The self-assembly, aggregation and phase transitions of food protein systems in one, two and three dimensions

Adsorption of Hydrophobin-Protein Mixtures at the Air-Water Interface: The Impact of pH and Electrolyte

The adsorption of the proteins β-casein, β-lactoglobulin, and hydrophobin, and the protein mixtures of β-casein/hydrophobin and β-lactoglobulin/hydrophobin have been studied at the air-water interface by neutron reflectivity, NR. Changing the solution pH from 7 to 2.6 has relatively little impact on the adsorption of hydrophobin or β-lactoglobulin, but results in a substantial change in the structure of the adsorbed layer of β-casein. In β-lactoglobulin/hydrophobin mixtures, the adsorption is dominated by the hydrophobin adsorption, and is independent of the hydrophobin or β-lactoglobulin concentration and solution pH. At pH 2.6, the adsorption of the β-casein/hydrophobin mixtures is dominated by the hydrophobin adsorption over the range of β-casein concentrations studied. At pH 4 and 7, the adsorption of β-casein/hydrophobin mixtures is dominated by the hydrophobin adsorption at low β-casein concentrations. At higher β-casein concentrations, β-casein is adsorbed onto the surface monolayer of hydrophobin, and some interpenetration between the two proteins occurs. These results illustrate the importance of pH on the intermolecular interactions between the two proteins at the interface. This is further confirmed by the impact of PBS, phosphate buffered saline, buffer and CaCl2 on the coadsorption and surface structure. The results provide an important insight into the adsorption properties of protein mixtures and their application in foam and emulsion stabilization.

Sulfate anion delays the self-assembly of human insulin by modifying the aggregation pathway

The understanding of the molecular mechanisms underlying protein self-assembly and of their dependence on solvent composition has implications in a large number of biological and biotechnological systems. In this work, we characterize the aggregation process of human insulin at acidic pH in the presence of sulfate ions using a combination of Thioflavin T fluorescence, dynamic light scattering, size exclusion chromatography, Fourier transform infrared spectroscopy, and transmission electron microscopy. It is found that the increase of sulfate concentration inhibits the conversion of insulin molecules into aggregates by modifying the aggregation pathway. At low sulfate concentrations (0-5 mM) insulin forms amyloid fibrils following the nucleated polymerization mechanism commonly observed under acidic conditions in the presence of monovalent anions. When the sulfate concentration is increased above 5 mM, the sulfate anion induces the salting-out of ∼18-20% of insulin molecules into reversible amorphous aggregates, which retain a large content of α-helix structures. During time these aggregates undergo structure rearrangements into β-sheet structures, which are able to recruit monomers and bind to the Thioflavin T dye. The alternative aggregation mechanism observed at large sulfate concentrations is characterized by a larger activation energy and leads to more polymorphic structures with respect to the self-assembly in the presence of chloride ions. The system shown in this work represents a case where amorphous aggregates on pathway to the formation of structures with amyloid features could be detected and analyzed.

Colloidal Stability & Conformational Changes in β-Lactoglobulin: Unfolding to Self-Assembly

A detailed understanding of the mechanism of unfolding, aggregation, and associated rheological changes is developed in this study for β-Lactoglobulin at different pH values through concomitant measurements utilizing dynamic light scattering (DLS), optical microrheology, Raman spectroscopy, and differential scanning calorimetry (DSC). The diffusion interaction parameter kD emerges as an accurate predictor of colloidal stability for this protein consistent with observed aggregation trends and rheology. Drastic aggregation and gelation were observed at pH 5.5. Under this condition, the protein's secondary and tertiary structures changed simultaneously. At higher pH (7.0 and 8.5), oligomerizaton with no gel formation occurred. For these solutions, tertiary structure and secondary structure transitions were sequential. The low frequency Raman data, which is a good indicator of hydrogen bonding and structuring in water, has been shown to exhibit a strong correlation with the rheological evolution with temperature. This study has, for the first time, demonstrated that this low frequency Raman data, in conjunction with the DSC endotherm, can be been utilized to deconvolve protein unfolding and aggregation/gelation. These findings can have important implications for the development of protein-based biotherapeutics, where the formulation viscosity, aggregation, and stability strongly affects efficacy or in foods where protein structuring is critical for functional and sensory performance.

