A Unique Temperature-Induced Reverse Supramolecular Chirality-Assisted Gel-to-Gel Transition.

Supramolecular hydrogels typically undergo a gel-to-sol transition with heat, as intermolecular interactions within the gel weaken. Although gel-to-gel transitions during heating are rare, they may occur due to minor rearrangements caused by thermal forces in the supramolecular self-assembled structure. Here, an unprecedented temperature-induced gel-to-gel transition assisted by supramolecular chiral inversion in a hydrogel system is presented. The transition results from a left-handed M-type helix to a right-handed P-type helix, attributed to the π-system-conjugated amino acid, l-Tyrosine (Fm- l-Tyr). Upon solvent dilution, Fm-l-Tyr induces translucent hydrogel formed by entangled fibers with a kinetically stable left-handed M-type supramolecular helix. At 70 °C, hydrogel transforms into an opaque gel with a reverse supramolecular chirality yielding a thermodynamically stable right-handed P-type helix. Supramolecular chiral inversion is substantiated by two chiroptical methods. This unique gel-to-gel transition, accompanied by chiral inversion, is anticipated to attract attention, especially for applications sensitive to chirality.

A bio-inspired silkworm 3D cocoon-like hierarchical self-assembled structure from π-conjugated natural aromatic amino acids.

The formation of spontaneous 3D self-assembled hierarchical structures from 1D nanofibers is a significant breakthrough in materials science. Overcoming the major challenges associated with developing these 3D structures, such as uncontrolled self-assembly, complex procedures, and machinery, has been a formidable task. However, the current discovery reveals that simple π-system (fluorenyl)-functionalized natural aromatic amino acids, phenylalanine (Fmoc-F) and tyrosine (Fmoc-Y), can form bio-inspired 3D cocoon-like structures. These structures are composed of entangled 1D nanofibers created through supramolecular self-assembly using a straightforward one-step process of solvent casting. The self-assembly process relies on π-π stacking of the fluorenyl (π-system) moieties and intermolecular hydrogen bonding between urethane amide groups. The cocoon-like structures are versatile and independent of concentration, temperature, and humidity, making them suitable for various applications. This discovery has profound implications for materials science and the developed advanced biomaterials, such as Fmoc-F and Fmoc-Y, can serve as flexible foundational components for constructing 3D fiber-based structures.

Proton-Conductive Melanin-Like Fibers through Enzymatic Oxidation of a Self-Assembling Peptide.

Melanin pigments have various properties that are of technological interest including photo- and radiation protection, rich coloration, and electronic functions. Nevertheless, laboratory-based synthesis of melanin and melanin-like materials with morphologies and chemical structures that are specifically optimized for these applications, is currently not possible. Here, melanin-like materials that are produced by enzymatic oxidation of a supramolecular tripeptide structures that are rich in tyrosine and have a 1D morphology are demonstrated, that are retained during the oxidation process while conducting tracks form through oxidative tyrosine crosslinking. Specifically, a minimalistic self-assembling peptide, Lys-Tyr-Tyr (KYY) with strong propensity to form supramolecular fibers, is utilized. Analysis by Raman spectroscopy shows that the tyrosines are pre-organized inside these fibers and, upon enzymatic oxidation, result in connected catechols. These form 1D conducting tracks along the length of the fiber, which gives rise to a level of internal disorder, but retention of the fiber morphology. This results in highly conductive structures demonstrated to be dominated by proton conduction. This work demonstrates the ability to control oxidation but retain a well-defined fibrous morphology that does not have a known equivalent in biology, and demonstrate exceptional conductivity that is enhanced by enzymatic oxidation.

Fibrillar Network Dynamics during Oscillatory Rheology of Supramolecular Gels.

