Thermoreversible lysozyme hydrogels: properties and an insight into the gelation pathway

Structural characterization and developability assessment of sustained release hydrogels for rapid implementation during preclinical studies

Sustained-release formulations are important tools to convert efficacious molecules into therapeutic products. Hydrogels enable the rapid assessment of sustained-release strategies, which are important during preclinical development where drug quantities are limited and fast turnaround times are the norm. Most research in hydrogel-based drug delivery has focused around synthesizing new materials and polymers, with limited focus on structural characterization, technology developability and implementation. Two commercially available thermosensitive hydrogel systems, comprised of block copolymers of poly(lactic-co-glycolic acid)-b-poly(ethylene glycol)-b-poly(lactic-co-glycolic acid) (PLGA) and poly(lactide-co-caprolactone)-b-poly(ethyleneglycol)-b-poly(lactide-co-caprolactone) (PLCL), were evaluated during this study. The two block copolymers described in the study were successfully formulated to form hydrogels which delayed the release of lysozyme (> 20 days) in vitro. Characterization of formulation attributes of the hydrogels like T_sol-gel temperature, complex viscosity and injection force showed that these systems are amenable to rapid implementation in preclinical studies. Understanding the structure of the gel network is critical to determine the factors controlling the release of therapeutics out of these gels. The structures were characterized via the gel mesh sizes, which were estimated using two orthogonal techniques: small angle X-ray scattering (SAXS) and rheology. The mesh sizes of these hydrogels were larger than the hydrodynamic radius (size) of lysozyme (drug), indicating that release through these gels is expected to be diffusive at all time scales rather than sub-diffusive. In vitro drug release experiments confirm that diffusion is the dominating mechanism for lysozyme release; with no contribution from degradation, erosion, relaxation, swelling of the polymer network or drug-polymer interactions. PLGA hydrogel was found to have a much higher complex viscosity than PLCL hydrogel, which correlates with the slower diffusivity and release of lysozyme seen from the PLGA hydrogel as compared to PLCL hydrogel. This is due to the increased frictional drag experienced by the lysozyme molecule in the PLGA hydrogel network, as described by the hydrodynamic theory.

Amyloid Self-Assembly of Lysozyme in Self-Crowded Conditions: The Formation of a Protein Oligomer Hydrogel

A method is designed to quickly form protein hydrogels, based on the self-assembly of highly concentrated lysozyme solutions in acidic conditions. Their properties can be easily modulated by selecting the curing temperature. Molecular insights on the gelation pathway, derived by in situ FTIR spectroscopy, are related to calorimetric and rheological results, providing a consistent picture on structure–property correlations. In these self-crowded samples, the thermal unfolding induces the rapid formation of amyloid aggregates, leading to temperature-dependent quasi-stationary levels of antiparallel cross β-sheet links, attributed to kinetically trapped oligomers. Upon subsequent cooling, thermoreversible hydrogels develop by the formation of interoligomer contacts. Through heating/cooling cycles, the starting solutions can be largely recovered back, due to oligomer-to-monomer dissociation and refolding. Overall, transparent protein hydrogels can be easily formed in self-crowding conditions and their properties explained, considering the formation of interconnected amyloid oligomers. This type of biomaterial might be relevant in different fields, along with analogous systems of a fibrillar nature more commonly considered.

Recent advances in the structure, synthesis, and applications of natural polymeric hydrogels

Hydrogels, polymeric network materials, are capable of swelling and holding the bulk of water in their three-dimensional structures upon swelling. In recent years, hydrogels have witnessed increased attention in food and biomedical applications. In this paper, the available literature related to the design concepts, types, functionalities, and applications of hydrogels with special emphasis on food applications was reviewed. Hydrogels from natural polymers are preferred over synthetic hydrogels. They are predominantly used in diverse food applications for example in encapsulation, drug delivery, packaging, and more recently for the fabrication of structured foods. Natural polymeric hydrogels offer immense benefits due to their extraordinary biocompatible nature. Hydrogels based on natural/edible polymers, for example, those from polysaccharides and proteins, can serve as prospective alternatives to synthetic polymer-based hydrogels. The utilization of hydrogels has so far been limited, despite their prospects to address various issues in the food industries. More research is needed to develop biomimetic hydrogels, which can imitate the biological characteristics in addition to the physicochemical properties of natural materials for different food applications.

