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.

Self-assembly and gelation properties of glycine/leucine Fmoc-dipeptides

Self-assembly of aromatic peptide amphiphiles is known to be driven by a combination of π-π stacking of the aromatic moieties and hydrogen bonding between the peptide backbones, with possible stabilisation from the amino acid side chains. Phenylalanine-based Fmoc-dipeptides have previously been reported for their characteristic apparent pKa transitions, which were shown to coincide with significant structural and morphological changes that were peptide sequence dependent. Here, phenylalanine was replaced by leucine and the effect on the self-assembling behaviour of Fmoc-dipeptides was measured using potentiometry, fluorescence and infrared spectroscopy, transmission electron microscopy, X-ray scattering and shear rheometry. This study provides additional cues towards the elucidation of the sequence-structure relationship in self-assembling aromatic peptide amphiphiles.

Enzyme-responsive Drug-delivery Systems

This chapter offers an overview of recent advances in enzyme-responsive materials potentially useful for drug delivery. The systems already developed provide new insights into the chemical design rules and response dynamics achievable by exploiting enzymatic catalysis as selective triggers in controlled release. The first section provides a general introduction about the role of enzymes in diseased states and examples where molecular therapeutics have been developed specifically to interfere with biochemical processes. The parameters to consider in order to develop enzyme-responsive drug-delivery systems are then discussed. Different approaches to design hydrogels, micelles and silica nanocontainers with moieties that can be substrates of enzymes are described with the help of relevant examples that highlight their performance. The research in this area is gaining momentum at a significant pace and it is likely that the first therapeutic enzyme responsive materials will reach the clinic in the next decade.

Transient fibril structures facilitating nonenzymatic self-replication

An emerging new direction of research focuses on developing "self-synthesizing materials", those supramolecular structures that can promote their own formation by accelerating the synthesis of building blocks and/or an entire assembly. It was postulated recently that practical design of such systems can benefit from the ability to control the assembly of amphiphilic molecules into nanostructures. We describe here the self-assembly pathway of short amphiphilic peptides into various forms of soluble β-sheet structures--β-plates, fibrils, and hollow nanotubes--and their consequent activity as autocatalysts for the synthesis of monomeric peptides from simpler building blocks. A detailed kinetic analysis of both the self-assembly and self-replication processes allows us to suggest a full model and simulate the replication process, revealing that only specific structures, primarily fibrils that are stable within the solution for a time shorter than a few hours, can be active as catalysts. Interestingly, we have found that such a process also induces fibril reproduction, in a mechanism very similar to the propagation of prion proteins by transmission of misfolded states.

Enzyme responsive materials: design strategies and future developments

Enzyme responsive materials (ERMs) are a class of stimuli responsive materials with broad application potential in biological settings. This review highlights current and potential future design strategies for ERMs and provides an overview of the present state of the art in the area.

Modeling the self-assembly of peptide amphiphiles into fibers using coarse-grained molecular dynamics

We have studied the self-assembly of peptide amphiphiles (PAs) into a cylindrical micelle fiber starting from a homogeneous mixture of PAs in water using coarse-grained molecular dynamics simulations. Nine independent 16 μs runs all show spontaneous fiber formation in which the PA molecules first form spherical micelles, and then micelles form a three-dimensional network via van der Waals interactions. As the hydrophobic core belonging to the different micelles merge, the three-dimensional network disappears and a fiber having a diameter of ∼80 Å appears. In agreement with atomistic simulation results, water molecules are excluded from the hydrophobic core and penetrate to ∼15 Å away from the axis of fiber. About 66% of the surface of fiber is covered with the IKVAV epitope, and ∼92% of the epitope is exposed to water molecules.

Morphology transformation via pH-triggered self-assembly of peptides

Three flexible peptides (P1: (C(17)H(35)CO-NH-GRGDG)(2)KG; P2: (Fmoc-GRGDG)(2)KG; P3: (CH(3)CO-NH-GRGDG)(2)KG) self-assembled to form a variety of morphologically distinct assemblies at different pHs. P1 formed nanofibers at pH 3, then self-assembled into nanospheres with pH up to 6 and further changed to lamellar structures when the pH value was further increased to 10. P2 aggregated into an entwined network structure at pH 3, and then self-assembled into well-defined nanospheres, lamellar structures, and vesicles via adjusting the pH value. However, P3 did not self-assemble into well-ordered nanostructures, presumably due to the absence of a large hydrophobic group. The varying self-assembly behaviors of the peptides at different pHs are attributed to molecular conformational changes. These self-assembled supramolecular materials might contribute to the development of new peptide-based biomaterials.

