Pickering Stabilized Peptide Gel Particles as Tunable Microenvironments for Biocatalysis

We demonstrate the preparation of peptide gel microparticles that are emulsified and stabilized by SiO₂ nanoparticles. The gels are composed of aromatic peptide amphiphiles 9-fluorenyl­methoxy­carbonyl­diphenylalanine (Fmoc-FF) coassembled with Fmoc-amino acids with different functional groups (S: serine; D: aspartic acid; K: lysine; and Y: tyrosine). The gel phase provides a highly hydrated matrix, and peptide self-assembly endows the matrix with tunable chemical environments which may be exploited to support and stabilize proteins. The use of Pickering emulsion to stabilize these gel particles is advantageous through avoidance of surfactants that may denature proteins. The performance of enzyme lipase B immobilized in pickering/gel microparticles with different chemical functionalities is investigated by studying transesterification in heptane. We show that the use of Pickering particles enhances the performance of the enzyme, which is further improved in gel-phase systems, with hydrophilic environment provided by Fmoc-FF/S giving rise to the best catalytic performance. The combination of a tunable chemical environment in gel phase and Pickering stabilization described here is expected to prove useful for areas where proteins are to be exploited in technological contexts such as biocatalysis and also in other areas where protein performance and activity are important, such as biosensors and bioinspired solar fuel devices

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 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

Stable Emulsions Formed by Self-Assembly of Interfacial Networks of Dipeptide Derivatives

We demonstrate the use of dipeptide amphiphiles that, by hand shaking of a biphasic solvent system for a few seconds, form emulsions that remain stable for months through the formation of nanofibrous networks at the organic/aqueous interface. Unlike absorption of traditional surfactants, the interfacial networks form by self-assembly through π-stacking interactions and hydrogen bonding. Altering the dipeptide sequence has a dramatic effect on the properties of the emulsions formed, illustrating the possibility of tuning emulsion properties by chemical design. The systems provide superior long-term stability toward temperature and salts compared to with sodium dodecyl sulfate (SDS) and can be enzymatically disassembled causing on-demand demulsification under mild conditions. The interfacial networks facilitate highly tunable and stable encapsulation and compartmentalization with potential applications in cosmetics, therapeutics, and food industry

Assessing the Utility of Infrared Spectroscopy as a Structural Diagnostic Tool for β‑Sheets in Self-Assembling Aromatic Peptide Amphiphiles

β-Sheets are a commonly found structural motif in self-assembling aromatic peptide amphiphiles, and their characteristic “amide I” infrared (IR) absorption bands are routinely used to support the formation of supramolecular structure. In this paper, we assess the utility of IR spectroscopy as a structural diagnostic tool for this class of self-assembling systems. Using 9-fluorene-methyloxycarbonyl dialanine (Fmoc-AA) and the analogous 9-fluorene-methylcarbonyl dialanine (Fmc-AA) as examples, we show that the origin of the band around 1680–1695 cm^–1 in Fourier transform infrared (FTIR) spectra, which was previously assigned to an antiparallel β-sheet conformation, is in fact absorption of the stacked carbamate group in Fmoc-peptides. IR spectra from ¹³C-labeled samples support our conclusions. In addition, DFT frequency calculations on small stacks of aromatic peptides help to rationalize these results in terms of the individual vibrational modes

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

We demonstrate the self-assembly of <i>bola</i>-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

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