior, our understanding of the gel formation mechanism of a low-molecular-weight gelator (LMWG) is incomplete. Here, we report a combined experimental and computational study on a LMWG, di-Fmoc-l-lysine, that has two aromatic moieties and multiple hydrogen bond donors and acceptors. Gelation in various organic solvent-water mixtures was obtained through the solvent-triggered technique. We show that an approach based on approximate cohesive energy density derived from density functional theory (DFT) calculations can capture the experimental solubility trend of LMWGs in different organic solvents. Furthermore, DFT calculations indicate parallel and helical structures to be the preferred structural motifs for gelator dimers. We believe that these motifs can potentially lead to fiber formation as observed with microscopy. Our work provides a relatively simple yet effective approach to quantify interactions between solvents and complex gelators that can help rationalize solubility and gelation behavior.

Unexpected aggregation induced circular dichroism, circular polarized luminescence and helical assembly from achiral hexaphenylsilole (HPS)

This work provides the finding of aggregation-induced circular dichroism (AICD), optical properties and helical assemblies of π–π conjugated molecule hexaphenylsilole (HPS).

Dynamic Evolution of Coaxial Nanotoruloid in the Self-Assembled Naphthyl-Containing l-Glutamide

Supramolecular gelation provides an efficient way of fabricating functional soft materials with various nanostructures. Amphiphiles containing naphthyl group and dialkyl l-glutamide with a methylene spacer, 1NALG and 2NALG, have been designed and their self-assembly in various organic solvents were investigated. Both of these compounds formed organogels in organic solvents. In the case of the alcohol solvents, the initially formed organogel underwent gel-precipitate transformation, which process was monitored by the UV-vis, CD spectra, and SEM observation. It was revealed that both the compounds formed the nanofiber structures in gel phases. Interestingly, in alcohol solvents, during the phase transition from the gel to precipitates, the nanofibers gradually transformed into a series of long coaxial solid nanotoruloid, a unique nanostructure that has never been observed in other self-assembly systems. In addition, during the gel formation, the nanofibers with supramolecular chirality or M-chirality were obtained. However, the coaxial nanotoruloid showed an inversed P-chirality. Comprehensive analysis based on various data and the gelator structure, substituent position, type of organic solvents, it was suggested that the synergistic interactions between the amide H-bond and π-π stacking of the naphthyl groups played important roles in the formation of the gels as well as the nanofiber, while the H-bonding ability of alcohol to the amide group can subtly regulate the gelator-gelator interactions and lead to the dynamic and hierarchical evolution of the unique nanostructures.

Controlled self-organization of cyanostilbene: emission tuning and photo-responsiveness

Cyanostilbene modified with dimethylaniline (CMD) could self-assemble into vesicles and fibrous morphologies depending on the solvophobic properties. Furthermore, morphology of well-defined nanostructures could be changed with enhanced emission triggered by the photo-isomerization of cyanostilbene. The present system has potential for building luminescent color conversion materials.

Solvent-polarity-tuned nanostructures assembled from modified octadecylcarbamate with an anthracen moiety

A-9-YMOC tends to form nanofibers whereas it self-assembles in polar solvents and nanoflowers in nonpolar solvents with different molecular arrangements.