Supramolecular gels are three-dimensional network structures formed by the hierarchical self-assembly of small molecules through weak noncovalent interactions. On the basis of the various interactions contributed by specific functional groups/moieties, gels with different architectures can be constructed that are smart to the external stimuli such as pH, type of solvent, stress, temperature, etc. In the present work, we explore the oscillatory shear response of supramolecular self-assembled systems formed by the low-molecular-weight (LMW) gelator based on difunctionalized amino acid, florenylmethoxycarbonyl (Fmoc)-lysine(Fmoc), Fm-K(Fm) in aqueous buffer solution, at two different pH (6 and 7.4). Small amplitude oscillatory shear (SAOS) reported weak frequency dependence of moduli indicating a gel-like network structure. Large amplitude oscillatory shear (LAOS) indicated flow regimes dictated by yielding and subsequent network dynamics analogous to cagelike soft glassy events reported for colloidal systems. The three interval thixotropy test (3iTT) indicated recovery of moduli due to regelation contributed by the reversible interactions. A generalized network model framework is utilized to investigate the transient network characteristics of the Fm-K(Fm) gels. The "network creation" and "network loss" rates were chosen as exponential functions of scaled shear stress (= |τ12(t)G|) to effectively describe the complex response. On the basis of the insights, possible mechanisms to explain the differences/similarities in the response at different pH are speculated. It is further illustrated that the modeling strategy can be extended to supramolecular gels of different classes because of the commonality/universality of their response features under oscillatory shear.

Effect of Solution Properties and Operating Parameters on Needleless Electrospinning of Poly(Ethylene Oxide) Nanofibers Loaded with Bovine Serum Albumin.

BACKGROUND: This paper presents the effect of solution properties and operating parameters of polyethylene oxide (PEO) based nanofiber using a wire electrode-based needleless electrospinning. METHODS: The feed solution was prepared using a PEO dissolved in water or a water-ethanol mixture. The PEO solution is blended with Bovine Serum Albumin protein (BSA) as a model drug to study the effect of the electrospinning process on the stability of the loaded protein. The polymer solution properties such as viscosity, surface tension, and conductivity were controlled by adjusting the solvent and salt content. The morphology and fiber size distribution of the nanofiber was analyzed using scanning electron microscopy. RESULTS: The results show that the issue of a beaded nanofiber can be eliminated either by increasing the solution viscosity or by the addition of salt and ethanol to the PEO-water system. The addition of salt and solvent produced a high frequency of smaller fiber diameter ranging from 100 to 150 nm. The encapsulation of BSA in PEO nanofiber was characterized by three different spectroscopy techniques (i.e. circular dichroism, Fourier transform infrared, and fluorescence) and the results showed the BSA is well encapsulated in the PEO matrix with no changes in the protein structure. CONCLUSION: This work may serve as a useful guide for a drug delivery industry to process a nanofiber at a large and continuous scale with a blend of drugs in nanofiber using a wire electrode electrospinning.

Biocytin-Based pH-Stimuli Responsive Supramolecular Multivariant Hydrogelator for Potential Applications.

Self-assembly is one of the important and fruitful methods to develop various biomaterials, especially hydrogels, from biologically important small molecules. Two such molecules are d-biotin (B-vitamin), which plays several vital roles in biology and is used in the biomedical field, and biocytin, which is used as an intracellular marker and also to identify neurons. Despite the significance of these biomolecules, their usage in the form of hydrogel is very limited. Herein, for the first time, a pH-stimuli responsive supramolecular hydrogel based on biocytin, an amidation product of d-biotin and l-lysine, has been developed to extend the use of d-biotin, biocytin, and l-lysine in the biomedical field. The newly developed hydrogelator such as Nα-(fluorenylmethoxycarbonyl)-d-biocytin) [FmB] exhibited pH-dependent hydrogelation and exhibited a thixotropic and thermally reversible nature at a physiological pH, while it displayed a supergelator nature at an acidic pH. Electron microscopic analysis of FmB hydrogel indicated the formation of a fibrous network structure. Various biophysical measurements including CD, FT-IR, fluorescence, and powder XRD demonstrated that intermolecular interactions such as aromatic and hydrogen bonding stabilize the fibrous structure of FmB. Importantly, the FmB hydrogel was found to be viable for the cells and displayed specific bactericidal activity against Escherichia coli. Since making hydrogels directly with drugs or vitamins rather than inclusion in other hydrogels (matrix) may provide new biomaterials that can act as self-delivery systems, we believe that the newly developed multivariant biocytin-based supramolecular hydrogel can significantly advance the application of biotin and biocytin in various fields.