Thermoresponsive BSA hydrogels with phase tunability

Amyloids are fibrillar structures formed due to protein aggregation or misfolding when the molecules undergo a conformational change from α-helix to β-sheet. Although this self-assembly is associated with many neurodegenerative diseases in vivo, the highly ordered amyloidic structures formed in vitro are ideal scaffolds for many bionanotechnological applications. Amyloid fibrillar networks under specific stimuli can also form stable hydrogels. We have used bovine serum albumin (BSA) as a model amyloidogenic protein to obtain thermally-induced hydrogels that display tunable sol-gel-sol transitions spanning over minutes to days. High concentrations of BSA (14-22% w/v) were heated at 65 °C for less than 3 min without any cross-linking agent to yield soft, injectable gels that were non-toxic to mammalian cells. A detailed investigation of temperature, concentration, incubation time and ionic strength on the formation and reversal of these gels was carried out using visual inspection, rheology, electron microscopy, fluorescence spectroscopy, UV-visible spectroscopy and circular dichroism spectroscopy. The optimum gelation temperature (T_g) for phase reversal of BSA gels was found to lie between 60 and 70 °C. An increase in protein concentration led to a reduction in the gelation time and increase in the gel-to-rev sol transition time. Gels heated for longer duration than their minimum gelation time yielded irreversible gels suggesting that low incubation periods were favourable for partial protein denaturation and hydrogel formation. This was supported by time-resolved secondary and tertiary structural ensemble studies. Further, the hydrogel networks demonstrated a zero-order drug release kinetics and the rev sol was found to be cytocompatible with HaCaT skin cell lines. Overall, our approach demonstrates rapid, crosslinker-free thermoresponsive BSA gelation with wide tunability and control on the time and material property, ideal for topical drug delivery applications.

Role of Sheet-Edge Interactions in β-sheet Self-Assembling Peptide Hydrogels

Hydrogels' hydrated fibrillar nature makes them the material of choice for the design and engineering of 3D scaffolds for cell culture, tissue engineering, and drug-delivery applications. One particular class of hydrogels which has been the focus of significant research is self-assembling peptide hydrogels. In the present work, we were interested in exploring how fiber-fiber edge interactions affect the self-assembly and gelation properties of amphipathic peptides. For this purpose, we investigated two β-sheet-forming peptides, FEFKFEFK (F8) and KFEFKFEFKK (KF8K), the latter one having the fiber edges covered by lysine residues. Our results showed that the addition of the two lysine residues did not affect the ability of the peptides to form β-sheet-rich fibers, provided that the overall charge carried by the two peptides was kept constant. However, it did significantly reduce edge-driven hydrophobic fiber-fiber associative interactions, resulting in reduced tendency for KF8K fibers to associate/aggregate laterally and form large fiber bundles and consequently network cross-links. This effect resulted in the formation of hydrogels with lower moduli but faster dynamics. As a result, KF8K fibers could be aligned only under high shear and at high concentration while F8 hydrogel fibers were found to align readily at low shear and low concentration. In addition, F8 hydrogels were found to fragment at high concentration because of the high aggregation state stabilizing the fiber bundles, resulting in fiber breakage rather than disentanglement and alignment.

Arrested dynamics in a model peptide hydrogel system

We report here on a peptide hydrogel system, which in contrast to most other such systems, is made up of relatively short fibrillar aggregates, discussing resemblance with colloidal rods. The synthetic model peptides A₈K and A₁₀K, where A denotes alanine and K lysine, self-assemble in aqueous solutions into ribbon-like aggregates having an average length 〈L〉 on the order of 100 nm and with a diameter d≈ 6 nm. The aggregates can be seen as weakly charged rigid rods and they undergo an isotropic to nematic phase transition at higher concentrations. Translational motion perpendicular to the rod axis gets strongly hindered when the concentration is increased above the overlap concentration. Similarly, the rotational motion is hindered, leading to very long stress relaxation times. The peptide self-assembly is driven by hydrophobic interactions and due to a net peptide charge the system is colloidally stable. However, at the same time short range, presumably hydrophobic, attractive interactions appear to affect the rheology of the system. Upon screening the long range electrostatic repulsion, with the addition of salt, the hydrophobic attraction becomes more dominant and we observe a transition from a repulsive glassy state to an attractive gel-state of the rod-like peptide aggregates.

