Optimizing Biomimetic Gelators Constructed from Amino Acid Building Blocks

Tryptophan-Based Self-Assembling Peptides with Bacterial Flocculation and Antimicrobial Properties

Tryptophan as an aromatic amino acid with a hydrophobic indole group plays important roles in stabilizing protein structures and enhancing molecular bindings in nature, but was rarely used in the molecular design of self-assembling peptides or gelators. Therefore, we prepared a series of short peptides from Trp amino acids and examined the potential roles of Trp residues for regulating peptide self-assembly and gelation. The introduced Trp amino acids not only diversify the molecular structures of peptide gelators, but also promote aromatic and hydrogen-bonding interactions for supramolecular self-assembling and gelation, which generates self-assembled nanostructures with twisted helical morphologies and supramolecular hydrogels with low minimal gelation concentrations. More importantly, the self-assembling peptides with Trp residues displayed strong preference for interacting with the lipidic membranes of bacteria, which resulted in bacterial flocculation and the death of E. coli and S. aureus.

In Situ Generated Medical Devices

Medical devices play a major role in all areas of modern medicine, largely contributing to the success of clinical procedures and to the health of patients worldwide. They span from simple commodity products such as gauzes and catheters, to highly advanced implants, e.g., heart valves and vascular grafts. In situ generated devices are an important family of devices that are formed at their site of clinical function that have distinct advantages. Among them, since they are formed within the body, they only require minimally invasive procedures, avoiding the pain and risks associated with open surgery. These devices also display enhanced conformability to local tissues and can reach sites that otherwise are inaccessible. This review aims at shedding light on the unique features of in situ generated devices and to underscore leading trends in the field, as they are reflected by key developments recently in the field over the last several years. Since the uniqueness of these devices stems from their in situ generation, the way they are formed is crucial. It is because of this fact that in this review, the medical devices are classified depending on whether their in situ generation entails chemical or physical phenomena.

Dicyclohexylurea derivatives of amino acids as dye absorbent organogels and anion sensors

Dicyclohexyl urea (DCU) derivatives of amino acids Fmoc-Phe-DCU (M1), Fmoc-Phg-DCU (M2) and Fmoc-Gaba-DCU (M3) have been shown to form phase selective, thermoreversible and mechanically robust gels in a large range of organic solvents. This is the first report of low molecular weight gelators (LMWG) from DCU derivatives of amino acids. The self-assembly mechanism of the organogels has been probed using concentration dependent 1H NMR, DMSO titration 1H NMR, fluorescence, FTIR, PXRD and FESEM techniques. Self-assembly leading to gelation process is mainly driven by hydrophobicity and π-π stacking interactions in between Fmoc groups. Interestingly, the gels can absorb several kinds of organic dyes efficiently and can be reused for dye absorption for multiple cycles. Additionally, M1-M3 act as sensors for anions like fluoride, acetate and hydroxide, for which they have specific fluorescence response. Gel formation by M1-M3 is completely arrested in the presence of fluoride. The possible binding mode of fluoride has been delineated using DFT studies. Calculations suggest, involvement of urea NH in a six membered intramolecular hydrogen bond, rendering it unavailable for fluoride binding. Backbone -NH of the amino acids of M1-M3 is responsible for fluoride binding. The reported small, economically viable, synthetically facile molecules not only enrich the repertoire of LMWG molecules, but can have multifaceted applications.

Supramolecular assemblies of amide-derived organogels featuring rigid π-conjugated phenylethynyl frameworks

Organogels, being an important class of soft materials, have evolved to be one of the most attractive subjects bridging supramolecular chemistry and material sciences due to their structural diversity and associated physical properties. Myriad applications in fields such as optoelectronics, light harvesting, environmental science, and regenerative medicine are being envisaged. Supramolecular gels usually are formed through self-aggregation of small-molecule gelators to form entangled self-assembled fibrillar networks through a combination of non covalent interactions such as hydrogen bonding, π-π stacking, electrostatic forces, donor-acceptor interactions, metal coordination, solvophobic forces, and van der Waals interactions. This feature article discusses recent and current state of research on amide derived organogelators bearing rigid conjugated phenylethynyl building blocks. Selective examples from our works along with some closely related examples from literature have been highlighted to showcase the structural diversity and their potential applications in supramolecular chemistry and materials science.