A Low-Molecular-Weight Gelator Composed of Pyrene and Fluorene Moieties for Effective Charge Transfer in Supramolecular Ambidextrous Gel.

Charge-transfer (CT) gel materials obtained from low-molecular-weight (LMW) compounds through a supramolecular self-assembly approach have received fascinating attention by many researchers because of their interesting material property and potential applications. However, most of the CT gel materials constructed were of organogels while the construction of CT gels in the form of a hydrogel is a challenge because of the solubility issue in water, which considerably limits the use of CT hydrogels. Herein, for the first time, we report a new LMW gelator [Nα-(fluorenylmethoxycarbonyl)-Nε-(δ-butyric-1-pyrenyl)-l-lysine, (FmKPy)], composed of two functional moieties such as fluorenylmethoxycarbonyl and pyrene, which not only parade both hydro and organo (ambidextrous) supramolecular gel formation but also exhibit CT ambidextrous gels when mixed with an electron acceptor such as 2,4,7-trinitro-9-fluorenone (TNF). This finding is significant as the established CT organogelator in the literature did not form an organogel in the absence of an electron acceptor or lose their gelation property upon the addition of the acceptor. CT between pyrene and TNF was confirmed by the color change as well as the appearance of the CT band in the visible region of the absorption spectrum. CT between FmKPy and TNF was supported by the solvent dilution method using tetrahydrofuran as the gel breaker and pyrene fluorescence quenching in the case compound containing pyrene and TNF. The morphology of FmKPy ambidextrous gels indicates the fibrous nature while the self-assembled structure is primarily stabilized by π-π stacking among fluorenyl and pyrenyl moieties and hydrogen bonding between amide groups. The FmKPy-TNF CT ambidextrous gel retains the fibrous nature; however, the size of the fibers changed. In FmKPy-TNF CT gels, TNF is intercalated between pyrene moieties in the self-assembled structure as confirmed by fluorescence quenching and powder X-ray diffraction. The FmKPy ambidextrous gel exhibits significant properties such as low minimum gelation concentration (MGC), thixotropic nature, pH stimuli response, and high thermal stability. Upon the addition of TNF, the FmKPy-TNF CT ambidextrous gel maintains all these properties except MGC which increased for FmKPy-TNF. Because pyrene-based LMW organogels have been developed widely for many applications while their hydrogels were limited, the current finding of the pyrene-based ambidextrous fluorescent gel with the CT property provides a wide opportunity to use FmKPy as a soft material maker and also for potential applications in fields like surface coating, three-dimensional printing, and so forth.

Role of Intramolecular Aromatic π-π Interactions in the Self-Assembly of Di-l-Phenylalanine Dipeptide Driven by Intermolecular Interactions: Effect of Alanine Substitution.

Although the role of intermolecular aromatic π-π interactions in the self-assembly of di-l-phenylalanine (l-Phe-l-Phe, FF), a peptide that is known for hierarchical structure, is well established, the influence of intramolecular π-π interactions on the morphology of the self-assembled structure of FF has not been studied. Herein, the role of intramolecular aromatic π-π interactions is investigated for FF and analogous alanine (Ala)-containing dipeptides, namely, l-Phe-l-Ala (FA) and l-Ala-l-Phe (AF). The results reveal that these dipeptides not only form self-assemblies, but also exhibit remarkable differences in structural morphology. The morphological differences between FF and the analogues indicate the importance of intramolecular π-π interactions, and the structural difference between FA and AF demonstrates the crucial role of the nature of intramolecular side-chain interactions (aromatic-aliphatic or aliphatic-aromatic), in addition to intermolecular interactions, in deciding the final morphology of the self-assembled structure. The current results emphasise that intramolecular aromatic π-π interaction may not be essential to induce self-assembly in smaller peptides, and π (aromatic)-alkyl or alkyl-π (aromatic) interactions may be sufficient. This work also illustrates the versatility of aromatic and a combination of aromatic and aliphatic residues in dipeptides in the formation of structurally diverse self-assembled structures.

Hydrogelation Induced by Change in Hydrophobicity of Amino Acid Side Chain in Fmoc-Functionalised Amino Acid: Significance of Sulfur on Hydrogelation.

