Microstructure of β-Lactoglobulin/Pectin Coacervates Studied by Small-Angle Neutron Scattering

Advances in Small Angle Neutron Scattering on Polysaccharide Materials

Polysaccharide materials and biomaterials gain the focus of intense research owing to their great versatility in chemical structures and modification possibilities, as well as their biocompatibility, degradability, and sustainability features. This review focuses on the recent advances in the application of SANS on polysaccharide systems covering a broad range of materials such as nanoparticulate assemblies, hydrogels, nanocomposites, and plant-originating nanostructured systems. It motivates the use of SANS in its full potential by demonstrating the features of contrast variation and contrast matching methods and by reporting the methodologies for data analysis and interpretation. As these soft matter systems may be organized in multiple length scales depending on the interactions and chemical bonds between their components, SANS offers exceptional and unique opportunities for advanced characterization and optimization of new nanostructured polysaccharide materials.

Fluctuations, structure, and size inside coacervates

Aqueous solutions of oppositely charged macromolecules exhibit the ubiquitous phenomenon of coacervation. This subject is of considerable current interest due to numerous biotechnological applications of coacervates and the general premise of biomolecular condensates. Towards a theoretical foundation of structural features of coacervates, we present a field-theoretic treatment of coacervates formed by uniformly charged flexible polycations and polyanions in an electrolyte solution. We delineate different regimes of polymer concentration fluctuations and structural features of coacervates based on the concentrations of polycation and polyanion, salt concentration, and experimentally observable length scales. We present closed-form formulas for correlation length of polymer concentration fluctuations, scattering structure factor, and radius of gyration of a labelled polyelectrolyte chain inside a concentrated coacervate. Using random phase approximation suitable for concentrated polymer systems, we show that the inter-monomer electrostatic interaction is screened by interpenetration of all charged polymer chains and that the screening length depends on the individual concentrations of the polycation and the polyanion, as well as the salt concentration. Our calculations show that the scattering intensity decreases monotonically with scattering wave vector at higher salt concentrations, while it exhibits a peak at intermediate scattering wave vector at lower salt concentrations. Furthermore, we predict that the dependence of the radius of gyration of a labelled chain on its degree of polymerization generally obeys the Gaussian chain statistics. However, the chain is modestly swollen, the extent of which depending on polyelectrolyte composition, salt concentration, and the electrostatic features of the polycation and polyanion such as the degree of ionization.

pH-Dependent complexation and polyelectrolyte chain conformation of polyzwitterion-polycation coacervates in salted water

The phase behavior and chain conformational structure of biphasic polyzwitterion-polyelectrolyte coacervates in salted aqueous solution are investigated with a model weak cationic polyelectrolyte, poly(2-vinylpyridine) (P2VP), whose charge fraction can be effectively tuned by pH. It is observed that increasing the pH leads to the increase of the yielding volume fraction and the water content of dense coacervates formed between net neutral polybetaine and cationic P2VP in contrast to the decrease of critical salt concentration for the onset of coacervation, where the P2VP charge fraction is reduced correspondingly. Surprisingly, a single-molecule fluorescence spectroscopic study suggests that P2VP chains upon coacervation seem to adopt a swollen or an even more expanded conformational structure at higher pH. As the hydrophobicity of P2VP chains is accompanied by a reduced charge fraction by increasing the pH, a strong pH-dependent phase and conformational behaviors suggest the shift of entropic and enthalpic contribution to the underlying thermodynamic energy landscape and chain structural dynamics of polyelectrolyte coacervation involving weak polyelectrolytes in aqueous solution.

Effect of linear charge density of polysaccharides on interactions with α-amylase: Self-Assembling behavior and application in enzyme immobilization

The co-existence of polysaccharides and enzymes in the food matrix could form complexes that directly influence the catalytic efficacy of enzymes. This work investigated the self-assembly behaviors of α-amylase and charged polysaccharides and fabricated the α-amylase/polysaccharides complex coacervates. The results showed that the linear charge density of polysaccharides had a critical impact on the complex formation, structure, and enzyme protection under acidic conditions. At low pH, α-amylase formed compact and tight coacervates with the λ-carrageenan. However, α-amylase/pectin coacervates dissociated when the pH was lower than 3.0. The optimized binding ratio of α-amylase/λ-carrageenan was 12:1, and α-amylase/pectin was 4:1. Finally, the α-amylase/λ-carrageenan complex coacervates effectively immobilized the enzyme and almost 70% of enzyme activity remained in coacervates after exposure to pH3.0 for 1 h. This study demonstrates that the change in the linear charge density of polysaccharides could regulate the enzyme-catalyzed process in food processing by a simple and fine-controlled method.