Self-assembly of chiral trans-cyclobutane-containing β-dipeptides into ordered aggregates

Two chiral synthetic β-dipeptides have been constructed, one with two trans-cyclobutane residues and the other with one trans and one cis fragment, 1 and 2, respectively, and investigated to get insight into the non-covalent interactions responsible for their self-assembly to form ordered aggregates, as well into parameters such as their morphology and size. Experimental evidence of the formation of these assemblies was provided by spectroscopy, microscopy and X-ray diffraction experiments that suggest the formation of nanoscale helical aggregates. This process involves a conformational change in the molecules of each dipeptide with respect to the preferred conformation of the isolated molecules in solution. A high-resolution NMR spectroscopy study allowed the determination of the dynamics of the gelation process in [D(8)]toluene and the sol-gel transition temperature, which was around 270 K in this solvent at a concentration of 15 mM. NMR spectroscopy experiments also provided some information about conformational changes involved in the sol-gel transition and also suggested a different gel packing for each dipeptide. These observations have been nicely explained by computational studies. The self-assembly of the molecules has been modelled and suggested a head-to-head molecular arrangement for 1 and a head-to-tail arrangement for 2 to give helical structures corresponding to hydrogen-bonded single chains. These chains interact with one another in an antiparallel way to afford bundles, the significant geometry parameters of which fit well to the main peaks observed in wide-angle X-ray diffraction spectra of the aggregates in the solid state.

Metathesis within self-assembled gels: transcribing nanostructured soft materials into a more robust form

This article reports the covalent capture of self-assembled gel-phase materials using alkene metathesis. Gels assembled from a gelator functionalized with peripheral alkene groups were reacted with Grubbs' second generation catalyst, added as a solution to the top of the gel and allowed to diffuse into the material for 24 h. Using this approach, the fibrillar self-assembled network was covalently captured, yielding a large amount of insoluble material that was robust, thermally stable, and highly swellable in solvents compatible with the gelator. Scanning electron microscopy demonstrated that the insoluble metathesized material contained nanoscale fibers, which were aligned into rigid fiber bundles on drying. When the gelator was assembled in the presence of a second non-cross-linkable gelator, self-sorting took place, giving rise to two independent gelator networks. Metathesis then generated an insoluble material in which the individual gel fibers of the cross-linkable gelator were captured, whereas the nonreactive gelator could be washed away. Intriguingly, using this approach appeared to hinder the alignment of gel fibers into rigid fiber bundles. Instead, individual, well-defined, robust gelator nanofibers were visualized in the dried materials. In addition, the material synthesized this way appeared to be even more highly porous and swellable on the addition of solvent. In summary, this article demonstrates that metathesis is an effective way to capture nanostructured gel-phase materials covalently, with the judicious choice of additives helping to control the morphology and behavior of the materials generated. This approach to nanofabrication could ultimately give rise to nanostructured polymeric materials with a wide range of applications.

Tailoring the properties of guanosine-based supramolecular hydrogels

We demonstrate that very stable hydrogels can be formed in aqueous potassium chloride solution by mixing a well-known gelator (guanosine, G) with a nongelator of similar structure (2',3',5'-tri-O-acetylguanosine, TAcG), and through a variety of characterization methods including rheology, small-angle neutron scattering, differential scanning calorimetry, and atomic force microscopy, we report substantial progress toward elucidating the factors that control the structure and stability of this fibrous gel system. The results suggest that the tailorability, long lifetime stability, and thermomechanical behavior of these gels derives from a reduction in the driving force toward crystallization with increased hydrophobic (TAcG) content, accompanied by a simultaneous decrease in fiber length and an increase in fiber width.