Although a few Fmoc-functionalised amino acids (Fmoc-AA) are capable of forming hydrogels, the exact levels of hydrophobicity, hydrogen bonding, and ionic nature of the Fmoc-AA gelator required for hydrogel formation remains uncertain. Here, the role of hydrophobicity of amino acid side chain, particularly in the formation of hydrogel, was studied by using Fmoc-norleucine (Fmoc-Nle) and its simple sulfur analogues such as Fmoc-methionine (Fmoc-M) in which the γCH2 of Fmoc-Nle is replaced by sulfur. Results indicate that Fmoc-M forms thermally reversible hydrogels in water (pH ca. 6.8), whereas Fmoc-Nle fails to display any gelation under similar conditions. The result suggests that substitution of the sulfur atom likely reduces the hydrophobicity of the alkyl side chain in Fmoc-Nle to the optimum level, which is sufficient to induce supramolecular hydrogelation in Fmoc-M. The difference in the self-association behaviour of Fmoc-M and Fmoc-Nle emphasise the importance of weak noncovalent interaction between side chains (in addition to the hydrogen-bond and aromatic interactions) to stabilise supramolecular self-assembly of Fmoc-functionalised compounds. The current observations provide a lead to the design of new sulfur-based low molecular weight gelators for various potential applications.

An additional fluorenylmethoxycarbonyl (Fmoc) moiety in di-Fmoc-functionalized L-lysine induces pH-controlled ambidextrous gelation with significant advantages.

In recent years, several fluorenylmethoxycarbonyl (Fmoc)-functionalized amino acids and peptides have been used to construct hydrogels, which find a wide range of applications. Although several hydrogels have been prepared from mono Fmoc-functionalized amino acids, herein, we demonstrate the importance of an additional Fmoc-moiety in the hydrogelation of double Fmoc-functionalized L-lysine [Fmoc(Nα)-L-lysine(NεFmoc)-OH, (Fmoc-K(Fmoc))] as a low molecular weight gelator (LMWG). Unlike other Fmoc-functionalized amino acid gelators, Fmoc-K(Fmoc) exhibits pH-controlled ambidextrous gelation (hydrogelation at different pH values as well as organogelation), which is significant among the gelators. Distinct fibrous morphologies were observed for Fmoc-K(Fmoc) hydrogels formed at different pH values, which are different from organogels in which Fmoc-K(Fmoc) showed bundles of long fibers. In both hydrogels and organogels, the self-assembly of Fmoc-K(Fmoc) was driven by aromatic π-π stacking and hydrogen bonding interactions, as evidenced from spectroscopic analyses. Characterization of Fmoc-K(Fmoc) gels using several biophysical methods indicates that Fmoc-K(Fmoc) has several advantages and significant importance as a LMWG. The advantages of Fmoc-K(Fmoc) include pH-controlled ambidextrous gelation, pH stimulus response, high thermal stability (∼100 °C) even at low minimum hydrogelation concentration (0.1 wt%), thixotropic property, high kinetic and mechanical stability, dye removal properties, cell viability to the selected cell type, and as a drug carrier. While single Fmoc-functionalized L-lysine amino acids failed to exhibit gelation under similar experimental conditions, the pH-controlled ambidextrous gelation of Fmoc-K(Fmoc) demonstrates the benefit of a second Fmoc moiety in inducing gelation in a LMWG. We thus strongly believe that the current findings provide a lead to construct or design various new synthetic Fmoc-based LMW organic gelators for several potential applications.

"Cross-linked fibrous" spherulites formed from a low molecular weight compound, Fmoc-functionalized phenolic amino acid.

While biomacromolecules such as proteins are shown to form fibrous spherulites, which are generally "semicrystalline" in nature, here we show that a simple, low molecular weight compound such as fluorenylmethoxycarbonyl-functionalized phenolic amino acid (Fmoc-l-tyrosine) can form "fibrous" spherulites with highly "cross-linked" microfibrils using the supramolecular self-assembly process.

Effect of aqueous ethanol on the triple helical structure of collagen.