Probing Globular Protein Self-Assembling Dynamics by Heterodyne Transient Grating Experiments

In this work, we studied the propagation of ultrasonic waves of lysozyme solutions characterized by different degrees of aggregation and networking. The experimental investigation was performed by means of the transient grating (TG) spectroscopy as a function of temperature, which enabled measurement of the ultrasonic acoustic proprieties over a wide time window, ranging from nanoseconds to milliseconds. The fitting of the measured TG signal allowed the extraction of several dynamic properties, here we focused on the speed and the damping rate of sound. The temperature variation induced a series of processes in the lysozyme solutions: Protein folding-unfolding, aggregation and sol–gel transition. Our TG investigation showed how these self-assembling phenomena modulate the sound propagation, affecting both the velocity and the damping rate of the ultrasonic waves. In particular, the damping of ultrasonic acoustic waves proved to be a dynamic property very sensitive to the protein conformational rearrangements and aggregation processes.

The physical basis of fabrication of amyloid-based hydrogels by lysozyme

Schematic of heating- and cooling-induced transitions between HEWL states, and the subsequent formation of the hydrogel.

Tough Supramolecular Hydrogel Based on Strong Hydrophobic Interactions in a Multiblock Segmented Copolymer

We report the preparation and structural and mechanical characterization of a tough supramolecular hydrogel, based exclusively on hydrophobic association. The system consists of a multiblock, segmented copolymer of hydrophilic poly(ethylene glycol) (PEG) and hydrophobic dimer fatty acid (DFA) building blocks. A series of copolymers containing 2K, 4K, and 8K PEG were prepared. Upon swelling in water, a network is formed by self-assembly of hydrophobic DFA units in micellar domains, which act as stable physical cross-link points. The resulting hydrogels are noneroding and contain 75-92 wt % of water at swelling equilibrium. Small-angle neutron scattering (SANS) measurements showed that the aggregation number of micelles ranges from 2 × 102 to 6 × 102 DFA units, increasing with PEG molecular weight. Mechanical characterization indicated that the hydrogel containing PEG 2000 is mechanically very stable and tough, possessing a tensile toughness of 4.12 MJ/m3. The high toughness, processability, and ease of preparation make these hydrogels very attractive for applications where mechanical stability and load bearing features of soft materials are required.

Formation and functional properties of protein-polysaccharide electrostatic hydrogels in comparison to protein or polysaccharide hydrogels

Protein and polysaccharide mixed systems have been actively studied for at least 50years as they can be assembled into functional particles or gels. This article reviews the properties of electrostatic gels, a recently discovered particular case of associative protein-polysaccharide mixtures formed through associative electrostatic interaction under appropriate solution conditions (coupled gel). This review highlights the factors influencing gel formation such as protein-polysaccharide ratio, biopolymer structural characteristics, final pH, ionic strength and total solid concentration. For the first time, the functional properties of protein-polysaccharide coupled gels are presented and discussed in relationship to individual protein and polysaccharide hydrogels. One of their outstanding characteristics is their gel water retention. Up to 600g of water per g of biopolymer may be retained in the electrostatic gel network compared to a protein gel (3-9g of water per g of protein). Potential applications of the gels are proposed to enable the food and non-food industries to develop new functional products with desirable attributes or new interesting materials to incorporate bioactive molecules.

Hydrophilic polyurethane matrix promotes chondrogenesis of mesenchymal stem cells

Segmental polyurethanes exhibit biphasic morphology and can control cell fate by providing distinct matrix guided signals to increase the chondrogenic potential of mesenchymal stem cells (MSCs). Polyethylene glycol (PEG) based hydrophilic polyurethanes can deliver differential signals to MSCs through their matrix phases where hard segments are cell-interactive domains and PEG based soft segments are minimally interactive with cells. These coordinated communications can modulate cell-matrix interactions to control cell shape and size for chondrogenesis. Biphasic character and hydrophilicity of polyurethanes with gel like architecture provide a synthetic matrix conducive for chondrogenesis of MSCs, as evidenced by deposition of cartilage-associated extracellular matrix. Compared to monophasic hydrogels, presence of cell interactive domains in hydrophilic polyurethanes gels can balance cell-cell and cell-matrix interactions. These results demonstrate the correlation between lineage commitment and the changes in cell shape, cell-matrix interaction, and cell-cell adhesion during chondrogenic differentiation which is regulated by polyurethane phase morphology, and thus, represent hydrophilic polyurethanes as promising synthetic matrices for cartilage regeneration.