Effect of glycine substitution on Fmoc-diphenylalanine self-assembly and gelation properties

We have investigated the self-assembly behavior of fluorenyl-9-methoxycarbonyl (Fmoc)-FG, Fmoc-GG, and Fmoc-GF and compared it to that of Fmoc-FF using potentiometry, fluorescence and infrared spectroscopy, transmission electron microscopy, wide-angle X-ray scattering, and oscillatory rheometry. Titration experiments revealed a substantially shifted apparent pK(a) transition for Fmoc-FG, Fmoc-GG, and Fmoc-GF. The apparent pK(a) values observed correlated with the hydrophobicity (log P) of the Fmoc-dipeptide molecules. Fmoc-GG and Fmoc-GF were found to self-assemble only in their protonated form (below their apparent pK(a)), while Fmoc-FG formed self-assembled structures above and below its apparent pK(a). Fmoc-GG and Fmoc-FG were found to form hydrogels below their apparent pK(a) transitions in agreement with the entangled fibers morphologies revealed by TEM. Unlike Fmoc-FF and Fmoc-GG, Fmoc-FG showed unusual gelation behavior as gels were found to form upon heating. Fmoc-GF formed precipitates instead of a hydrogel below its apparent pK(a) in agreement with the formation of micrometer scale sheetlike structures observed by TEM. The fact that all four Fmoc-dipeptides were found to self-assemble suggests that the main driving force behind the self-assembly process is a combination of the hydrophobic and π-π interactions of the fluorenyl moieties with a secondary role for hydrogen bonding of the peptidic components. The nature of the peptidic tail was found to have a pronounced effect on the type of self-assembled structure formed. This work indicates that the substitution of phenylalanine by glycine significantly impacts on the mode of assembly and illustrates the versatility of aromatic peptide amphiphiles in the formation of structurally diverse nanostructures.

Enzyme-directed assembly and manipulation of organic nanomaterials

Enzymes are the prime protagonists in the chemistry of living organisms. As such, chemists and biologists have long been fascinated by the array of highly selective transformations possible under biological conditions that are facilitated by enzyme-catalyzed reactions. Moreover, enzymes are involved in replicating, repairing and transmitting information in a highly selective and organized fashion through detection and signal amplification cascades. Indeed, because of their selectivity and potential for use outside of biological systems, enzymes have found immense utility in various biochemical assays and are increasingly finding applications in the preparation of small molecules. By contrast, the use of enzymatic reactions to prepare and build supramolecular and nanoscale materials is relatively rare. In this article, we seek to highlight efforts over the past 10 years at taking advantage of enzymatic reactions to assemble and manipulate complex soft, organic materials on the nanoscale. It is tantalizing to think of these processes as mimics of natural systems where enzymes are used in the assembly and transformation of the most complex nanomaterials known, for example, virus capsid assemblies and the myriad array of nanoscale biomolecular machinery.

Supramolecular hydrogels based on the epitope of potassium ion channels

Imparting aromatic-aromatic interactions to the potassium binding epitope affords a supramolecular hydrogelator that responds to the K(+) concentration by self-assembly into nanofibers of different widths and crosslinking patterns, which illustrates a simple approach to generate biomimic materials based on tunable, hierarchical self-assembly of small molecules.

Efficient and facile formation of two-component nanoparticles via aromatic moiety directed self-assembly

Here we present a two-component self-assembling system employing the interaction of aromatic groups (Fmoc) to construct nanoparticles. Spherical particles of around 70 nm were formed spontaneously by a simple trigonal Fmoc-conjugate, which were then stabilized by Fmoc-dipeptides at physiological pH in aqueous solution. These novel particles, being well-tolerated by cells, capable of encapsulating hydrophobic compounds, and readily decorated by short peptides, are promising carriers for drug delivery.