Geometry of flexible filament cohesion: better contact through twist?

Cohesive interactions between filamentous molecules have broad implications for a range of biological and synthetic materials. While long-standing theoretical approaches have addressed the problem of inter-filament forces from the limit of infinitely rigid rods, the ability of flexible filaments to deform intra-filament shape in response to changes in inter-filament geometry has a profound affect on the nature of cohesive interactions. In this paper, we study two theoretical models of inter-filament cohesion in the opposite limit, in which filaments are sufficiently flexible to maintain cohesive contact along their contours, and address, in particular, the role played by helical-interfilament geometry in defining interactions. Specifically, we study models of featureless, tubular filaments interacting via: (1) pair-wise Lennard-Jones (LJ) interactions between surface elements and (2) depletion-induced filament binding stabilized by electrostatic surface repulsion. Analysis of these models reveals a universal preference for cohesive filament interactions for non-zero helical skew, and further, that in the asymptotic limit of vanishing interaction range relative to filament diameter, the skew-dependence of cohesion approaches a geometrically defined limit described purely by the close-packing geometry of twisted tubular filaments. We further analyze non-universal features of the skew-dependence of cohesion at small-twist for both potentials, and argue that in the LJ model the pair-wise surface attraction generically destabilizes parallel filaments, while in the second model, pair-wise electrostatic repulsion in combination with non-pairwise additivity of depletion leads to a meta-stable parallel state.

Formation and functionality of self-assembled whey protein microgels

Whey proteins spontaneously form spherical particles when heated in aqueous solutions at conditions where their net charge density is below a critical value. The particles are microgels consisting of a hydrated crosslinked network of proteins with a diameter between 100nm and 1μm. Stable suspensions of these microgels can be formed in a narrow range of conditions when the protein charge density is low enough to induce their formation, but high enough to inhibit further association into larger clusters or macroscopic gels. The formation of microgels and their application to stabilize emulsions and foams; form core-shell particles; form gels; or modify the texture of polysaccharide solutions and gels are reviewed.

Implications of protein polymorphism on protein phase behaviour

The phase behaviour of small globular proteins is often modeled by approximating them as spherical particles with fixed internal structure. However, changes in the local environment of a protein can lead to changes in its conformation rendering this approximation invalid. We present a simple two-state model in which protein conformation is not conserved and where the high-energy, non-native state is stabilised by pair-wise attractive interactions. The resulting phase behaviour is remarkably complex, non-universal and exhibits re-entrance. The model calculations show a demarcation between a regime where conformational transitioning is largely enslaved by phase separation and one where this is not the case. In the latter regime, which is characterised by a large free energy difference between the native and the non-native state, we deduce that the kinetics of the phase transition strongly depend on the average conformation of the proteins prior to their condensation. For condensation to occur in this regime within a dispersion of native proteins, nucleation of a cluster of proteins in the non-native state is required. We argue that our theory supports the distinction between common phase separation and the nucleated assembly of non-native supramolecular aggregates in protein dispersions.

Structural heterogeneity of milk casein micelles: a SANS contrast variation study

We examine the internal structure of milk casein micelles using the contrast variation method in Small-Angle Neutron Scattering (SANS). Experiments were performed with casein dispersions of different origins (i.e., milk powder or fresh milk) and extended to very low q-values (∼9 × 10(-4) Å(-1)), thus making it possible to precisely determine the apparent gyration radius Rg at each contrast. From the variation of I(q → 0) with contrast, we determine the distribution of composition of all the particles in the dispersions. As expected, most of these particles are micelles, made of casein and calcium phosphate, with a narrow distribution in compositions. These micelles always coexist with a very small fraction of fat droplets, with sizes in the range of 20-400 nm. For the dispersions prepared from fresh milk, which were purified under particularly stringent conditions, the number ratio of fat droplets to casein micelles is as low as 1 to 10(6). In that case, we are able to subtract from the total intensity the contribution of the fat droplets and in this way obtain the contribution of the micelles only. We then analyze the variation of this contribution with contrast using the approach pioneered by H. B. Stuhrmann. We model the casein micelle as a core-shell spherical object, in which the local scattering length density is determined by the ratio of calcium phosphate nanoclusters to proteins. We find that models in which the shell has a lower concentration of calcium phosphate than the core give a better agreement than models in which the shell has a higher density than the core.

