Sisir Debnath

Organogelation and Hydrogelation of Low-Molecular-Weight Amphiphilic Dipeptides: pH Responsiveness in Phase-Selective Gelation and Dye Removal†

The search for efficient low-molecular-weight gelators (LMWGs) with possible structure-activity correlation is on the rise. The present work reports a novel set of amphiphilic dipeptide-based carboxylic acids capable of efficiently gelating organic solvents. More interestingly, their sodium salts showed enhanced efficiency in organogelation with the additional ability to gelate water. Electrostatic interactions present in the aggregation of the sodium carboxylates of amphiphilic dipeptides seem to be important because some of the nongelator carboxylic acids turned out to be excellent gelators upon salt formation. The combinations and sequence of the amino acids in the dipeptide moiety were systematically altered to understand the collective importance of the nonpolar aliphatic/aromatic substitution in amino acids in the self-assembling behavior of amphiphiles. Almost a 20-fold enhancement in the gelation ability was observed on reversing the sequence of the amino acid residues, and in some cases, nongelators were transformed to efficient gelators. Spectroscopic and microscopic studies of these thermoreversible organo/hydrogels revealed that balanced participation of the noncovalent interactions including hydrogen bonding and van der Waals interactions are crucial for organo/hydrogelation. These dipeptides selectively gelate organic solvents from their mixtures with water, and the xerogels prepared from these organogels showed time-dependent adsorption of dyes such as crystal violet. The most remarkable feature of these gelators is the pH responsiveness, which was aptly utilized for the pH-dependent phase-selective gelation of either solvent in a biphasic mixture of oil and water. The dissimilar gelation ability of the acid and its salt originating from the pH responsiveness of the amphiphilic dipeptide was employed in the instant removal of large amounts of dyes for wastewater treatment.

Peptide Nanofibers with Dynamic Instability through Nonequilibrium Biocatalytic Assembly

We demonstrate the formation of supramolecular peptide nanofibers that display dynamic instability; i.e., they are formed by competing assembly and disassembly reactions, where assembly is favored away from equilibrium. The systems are based on competitive catalytic transacylation and hydrolysis, producing a self-assembling aromatic peptide amphiphile from amino acid precursors that temporarily exceeds the critical gelation concentration, until the competing hydrolytic reaction takes over. Analysis by atomic force microscopy shows consecutive nanofiber formation and shortening. The process results in macroscopically observable temporary hydrogelation, which may be repeated upon refueling the system with further addition of the chemically activated amino acid precursor. Nonequilibrium nanostructures open up opportunities for mimicry of the behavior of dynamic gels found in natural systems and provide components for future adaptive nanotechnologies.

Hydrogelation Through Self-Assembly of Fmoc-Peptide Functionalized Cationic Amphiphiles: Potent Antibacterial Agent

The present work reports a new class of antibacterial hydrogelators based on anti-inflammatory N-fluorenyl-9-methoxycarbonyl (Fmoc) amino acid/peptides functionalized cationic amphiphiles. These positively charged hydrogelators were rationally designed and developed by the incorporation of a pyridinium moiety at the C-terminal of Fmoc amino acid/peptides, because the pyridinium-based amphiphiles are a known antibacterial agent due to their cell membrane penetration properties. The Fmoc amino acid/peptide-based cationic amphiphiles efficiently gelate (minimum gelation concentration approximately 0.6-2.2%, w/v) water at room temperature. Judicious variation of amino acid and their sequences revealed the architectural dependence of the molecules on their gelation ability. Several microscopic techniques like field-emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM) were used to obtain the visual insight of the morphology of the gel network. A number of spectroscopic techniques like circular dichroism, FTIR, photoluminescence, and XRD were utilized to know the involvement of several noncovalent interactions and participation of the different segments of the molecules during gelation. Spectroscopic results showed that the pi-pi interaction and intermolecular hydrogen bonding are the major responsible factors for the self-assembled gelation process that are oriented through an antiparallel beta-sheet arrangement of the peptide backbone. These Fmoc-based cationic molecules exhibited efficient antibacterial activity against both Gram-positive and Gram-negative bacteria.

Head group modulated pH-responsive hydrogel of amino acid-based amphiphiles: entrapment and release of cytochrome c and vitamin B12.