Protein microgels from amyloid fibril networks

Nanofibrillar forms of proteins were initially recognized in the context of pathology, but more recently have been discovered in a range of functional roles in nature, including as active catalytic scaffolds and bacterial coatings. Here we show that protein nanofibrils can be used to form the basis of monodisperse microgels and gel shells composed of naturally occurring proteins. We explore the potential of these protein microgels to act as drug carrier agents, and demonstrate the controlled release of four different encapsulated drug-like small molecules, as well as the component proteins themselves. Furthermore, we show that protein nanofibril self-assembly can continue after the initial formation of the microgel particles, and that this process results in active materials with network densities that can be modulated in situ. We demonstrate that these materials are nontoxic to human cells and that they can be used to enhance the efficacy of antibiotics relative to delivery in homogeneous solution. Because of the biocompatibility and biodegradability of natural proteins used in the fabrication of the microgels, as well as their ability to control the release of small molecules and biopolymers, protein nanofibril microgels represent a promising class of functional artificial multiscale materials generated from natural building blocks.

A shape memory hydrogel induced by the interactions between metal ions and phosphate

A novel ferric-phosphate induced shape memory (SM) hydrogel is prepared by the one-step copolymerization of isopropenyl phosphonic acid (IPPA) and acrylamide (AM) in the presence of a crosslinker polyethylene glycol diacrylate (PEGDA). Different from the traditional SM hydrogels, our SM hydrogel can be processed into various shapes as needed and recovers to its original form in ‘multiconditions’ such as in the presence of a reducing agent or in the presence of a competitive complexing agent. This unique feature is attributed to the fact that the oxidized ferric ions show a high complexation ability with phosphate groups of IPPA, which acts as a physical crosslinker to form the secondary networks within the hydrogels to induce the shape memory effect. The memory behavior was totally reversible, owing to Fe3+ that can be reduced to Fe2+ and extracted by the complexing agent. Particularly, the SM hydrogels exhibit controllable and good mechanical characteristics by introduction of the ferric ions, i.e., the elastic modulus can increase from 2 kPa to 70 kPa dramatically. Learning from biological systems, phosphate-metal ion based hydrogels could become an attractive candidate for various biomedical and environmental applications.

Cooperative self-assembly of peptide gelators and proteins

Molecular self-assembly provides a versatile route for the production of nanoscale materials for medical and technological applications. Herein, we demonstrate that the cooperative self-assembly of amphiphilic small molecules and proteins can have drastic effects on supramolecular nanostructuring of resulting materials. We report that mesoscale, fractal-like clusters of proteins form at concentrations that are orders of magnitude lower compared to those usually associated with molecular crowding at room temperature. These protein clusters have pronounced effects on the molecular self-assembly of aromatic peptide amphiphiles (fluorenylmethoxycarbonyl- dipeptides), resulting in a reversal of chiral organization and enhanced order through templating and binding. Moreover, the morphological and mechanical properties of the resultant nanostructured gels can be controlled by the cooperative self-assembly of peptides and protein fractal clusters, having implications for biomedical applications where proteins and peptides are both present. In addition, fundamental insights into cooperative interplay of molecular interactions and confinement by clusters of chiral macromolecules is relevant to gaining understanding of the molecular mechanisms of relevance to the origin of life and development of synthetic mimics of living systems.

In Situ Fibril Formation of κ-Casein by External Stimuli within Multilayer Thin Films

We have developed the in situ fibrillation of κ-casein, employed as amyloid precursor, within multilayer films consisting of κ-casein and poly(acrylic acid) (PAA) prepared by the layer-by-layer (LbL) deposition. The fibrillation of κ-casein within the multilayered films is strongly dependent on the extent of intermolecular interactions between κ-casein and PAA. When films constructed initially at pH 3 were heat treated at the same pH, κ-casein did not transform into fibrils. However, when the films were subjected to heat treatment at pH 5, κ-casein was transformed into fibrils within multilayer films due to weakened intermolecular interactions between κ-casein and PAA. We also noted that the multilayer film was swollen at pH 5 by the charge imbalance within the film, which we believe gives enough mobility for κ-caseins to form fibrils with adjacent κ-caseins within the multilayer. The fibrils were found to be uniformly distributed across the entire film thickness, and the aspect ratio as well as the number density of fibrils increased as a function of incubation time. The present study reveals a strategy to realize in situ nanocomposites within LbL multilayer films simply by triggering the formation of protein fibrils by controlling the intermolecular interactions between amyloid precursors and polyelectrolytes (PEs).