Charge-induced patchy attractions between proteins

Static light scattering (SLS) combined with structure-based Monte Carlo (MC) simulations provide new insights into mechanisms behind anisotropic, attractive protein interactions. A nonmonotonic behavior of the osmotic second virial coefficient as a function of ionic strength is here shown to originate from a few charged amino acids forming an electrostatic attractive patch, highly directional and complementary. Together with Coulombic repulsion, this attractive patch results in two counteracting electrostatic contributions to the interaction free energy which, by operating over different length scales, is manifested in a subtle, salt-induced minimum in the second virial coefficient as observed in both experiment and simulations.

Mechanical deformation mechanisms and properties of amyloid fibrils

Amyloid fibrils have recently received attention due to their remarkable mechanical properties, which are highly correlated with their biological functions. We have studied the mechanical deformation mechanisms and properties of amyloid fibrils as a function of their length scales by using atomistic simulations. It is shown that the length of amyloid fibrils plays a role in their deformation and fracture mechanisms in such a way that the competition between shear and bending deformations is highly dependent on the fibril length, and that as the fibril length increases, so does the bending strength of the fibril while its shear strength decreases. The dependence of rupture force for amyloid fibrils on their length is elucidated using the Bell model, which suggests that the rupture force of the fibril is determined from the hydrogen bond rupture mechanism that critically depends on the fibril length. We have measured the toughness of amyloid fibrils, which is shown to depend on the fibril length. In particular, the toughness of the fibril with its length of ∼3 nm is estimated to be ∼30 kcal mol(-1) nm(-3), comparable to that of a spider silk crystal with its length of ∼2 nm. Moreover, we have shown the important effect of the pulling rate on the mechanical deformation mechanisms and properties of amyloid fibril. It is found that as the pulling rate increases, so does the contribution of the shear effect to the elastic deformation of the amyloid fibril with its length of <10 nm. However, we found that the deformation mechanism of the amyloid fibril with its length of >15 nm is almost independent of the pulling rate. Our study sheds light on the role of the length scale of amyloid fibrils and the pulling rate in their mechanical behaviors and properties, which may provide insights into how the excellent mechanical properties of protein fibrils can be determined.

Re-entrant isotropic-nematic phase behavior in polymer-depleted amyloid fibrils

Amyloid fibrils dispersed in water exhibit both isotropic (I) and nematic (N) phases, depending on concentration, but their coexistence, expected from the first order nature of the I - N thermodynamic transition is seldom observed. By adding a non-absorbing polymer to an amyloid fibrils suspension, we report, for the first time, an unusual closed-loop phase behavior. The phase diagrams reveal that the I + N coexistence does emerge, but only when the depleting polymer is present at intermediate concentrations. We combine depletion potentials in the dilute and semi-dilute polymer regime with the DLVO theory and the principle of equivalent law of corresponding states to calculate variations of the second virial coefficient with increasing polymer concentrations. We conclude that the decrease of the depletion potential range in the semi-dilute regime plays a pivotal role in the observed re-stabilization, leading to a closure of the I + N coexistence region.

The effect of protein aggregate morphology on phase separation in mixtures with polysaccharides

The morphology of aggregates formed by heating the globular protein β-lactoglobulin (β-lg) changes with the addition of a small amount of CaCl2, from small strands to larger spherical aggregates (microgels). We investigated the effect of this morphological transition on the structure of mixtures of β-lg aggregates with the polysaccharide κ-carrageenan (κ-car), using confocal laser scanning microscopy and dynamic light scattering. The change in the morphology of the β-lg aggregates strongly reduced the κ-car concentration at which the system phase separated. As a consequence a dramatic change in the structure of the mixtures occurred over a narrow range of the CaCl2 concentration. Phase separation leads to the formation of micron-sized protein rich domains that have a tendency to stick together in large flocs. There is a big difference between the protein concentrations in the two phases, but the κ-car concentration is only weakly lower in the protein rich phase. A comparison is made between mixtures prepared at room temperature, after separately heating β-lg, and heated mixtures of native β-lg and κ-car. The micro-phase separated structure of the two systems is similar, but the aggregates disperse upon dilution in the former case, while they are covalently bound within the domains in the latter case. Other, more subtle, differences were also observed. The results explain the very high sensitivity of the structure of β-lg/κ-car mixtures to calcium ions.