The present study describes the rational design and synthesis of amino acid-based amphiphilic hydrogelators, which were systemically fine-tuned at the head group to develop pH-responsive hydrogels. To understand the basic structural requirements of a low molecular weight amphiphilic hydrogelator, 10 analogous amphiphiles based on L-phenylalanine and L-tyrosine with structurally related head group were synthesized. Among them, three with quaternary ammonium substitution at the head group formed transparent hydrogels at room temperature while others were unable to gelate water. To establish correlations between the head group architecture of the gelators and their supramolecular arrangements, a variety of spectroscopic and microscopic techniques were investigated that showed that a balance between hydrophilicity and hydrophobicity is required to achieve hydrogelation. Interestingly, the gelator with tyrosinate in its head group showed remarkable response toward external pH. All hydrogels including the pH-responsive one were used in the controlled and/or pH-triggered release of entrapped (with in hydrogels) vitamin B12 and cytochrome c at different pHs. Since the hydrogels were formed at room temperature without heating, this could be very important during the entrapment of biomolecules such as proteins because of their heat sensitivity. At biological pH (7.4), the release of entrapped biomolecules from all three hydrogels was caused by diffusion through the gel structure, but at endosomal pH (approximately 5.5) and further lower pH, the release rate of biomolecules from the pH-responsive hydrogel with tyrosinate head group (pKa approximately equal to 7.2) increased by 9-10-fold compared to that observed at physiological pH, because of gel dissolution. Retention of the structure and activity of released biomolecule has established the prospect of the hydrogel as an efficient drug delivery vehicle.

Aromatic peptide amphiphiles: significance of the Fmoc moiety

Aromatic peptide amphiphile hydrogelators commonly utilise the fluorenyl-9-methoxycarbonyl moiety as an N-terminal capping group. Material properties and spectroscopic techniques show the influence of alternative linkers between the fluorenyl moiety and the peptide. This study establishes whether methoxycarbonyl is an optimal or mainly convenient linker, for this class of self-assembling systems.

Differential Self-Assembly and Tunable Emission of Aromatic Peptide Bola-Amphiphiles Containing Perylene Bisimide in Polar Solvents Including Water

We demonstrate the self-assembly of bola-amphiphile-type conjugates of dipeptides and perylene bisimide (PBI) in water and other polar solvents. Depending on the nature of the peptide used (glycine-tyrosine, GY, or glycine-aspartic acid, GD), the balance between H-bonding and aromatic stacking can be tailored. In aqueous buffer, PBI-[GY]2 forms chiral nanofibers, resulting in the formation of a hydrogel, while for PBI-[GD]2 achiral spherical aggregates are formed, demonstrating that the peptide sequence has a profound effect on the structure formed. In water and a range of other polar solvents, self-assembly of these two PBI-peptides conjugates results in different nanostructures with highly tunable fluorescence performance depending on the peptide sequence employed, e.g., fluorescent emission and quantum yield. Organogels are formed for the PBI-[GD]2 derivative in DMF and DMSO while PBI-[GY]2 gels in DMF. To the best of our knowledge, this is the first successful strategy for using short peptides, specifically, their sequence/structure relationships, to manipulate the PBI nanostructure and consequent optical properties. The combination of controlled self-assembly, varied optical properties, and formation of aqueous and organic gel-phase materials may facilitate the design of devices for various applications related to light harvesting and sensing.

Insights into the coassembly of hydrogelators and surfactants based on aromatic peptide amphiphiles

The coassembly of small molecules is a useful means of increasing the complexity and functionality of their resultant supramolecular constructs in a modular fashion. In this study, we explore the assembly and coassembly of serine surfactants and tyrosine-leucine hydrogelators, capped at the N-termini with either fluorenyl-9-methoxycarbonyl (Fmoc) or pyrene. These systems all exhibit self-assembly behavior, which is influenced by aromatic stacking interactions, while the hydrogelators also exhibit β-sheet-type arrangements, which reinforce their supramolecular structures. We provide evidence for three distinct supramolecular coassembly models; cooperative, disruptive, and orthogonal. The coassembly mode adopted depends on whether the individual constituents (I) are sufficiently different, such that effective segregation and orthogonal assembly occurs; (II) adhere to a communal mode of self-assembly; or (III) act to compromise the assembly of one another via incorporation and disruption. We find that a greater scope for controllable coassembly exists within orthogonal systems; which show minimal relative changes in the native gelators supramolecular structure by Fourier transform infrared spectroscopy (FTIR), circular dichroism (CD), and fluorescence spectroscopy. This is indicative of the segregation of orthogonal coassembly constituents into distinct domains, where surfactant chemical functionality is presented at the surface of the gelators supramolecular fibers. Overall, this work provides new insights into the design of modular coassembly systems, which have the potential to augment the chemical and physical properties of existing gelator systems.