Pharmaceutical organogels prepared from aromatic amino acid derivatives

Organogels are semi-solid systems in which an organic liquid phase is immobilized by a 3-dimensional network composed of self-assembled gelator molecules. Although there is a large variety of organogel systems, relatively few have been investigated in the field of drug delivery, owing mostly to the lack of information on their biocompatibility and toxicity. In this work, organogelator-biocompatible structures based on aromatic amino acids, namely, tyrosine, tryptophan, and phenylalanine were synthesized by derivatization with aliphatic chains. Their ability to gel an injectable vegetable oil (i.e. safflower oil) and to sustain the release of a model anti-Alzheimer drug (i.e. rivastigmine) was then evaluated. Organogels and molecular packing were characterized by differential scanning calorimetry, rheology analysis, Fourier-transform infrared spectroscopy and X-ray crystallography. The amino acid derivatives were able to gel safflower oil through van der Waals interactions and H-bonds. Tyrosine-derivatives produced the strongest gels while tryptophan was associated with poor gelling properties. The superior gelling ability of tyrosine derivatives could be explained by their well-structured 2-dimensional packing in the network. The addition of an optimal N-methyl-2-pyrrolidone amount to tyrosine gels fluidized the network and allowed their injection through conventional needles. Upon contact with an aqueous medium, the gels formed in situ and released entrapped rivastigmine in a sustained fashion.

Fluorescent magnesium(II) coordination polymeric hydrogel

A pH and mechano-responsive coordination polymeric gel was developed without the use of long chain hydrophobic groups. The hydrogel was synthesised by reacting the aqueous solution of Mg2+ with the basic aqueous solution of N-(7-hydroxyl-4-methyl-8-coumarinyl)-alanine. The gelation is attributed to the self-aggregation of 1D coordination polymers to form 3D nanostructures through non-covalent interactions to entrap water molecules. The freeze-dried hydrogel exhibits a fibrillar network structure with a uniform ribbon shape. UV/vis absorption studies illustrate that the hydrogel displays a typical pi-pi* transition. The fluorescence intensity of the hydrogel is enhanced drastically with a longer lifetime upon gel formation. Mechanical analysis including dynamic oscillation on shear, steady shear and creep (retardation-relaxation) testing have been performed to elucidate the supramolecular nature of the 3D assembly. Together with the viscoelastic properties and biocompatibility, the Mg2+ hydrogel may find utility as a novel soft material in biomedical applications.

Pentapeptide based organogels: the role of adjacently located phenylalanine residues in gel formation

A terminally protected self-assembling pentapeptide Boc-Leu(1)-Val(2)-Phe(3)-Phe(4)-Ala(5)-OMe 1 bearing sequence similarity with Aβ17-21 (the fragment 17-21 of the amyloid β-peptide Aβ42) forms thermoreversible transparent gels in various organic solvents including benzene, toluene, m-xylene and 1,2-dichlorobenzene. A series of its variants have been synthesized in order to study the role of adjacently located phenylalanine residues and the protecting groups for gelation in different organic solvents. Replacement of any of the Phe residues of the Phe-Phe segment with any other hydrophobic α-amino acid residue drastically changes the gel forming properties indicating that both Phe residues have an important role in gel formation. These gels are characterised using field emission scaning electron microscopy (FE-SEM), transmission electron microscopy (TEM), FT-IR and wide angle X-ray scattering (WAXS) studies. WAXS studies of the peptide 1-benzene gel indicate that π-π interaction is responsible for gel formation and it reveals the necessity of the Phe residues in gel formation. Transmission electron microscopy (TEM) of the gels reveals a nanofibrillar morphology, which is obtained from the self-assembled gelators in the gel phase. These gels bind with a physiological dye, Congo red, and show a green birefringence under cross polarizers, which is a characteristic feature of amyloid fibrils.