Bridging the gap between the nanostructural organization and macroscopic interfacial rheology of amyloid fibrils at liquid interfaces

The interfacial behavior of proteins and protein aggregates such as fibrils influences the bulk behavior of multiphase systems in foods, pharmaceuticals, and other technological applications. Additionally, it is an important factor in some biological processes such as the accumulation of amyloid fibrils at biological membranes in neurodegenerative diseases. Here, using β-lactoglobulin fibrils as a model system, we cover a large range of characteristic measuring length scales by combining atomic force microscopy, passive probe particle tracking, tensiometry, interfacial shear, and dilatational rheology in order to correlate the intricate structure of fibril-laden interfaces with their macroscopic adsorption kinetics and viscoelasticity. A subtle change in solution pH provokes pronounced changes in interfacial properties such as alignment, entanglement, multilayer formation, and fibril fracture, which can be resolved and linked across the various length scales involved.

Homogeneous percolation versus arrested phase separation in attractively-driven nanoemulsion colloidal gels

We elucidate mechanisms for colloidal gelation of attractive nanoemulsions depending on the volume fraction (ϕ) of the colloid. Combining detailed neutron scattering, cryo-transmission electron microscopy and rheological measurements, we demonstrate that gelation proceeds by either of two distinct pathways. For ϕ sufficiently lower than 0.23, gels exhibit homogeneous fractal microstructure, with a broad gel transition resulting from the formation and subsequent percolation of droplet-droplet clusters. In these cases, the gel point measured by rheology corresponds precisely to arrest of the fractal microstructure, and the nonlinear rheology of the gel is characterized by a single yielding process. By contrast, gelation for ϕ sufficiently higher than 0.23 is characterized by an abrupt transition from dispersed droplets to dense clusters with significant long-range correlations well-described by a model for phase separation. The latter phenomenon manifests itself as micron-scale "pores" within the droplet network, and the nonlinear rheology is characterized by a broad yielding transition. Our studies reinforce the similarity of nanoemulsions to solid particulates, and identify important qualitative differences between the microstructure and viscoelastic properties of colloidal gels formed by homogeneous percolation and those formed by phase separation.

Resistive switching behavior in gelatin thin films for nonvolatile memory application

This paper presents the characteristics of gelatin, which can cause reproducible resistive switching and bipolar resistive switching in aluminum (Al)/gelatin (35 nm)/ITO devices. The memory devices exhibited a high ON/OFF ratio of over 10(6) and a long retention time of over 10(5) seconds. The resistive switching mechanism was investigated using the high-angle dark field transmission electron microscopy image of Al/gelatin/ITO devices in the pristine high-resistance state (HRS) and then in returning to HRS after the RESET process. The energy-dispersive X-ray spectroscopy analysis revealed the aggregation of N and Al elements and the simultaneous presence of carbon and oxygen elements in the rupture of filament paths. Furthermore, via a current-sensing atomic force microscopy, we found that conduction paths in the ON-state are distributed in a highly localized area, which is associated with a carbon-rich filamentary switching mechanism. These results support that the chelation of N binding with Al ions improves the conductivity of the low-resistance state but not the production of metal filaments.

Mechanical properties of protein adsorption layers at the air/water and oil/water interface: a comparison in light of the thermodynamical stability of proteins

Over the last decades numerous studies on the interfacial rheological response of protein adsorption layers have been published. The comparison of these studies and the retrieval of a common parameter to compare protein interfacial activity are hampered by the fact that different boundary conditions (e.g. physico-chemical, instrumental, interfacial) were used. In the present work we review previous studies and attempt a unifying approach for the comparison between bulk protein properties and their adsorption films. Among many common food grade proteins we chose bovine serum albumin, β-lactoglobulin and lysozyme for their difference in thermodynamic stability and studied their adsorption at the air/water and limonene/water interface. In order to achieve this we have i) systematically analyzed protein adsorption kinetics in terms of surface pressure rise using a drop profile analysis tensiometer and ii) we addressed the interfacial layer properties under shear stress using an interfacial shear rheometer under the same experimental conditions. We could show that thermodynamically less stable proteins adsorb generally faster and yield films with higher shear rheological properties at air/water interface. The same proteins showed an analog behavior when adsorbing at the limonene/water interface but at slower rates.