Imidazolium Bromide-Based Ionic Liquid Assisted Improved Activity of Trypsin in Cationic Reverse Micelles

The present work reports the imidazolium-based ionic liquids (ILs) assisted enhancement in activity of water-pool solubilized enzyme trypsin in cationic reverse micelles of CTAB. A set of imidazolium ILs (1-alkyl-3-methyl imidazolium bromides) were prepared with varying lengths of their side arm which results in the differential location of these organic salts in the reverse micelles. The different ILs offered varied activating effects on the biocatalyst. The activity of trypsin improved approximately 30-300% in the presence of 0.1-10 mM of different ILs in reverse micelles of CTAB. Trypsin showed approximately 300% (4-fold) increment in its activity in the presence of IL 2 (1-ethyl-3-methyl imidazolium bromide, EMIMBr) compared to that observed in the absence of IL in CTAB reverse micelles. The imidazolium moiety of the IL, resembling the histidine amino acid component of the catalytic triad of hydrolases and its Br(-) counterion, presumably increases the nucleophilicity of water in the vicinity of the enzyme by forming a hydrogen bond that facilitates the enzyme-catalyzed hydrolysis of the ester. However, the ILs with increasing amphiphilic character had little to no effect on the activity of trypsin due to their increased distance from the biocatalyst, as they tend to get localized toward the interfacial region of the aggregates. Dynamic light scattering experimentation was carried out in the presence of ILs to find a possible correlation between the trypsin activity and the size of the aggregates. In concurrence with the observed highest activity in the presence of IL 2, the circular dichroism (CD) spectrum of trypsin in CTAB reverse micelles doped with IL 2 exhibited the lowest mean residue ellipticity (MRE), which is closest to that of the native protein in aqueous buffer.

Differential supramolecular organisation of Fmoc-dipeptides with hydrophilic terminal amino acid residues by biocatalytic self-assembly

The study of enzymatically triggered self-assembly of aromatic peptide amphiphiles has become increasingly popular in recent years and has lead to a variety of nanoscale architectures. As hydrophobic interactions have been recognised as a major driving force in their self-assembly, typically, the peptide components are found to be hydrophobic in nature, containing aromatic or aliphatic amino acid residues. In this article, we use subtilisin triggered self-assembly of four closely related Fmoc-dipeptide amphiphiles with terminal hydrophilic amino acid residues, YT, YS, YN and YQ, in order to introduce a new functionality to the self-assembled systems, and determine the influence of each amino acid side chain. We use microscopy techniques, rheology, fluorescence, FTIR and CD to demonstrate differences in molecular assembly, mechanical properties and nanoscale architecture as a direct result of the subtle molecular variance of each system. We demonstrate that the amino acid side chain in position two directly affects the molecular packing abilities in the supramolecular structure, with YT, YS and YN forming nanoscale fibres with mechanical properties being linked to the functionality of the amino acid side chain, and YQ forming spherical structures due to steric effects associated with the glutamine side chain prohibiting the adoption of the typical π–β assembly.

Biocatalytic self-assembly of nanostructured peptide microparticles using droplet microfluidics.

Uniformly-sized, nanostructured peptide microparticles are generated by exploiting the ability of enzymes to serve (i) as catalysts, to control self-assembly within monodisperse, surfactant-stabilized water-in-oil microdroplets, and (ii) as destabilizers of emulsion interfaces, to enable facile transfer of the produced microparticles to water. This approach combines the advantages of biocatalytic self-assembly with the compartmentalization properties enabled by droplet microfluidics. Firstly, using microfluidic techniques, precursors of self-assembling peptide derivatives and enzymes are mixed in the microdroplets which upon catalytic conversion undergo molecular self-assembly into peptide particles, depending on the chemical nature of the precursors. Due to their amphiphilic nature, enzymes adsorb at the water-surfactant-oil interface of the droplets, inducing the transfer of peptide microparticles from the oil to the aqueous phase. Ultimately, through washing steps, enzymes can be removed from the microparticles which results in uniformely-sized particles composed of nanostructured aromatic peptide amphiphiles.