Effect of sodium chloride on the thermodynamic, rheological, and microstructural properties of field pea protein isolate/chitosan complex coacervates

The effect of increasing sodium chloride concentration (c_NaCl, 0-0.4 M) on the formation and rheological and microstructural properties of field pea protein isolate (FPPI)/chitosan (Ch) complex coacervates was investigated. The maximum turbidity and zeta potential of FPPI/Ch mixtures consistently decreased with the increasing c_NaCl. The tertiary conformation of FPPI was altered to facilitate the aggregation of FPPI/Ch complexes via hydrophobic interactions. Changes in thermodynamic parameters during the titration of FPPI with Ch confirmed the addition of NaCl could cause the inhibition of electrostatic complexation and the induction of non-Coulombic interactions. FPPI/Ch complex coacervates exhibited first enhanced and then weakened viscoelastic properties and an initially tightened and then a loosened microstructure as the c_NaCl increased. In summary, appropriate c_NaCl favors the formation of FPPI/Ch complex coacervates with improved functionalities via the coordination of promoted hydrophobic interactions and inhibited electrostatic attractions, facilitating the application of this protein ingredient in food development.

Binary Solutions of Hyaluronan and Lactose-Modified Chitosan: The Influence of Experimental Variables in Assembling Complex Coacervates

A miscibility study between oppositely charged polyelectrolytes, namely hyaluronic acid and a lactose-modified chitosan, is here reported. Experimental variables such as polymers' weight ratios, pH values, ionic strengths and hyaluronic acid molecular weights were considered. Transmittance analyses demonstrated the mutual solubility of the two biopolymers at a neutral pH. The onset of the liquid-liquid phase separation due to electrostatic interactions between the two polymers was detected at pH 4.5, and it was found to be affected by the overall ionic strength, the modality of mixing and the polymers' weight ratio. Thorough Dynamic Light Scattering (DLS) measurements were performed to check the quality of the formed coacervates by investigating their dimensions, homogeneity and surface charge. The whole DLS results highlighted the influence of the hyaluronic acid molecular weight in affecting coacervates' dispersity and size.

Effect of charge density of polysaccharide on self-assembly behaviors of ovalbumin and sodium alginate

Similarities and differences of assembly for ovalbumin (OVA) and two kinds of sodium alginate (SA1 and SA2) varying in charge densities (λ_SA1: λ_SA2 ≈ 2:1) were investigated. The assembly processes of OVA/SA mixtures were characterized by phase diagram, particle size, and microstructure. Two differences between OVA/SA1 and OVA/SA2 mixtures in the phase diagram were distinctly observed. First, due to the higher charge density of SA1, the strong interaction between OVA and SA1 caused only pH_φ1 to be recorded. A higher linear charge density of SA1 narrowed the pH_φ1-pH_φ2 range at ratios of 2:1 and 1:1. Second, OVA/SA1 complexes formed a coacervate with a relatively strong resistance to ion-induced shielding effects. This maintained the smaller size (tighter structure) with a larger number of complexes in the coacervate without 250 mM NaCl. The regulating polysaccharides with different charge densities could control the soluble region of complexes and endow various size or morphology of the coacervate assembled by proteins and polysaccharides.

Complex coacervation in charge complementary biopolymers: Electrostatic versus surface patch binding

In this review, a number of systems are described to demonstrate the effect of polyelectrolyte chain stiffness (persistence length) on the coacervation phenomena, after we briefly review the field. We consider two specific types of complexation/coacervation: in the first type, DNA is used as a fixed substrate binding to flexible polyions such as gelatin A, bovine serum albumin and chitosan (large persistence length polyelectrolyte binding to low persistence length biopolymer), and in the second case, different substrates such as gelatin A, bovine serum albumin, and chitosan were made to bind to a polyion gelatin B (low persistence length substrate binding to comparable persistence length polyion). Polyelectrolyte chain flexibility was found to have remarkable effect on the polyelectrolyte-protein complex coacervation. The competitive interplay of electrostatic versus surface patch binding (SPB) leading to associative interaction followed by complex coacervation between these biopolymers is elucidated. We modelled the SPB interaction in terms of linear combination of attractive and repulsive Coulombic forces with respect to the solution ionic strength. The aforesaid interactions were established via a universal phase diagram, considering the persistence length of polyion as the sole independent variable.