The presence of an air-water interface affects formation and elongation of α-Synuclein fibrils

The aggregation of human α-Synuclein (α-Syn) into amyloid fibrils is related to the onset of multiple diseases termed synucleinopathies. Substantial evidence suggests that hydrophobic-hydrophilic interfaces promote the aggregation of amyloidogenic proteins and peptides in vitro. In this work the effect of the air-water interface (AWI) on α-Syn aggregation is investigated by means of thioflavin T binding measurements, dynamic light scattering, size-exclusion chromatography, electron microscopy, and atomic force microscopy. Measurements were performed with the monomeric protein alone or together with preformed seeds. In presence of the AWI, α-Syn aggregates readily into amyloid fibrils that remain adsorbed to the AWI. Instead, when the AWI is removed from the samples by replacing it with a solid-liquid interface, the interfacial aggregation of monomeric α-Syn is greatly reduced and no significant increase in ThT fluorescence is detected in the bulk, even at 900 μM concentration. Bulk aggregation is observed only when a sufficient amount of preformed seeds is added, and the initial slope of the kinetics scales with the amount of seeds as expected for first order kinetics. By contrast, in seeded experiments with the AWI, the initial slope is one order of magnitude lower and secondary nucleation pathways appear instead to be dominant. Thus, interfaces play multiple roles in the aggregation of α-Syn, influencing primary nucleation, aggregate elongation, and secondary nucleation processes. Interfacial effects must therefore be taken into account to achieve a complete understanding of protein aggregation events in vitro as well as in vivo.

Polymorphism complexity and handedness inversion in serum albumin amyloid fibrils

Protein-based amyloid fibrils can show a great variety of polymorphic structures within the same protein precursor, although the origins of these structural homologues remain poorly understood. In this work we investigate the fibrillation of bovine serum albumin--a model globular protein--and we follow the polymorphic evolution by a statistical analysis of high-resolution atomic force microscopy images, complemented, at larger length scales, by concepts based on polymer physics formalism. We identify six distinct classes of coexisting amyloid fibrils, including flexible left-handed twisted ribbons, rigid right-handed helical ribbons and nanotubes. We show that the rigid fibrils originate from flexible fibrils through two diverse polymorphic transitions, first, via a single-fibril transformation when the flexible left-handed twisted ribbons turn into the helical left-handed ribbons, to finally evolve into nanotube-like structures, and second, via a double-fibril transformation when two flexible left-handed twisted ribbons wind together resulting in a right-handed twisted ribbon, followed by a rigid right-handed helical ribbon polymorphic conformation. Hence, the change in handedness occurs with an increase in the level of the fibril's structural organization.

In-situ quantification of the interfacial rheological response of bacterial biofilms to environmental stimuli

Understanding the numerous factors that can affect biofilm formation and stability remain poorly understood. One of the major limitations is the accurate measurement of biofilm stability and cohesiveness in real-time when exposed to changing environmental conditions. Here we present a novel method to measure biofilm strength: interfacial rheology. By culturing a range of bacterial biofilms on an air-liquid interface we were able to measure their viscoelastic growth profile during and after biofilm formation and subsequently alter growth conditions by adding surfactants or changing the nutrient composition of the growth medium. We found that different bacterial species had unique viscoelastic growth profiles, which was also highly dependent on the growth media used. We also found that we could reduce biofilm formation by the addition of surfactants or changing the pH, thereby altering the viscoelastic properties of the biofilm. Using this technique we were able to monitor changes in viscosity, elasticity and surface tension online, under constant and varying environmental conditions, thereby providing a complementary method to better understand the dynamics of both biofilm formation and dispersal.