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 (13)C-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.

Heat-denatured lysozyme aggregation and gelation as revealed by combined dielectric relaxation spectroscopy and light scattering measurements

The dielectric behavior of native and heat-denatured lysozyme in ethanol-water solutions was examined in the frequency range from 1 MHz to 2 GHz, using frequency-domain dielectric relaxation spectroscopy. Because of the conformational changes on unfolding, dielectric methods provide information on the denaturation process of the protein and, at protein concentration high enough, on the subsequent aggregation and gelation. Moreover, the time evolution of the protein aggregation and gelation was monitored measuring, by means of dynamic light scattering methods, the diffusion coefficient of micro-sized polystyrene particles, deliberately added to the protein solution, which act as a probe of the viscosity of the microenvironment close to the particle surface. All together, our measurements indicate that heat-induced denaturation favors, at high concentrations, a protein aggregation process which evolves up to the full gelation of the system. These findings have a direct support from IR measurements of the absorbance of the amide I band that, because of the unfolding, indicate that proteins entangle each other, producing a network structure which evolves, in long time limit, in the gel.

Lysozyme pattern formation in evaporating drops

Liquid droplets containing suspended particles deposited on a solid, flat surface generally form ringlike structures due to the redistribution of solute during evaporation (the "coffee ring effect"). The forms of the deposited patterns depend on interactions between solute(s), solvent, and substrate. In this study, deposition patterns from droplets of a simplified model biological fluid (DI water + lysozyme) are examined by scanning probe and optical microscopy. The overall lysozyme residue morphology is complex (with both a perimeter "rim" and undulating interior) but varies little with concentration. However, the final packing of lysozyme molecules is strongly dependent on initial concentration.

Re-formation of fibrils from hydrolysates of β-lactoglobulin fibrils during in vitro gastric digestion

In this study, in vitro digestion of β-lactoglobulin (β-Lg) fibrils and the re-formation of fibril-like structures after prolonged enzymatic hydrolysis (up to 48 h) were investigated using matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS), thioflavin T fluorescence photometry, and transmission electron microscopy (TEM). Pure β-Lg fibrils that had been formed by heat treatment at pH 2.0 were rapidly hydrolyzed by pepsin in the simulated gastric fluid (pH 1.2), and some new peptides that were suitable for further fibril formation were produced. TEM showed that the new fibrils were long and straight but thinner than the original fibrils, and both TEM and MALDI-MS indicated that the peptides in the new fibrils were shorter/smaller than the peptides in the original fibrils. The formation of new fibrils was found to be affected more by pH than by enzyme activity or temperature.

Design properties of hydrogel tissue-engineering scaffolds

This article summarizes the recent progress in the design and synthesis of hydrogels as tissue-engineering scaffolds. Hydrogels are attractive scaffolding materials owing to their highly swollen network structure, ability to encapsulate cells and bioactive molecules, and efficient mass transfer. Various polymers, including natural, synthetic and natural/synthetic hybrid polymers, have been used to make hydrogels via chemical or physical crosslinking. Recently, bioactive synthetic hydrogels have emerged as promising scaffolds because they can provide molecularly tailored biofunctions and adjustable mechanical properties, as well as an extracellular matrix-like microenvironment for cell growth and tissue formation. This article addresses various strategies that have been explored to design synthetic hydrogels with extracellular matrix-mimetic bioactive properties, such as cell adhesion, proteolytic degradation and growth factor-binding.

In vitro digestion of beta-lactoglobulin fibrils formed by heat treatment at low pH

