Tuning Supramolecular Structure and Functions of Peptide bola-Amphiphile by Solvent Evaporation-Dissolution.

Solvent molecules significantly affect the supramolecular self-assembly, for example, in forming solvent-bridged hydrogen bonding networks. Even small changes in solvent composition can have dramatic impact on supramolecular assembly. Herein, we demonstrate the use of trace solvents (as low as 0.04%) to tune the morphology and consequent functions of supramolecular nanostructures based on an aromatic peptide bola-amphiphile. Specifically, perylene bisimide-(di)glycine-tyrosine (PBI-[GY]2) bola-amphiphile was shown to give rise to red-emitting nanofibers when assembled in water, while exposure to trace organic solvents such as tetrahydrofuran (THF) and others via solvent-evaporation followed by aqueous assembly gave rise to white-light-emitting nanospheres. Differential hydrogen bonding between water (donor and acceptor) and THF (acceptor only) impacts supramolecular organization, which was verified using a density functional theory (DFT) simulation. The tunable consequent surface hydrophobicity was utilized in staining the cytoplasm and membrane of cells, respectively. The trace-solvent effect achieved through evaporation-dissolution provides a methodology to mediate the morphologies and consequent functions for supramolecular biomaterials controlled by the self-assembly pathway.

Nanopropulsion by biocatalytic self-assembly.

A number of organisms and organelles are capable of self-propulsion at the micro- and nanoscales. Production of simple man-made mimics of biological transportation systems may prove relevant to achieving movement in artificial cells and nano/micronscale robotics that may be of biological and nanotechnological importance. We demonstrate the propulsion of particles based on catalytically controlled molecular self-assembly and fiber formation at the particle surface. Specifically, phosphatase enzymes (acting as the engine) are conjugated to a quantum dot (the vehicle), and are subsequently exposed to micellar aggregates (fuel) that upon biocatalytic dephosphorylation undergo fibrillar self-assembly, which in turn causes propulsion. The motion of individual enzyme/quantum dot conjugates is followed directly using fluorescence microscopy. While overall movement remains random, the enzyme-conjugates exhibit significantly faster transport in the presence of the fiber forming system, compared to controls without fuel, a non-self-assembling substrate, or a substrate which assembles into spherical, rather than fibrous structures upon enzymatic dephosphorylation. When increasing the concentration of the fiber-forming fuel, the speed of the conjugates increases compared to non-self-assembling substrate, although directionality remains random.

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.

Photothermal control of the gelation properties of nickel bis(dithiolene) metallogelators under near-infrared irradiation.

The proper functionalization of nickel bis(dithiolene) complexes by pendant cholesteryl fragments allows for the formation of extended networks of intertwined fibers providing robust gels. Furthermore, such nickel bis(dithiolene) complexes are also efficient photothermal centers in solution in the near infrared (NIR), with a photothermal conversion efficiency comparable to that of gold nanoparticles. This unprecedented association in one single molecule, of the two properties, i.e., gelation ability and photothermal effect, gives a highly efficient handle to modulate the gel stability through light irradiation in the NIR region, providing a novel approach to photoresponsive gels.

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 ß-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 gelator's 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 gelator's 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.

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.

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.

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.

Surfactant-stabilized small hydrogel particles in oil: hosts for remarkable activation of enzymes in organic solvents.

Hydrogels of amino acid based cationic surfactant having C(16) tails were used to immobilize heme proteins and enzyme. These hydrogel-entrapped proteins/enzyme showed remarkable activation when dispersed in organic solvent. The activation effect (ratio of the activity of the hydrogel-entrapped enzyme in organic solvent to the activity of the native enzyme in water) of cytochrome c increased up to 350-fold with varying protein and gelator concentration. Hydrogel-entrapped hemoglobin and horseradish peroxidase (HRP) also showed markedly improved activity in organic solvent. Alteration in the structure of the gelator and its supramolecular arrangement showed that the protein immobilized within amphiphilic networks with larger interstitial space exhibited higher activation. This striking activation of hydrogel-entrapped proteins stems from the following effects: 1) the hydrophilic domain of the amphiphilic networks facilitates accessibility of the enzyme to the water-soluble substrate. 2) the surfactant, as an integral part of the amphiphilic network, assists in the formation of a distinct interface through which reactants and products are easily transferred between hydrophilic and hydrophobic domains. 3) Surfactant gelators help in the dispersion and stabilization of gel matrix into small particles in organic solvent, which enhances the overall surface area and results in improved mass transfer. The activation was dramatically improved up to 675-fold in the presence of nongelating anionic surfactants that helped in disintegration of the gel into further smaller-sized particles. Interestingly, hydrogel-immobilized HRP exhibited about 2000-fold higher activity in comparison to the activity of the suspended enzyme in toluene. Structural changes of the entrapped enzyme and the morphology of the matrix were investigated to understand the mechanism of this activation.

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.

Effect of counterions on the activity of lipase in cationic water-in-oil microemulsions.

This paper delineates how the different counterions affect the physicochemical properties of the aqueous aggregates and thereby the lipase activities at the interface of cationic water-in-oil microemulsions. To this end, we have synthesized a series of cetyltrimethylammonium-based surfactants, 1-14, having aliphatic, aliphatic with aromatic substitution at the alpha position, and aromatic carboxylate anion as the counterion. The physicochemical characterizations of these aqueous aggregates were done by conductometric, tensiometric, fluorometric techniques to determine counterion binding (beta), critical micelle concentration (cmc), and micropolarity at the microenvironment. It has been found that the activity of lipase mainly increases with hydrophobicity (which is directly proportional to the counterion binding (beta) of the surfactant) of the counterion and reaches a maximum when the beta value is around 0.5. Increase in hydrophobicity as well as beta leads to the attachment of more counterions at interface resulting in enhancement of interfacial area. Consequently, the enzyme may attain flexible secondary conformation at the augmented surface area and also allow larger population of substrates and enzyme molecules at the interface leading to the enhancement in lipase activity. After an optimum value of beta, further increase probably produces a steric crowding at the interface, hindering the smooth occupancy of enzyme and the substrate in this region leading to decrease of enzyme activity, while molecular surface area of the counterion did not show any virtual influence on the lipase activity. Thus, the variation in the counterion structure and hydrophobicity plays a crucial role in modulating the lipase activity.