Collagen, the most abundant protein in mammals, is widely used for making biomaterials. Recently, organic solvents have been used to fabricate collagen-based biomaterials for biological applications. It is therefore necessary to understand the behavior of collagen in the presence of organic solvents at low (≤50%, v/v) and high (≥90%, v/v) concentrations. This study was conducted to examine how collagen reacts when exposed to low and high concentrations of ethanol, one of the solvents used to make collagen-based biomaterials. Solubility testing indicated that collagen remains in solution at low concentrations (≤50%, v/v) of ethanol but precipitates (gel-like) thereafter, irrespective of the method of addition of ethanol (single shot or gradual addition); this behavior is different from that observed recently with acetonitrile. Collagen retains its triple helix in the presence of ethanol but becomes thermodynamically unstable, with substantially reduced melting temperature, with increasing concentration of ethanol. It was also found that the CD ellipticity at 222 nm, characteristic of the triple-helical structure, does not correlate with the thermal stability of collagen. Time-dependent experiments reveal that the collagen triple helix is kinetically stable in the presence of 0-40% (v/v) ethanol at low temperature (5 °C) but highly unstable in the presence of ethanol at elevated temperature (~34 °C). These results indicate that when ethanol is used to process collagen-based biomaterials, such factors as temperature and duration should be done taking into account, to prevent extensive damage to the triple-helical structure of collagen.

1,1,1,3,3,3-Hexafluoro-2-propanol and 2,2,2-trifluoroethanol solvents induce self-assembly with different surface morphology in an aromatic dipeptide.

Peptide based self-assembled structures, especially those from smaller peptides, have attracted much research interest due to their potential applications as biomaterials. These structures have been produced using different solvents (one of the methods), including alcohols, except fluorinated alcohols, which are believed to support non-aggregated structures. Herein, we have studied the ability of 2,2,2-trifluoroethanol (TFE) and 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) solvents to induce self-assembly of an aromatic dipeptide, namely Tyr-Phe (YF). SEM images showed that HFIP and TFE can induce self-assembly with completely different morphologies, namely microribbons and microspheres, respectively, when YF is dried on a glass surface. Optical microscopic images showed that the microribbons possess birefringence under polarized light, whereas the microspheres do not, indicating that the self-assembled structures derived from HFIP solution are more highly ordered and crystalline in nature than those derived from TFE. Spectroscopic results indicated that the YF peptide adopts completely different conformations in these solvents. Time-dependent experiments suggested that the conformation of YF in HFIP is kinetically unstable and undergoes conformational change, whereas it is more stable in TFE, demonstrating that the modes of interaction of the TFE and HFIP solvents with the peptide are dissimilar. Different self-assembled structures were observed at different time intervals when YF was incubated in neat HFIP and 10% HFIP-90% TFE, establishing that the monomeric conformation plays a dominant role in deciding the final self-assembled structure (morphology) of YF. The current results demonstrate that TFE and HFIP solvents can produce self-assembled structures with different morphologies during solvent evaporation, despite their similar properties, such as secondary structural (α-helix) induction and preserving the peptide in its monomeric conformation in solution.

2,2,2-Trifluoroethanol disrupts the triple helical structure and self-association of type I collagen.

Collagen, a fibrous structural protein, is a major component of skin, tendon, bone, and other connective tissues. Collagen is one of the dominant biomaterials used for tissue engineering and drug delivery applications. 2,2,2-Trifluoroethanol (TFE) has been used as a co-solvent in the preparation of collagen based biomaterials, which are used for tissue engineering applications. However, the basic knowledge about the structural behavior of collagen in TFE is necessary for an adequate application of collagen as a carrier system. In this work, the effect of TFE on the structure and self-association of collagen has been studied in detail using different spectroscopic methods such as circular dichroism (CD), Fourier transform infrared (FTIR), and UV-Vis absorption. The results obtained from CD and FTIR suggest that collagen transform its structure from triple helix to predominantly unordered conformation with increasing concentration of TFE. Thermal melting studies reveal that the stability of collagen triple helix decreases even at low concentration of TFE. Turbidity measurements indicate that TFE, at higher concentrations, inhibits the collagen fibril formation which arises due to the self-association of collagen molecules. TFE has conventionally been known to promote the ordered structures in proteins and peptides. Destabilization of collagen triple helix by TFE is first of its kind information on the effect of TFE to disrupt the native conformation of proteins.