Extensive studies have been done on beta-lactoglobulin (beta-Lg) fibrils in the past decade due to their potential as functional food ingredients, gelling agents, and encapsulation devices etc. (van der Goot, A. J.; Peighambardoust, S. H.; Akkermans, C.; van Oosten-Manski, J. M. Creating novel structures in food materials: The role of well-defined shear flow. Food Biophys. 2008, 3(2), 120-125 and Loveday, S. M.; Rao, M. A.; Creamer, L. K.; Singh, H. Factors affecting rheological characteristics of fibril gels: The case of beta-lactoglobulin and alpha-lactalbumin. J. Food Sci. 2009, 74 (3), R47-R55). However, most of the studies focus on the formation and mechanism of the fibrils. Little is known about fibril digestibility to date. In this work, in vitro pepsin digestion of bovine beta-lactoglobulin (beta-Lg) fibrils in simulated gastric fluid was investigated using thioflavin T fluorescence photometry, sodium dodecyl sulfate-polyacrylamide gel electrophoresis, size-exclusion chromatography, matrix-assisted laser desorption/ionization mass spectrometry, and transmission electron microscopy (TEM). The fibrils were formed by heating beta-Lg solutions at 80 degrees C and pH 2.0 for 20 h. The fibrils were found to be digested completely by pepsin within 2 min, when long, straight fibrils were no longer observed by TEM. The peptides in the fibrils (2000-8000 Da) could be digested to smaller peptides (mostly <2000 Da) by pepsin. The peptides in the fibrils were believed to be more susceptible for pepsin to access and attack because of their hydrophobic nature. For comparison purposes, solutions of beta-Lg heated at neutral pH (pH 7.4) were also studied under the same conditions.

Rheological properties of peptide-based hydrogels for biomedical and other applications

Peptide-based hydrogels are an important class of biomaterials finding use in food industry and potential use in tissue engineering, drug delivery and microfluidics. A primary experimental method to explore the physical properties of these hydrogels is rheology. A fundamental understanding of peptide hydrogel mechanical properties and underlying molecular mechanisms is crucial for determining whether these biomaterials are potentially suitable for biotechnological uses. In this critical review, we cover the literature containing rheological characterization of the physical properties of peptide and polypeptide-based hydrogels including hydrogel bulk mechanical properties, gelation mechanisms, and the behavior of hydrogels during and after flow (219 references).

Alkali cold gelation of whey proteins. Part II: Protein concentration

The effect of the whey protein isolate (WPI) concentration on the sol-gel-sol transition in alkali cold gelation was investigated at pH 11.6-13 using oscillatory rheometry. The elastic modulus increases quickly with time to reach a local maximum (G'max), followed by a degelation step where the modulus decreases to a minimum value (G'min). Depending on the pH, a second gelation step will occur. At the end of the first gelation step around G'max, the system fulfilled the Winter-Chambon criterion of gelation. The analysis of the maximum moduli with the protein concentration shows that (i) there is a percolation concentration above which an elastic response is observed (approximately 6.8 wt %); (ii) there are two concentration regimes for G''max and G''max above this concentration, where we have considered power-law and percolation equations; (iii) there is a crossover concentration between the two regimes (at approximately 8 wt %) for both G'max and G''max when both moduli are equal, and this value is constant under all conditions tested (G'max=G''max approximately 4 Pa). Therefore, alkali cold gelation is better represented using two concentrations regimes than one, as observed for other biopolymers.

Scaling and self-organized criticality in proteins I

The complexity of proteins is substantially simplified by regarding them as archetypical examples of self-organized criticality (SOC). To test this idea and elaborate on it, this article applies the Moret-Zebende SOC hydrophobicity scale to the large-scale scaffold repeat protein of the HEAT superfamily, PR65/A. Hydrophobic plasticity is defined and used to identify docking platforms and hinges from repeat sequences alone. The difference between the MZ scale and conventional hydrophobicity scales reflects long-range conformational forces that are central to protein functionality.

Protein Fibrillar Hydrogels for three-Dimensional Tissue Engineering

Protein self-assembly into highly ordered fibrillar aggregates has attracted increasing attention over recent years, due primarily to its association with disease states such as Alzheimer's. More recently, however, research has focused on understanding the generic behavior of protein self-assembly where fibrillation is typically induced under harsh conditions of low pH and/or high temperature. Moreover the inherent properties of these fibrils, including their nanoscale dimension, environmental responsiveness, and biological compatibility, are attracting substantial interest for exploiting these fibrils for the creation of new materials. Here we will show how protein fibrils can be formed under physiological conditions and their subsequent gelation driven using the ionic strength of cell culture media while simultaneously incorporating cells homogeneously throughout the gel network. The fibrillar and elastic nature of the gel have been confirmed using cryo-transmission electron microscopy and oscillatory rheology, respectively; while cell culture work shows that our hydrogels promote cell spreading, attachment, and proliferation in three dimensions.