Sugar-coated proteins: the importance of degree of polymerisation of oligo-galacturonic acid on protein binding and aggregation

We have simplified the structural heterogeneity of protein-polysaccharide binding by investigating protein binding to oligosaccharides. The interactions between bovine beta-lactoglobulin A (βLgA) and oligo-galacturonic acids (OGAs) with various numbers of sugar residues have been investigated with a range of biophysical techniques. We show that the βLgA-OGA interaction is critically dependent on the length of the oligosaccharide. Isothermal titration calorimetry results suggest that a minimum length of 7 or 8 sugar residues is required in order to exhibit appreciable exothermic interactions with βLgA - shorter oligosaccharides show no enthalpic interactions at any concentration ratio. When titrating βLgA into OGAs with more than 7-8 sugar residues the sample solution also became turbid with increasing amounts of βLgA, indicating the formation of macroscopic assemblies. Circular dichroism, thioflavin T fluorescence and small angle X-ray/neutron scattering experiments revealed two structural regimes during the titration. When OGAs were in excess, βLgA formed discrete assemblies upon OGA binding, and no subsequent aggregation was observed. However, when βLgA was present in excess, multi-scale structures were formed and this eventually led to the separation of the solution into two liquid-phases.

Application of Monte Carlo simulation in addressing key issues of complex coacervation formed by polyelectrolytes and oppositely charged colloids

This paper reviews the recent advance of Monte Carlo (MC) simulation in addressing key issues of complex coacervation between polyelectrolytes and oppositely charged colloids. Readers were first supplied with a brief overview of current knowledge and experimental strategies in the study of complex coacervation. In the next section, the general MC simulation procedures as well as representative strategies applied in complex coacervation were summarized. The unique contributions of MC simulation in either capturing delicate features, easing the experimental trials or proving the concept were then elucidated through the following aspects: i) identify phase boundary and decouple interaction contributions; ii) clarify composition distribution and internal structure; iii) predict the influences of physicochemical conditions on complex coacervation; iv) delineate the mechanisms for "binding on the wrong side of the isoelectric point". Finally, current challenges as well as prospects of MC simulation in complex coacervation are also discussed. The ultimate goal of this review is to provide readers with basic guideline for synergistic design of experiments in combination with MC simulation, and deliver convincing interpretation and reliable prediction for the structure and behavior in polyelectrolyte-macroion complex coacervation.

Molecular and structural basis of low interfacial energy of complex coacervates in water

Complex coacervate refers to a phase-separated fluid, typically of two oppositely charged polyelectrolytes in solution, representing a complex fluid system that has been shown to be of essential interest to biological systems, as well as for soft materials processing owing to the expectation of superior underwater coating or adhesion properties. The significance and interest in complex coacervate fluids critically rely on its low interfacial tension with respect to water that, in turn, facilitates the wetting of macromolecular or material surfaces under aqueous conditions, provided there is attractive interaction between the polyelectrolyte constituents and the surface. However, the molecular and structural bases of these properties remain unclear. Recent studies propose that the formation of water-filled and bifluidic sponge-like nanostructured network, driven by the tuning of electrostatic interactions between the polyelectrolyte constituents or their complexes may be a common feature of complex coacervate fluids that display low fluid viscosity and low interfacial tension, but more studies are needed to verify the generality of these observations. In this review, we summarize representative studies of interfacial tension and ultrastructures of complex coacervate fluids. We highlight that a consensus property of the complex coacervate fluid is the observation of high or even bulk-like water dynamics within the dense complex coacervate phase that is consistent with a low cohesive energy fluid. Our own studies on this subject are enabled by the application of magnetic resonance relaxometry methods relying on spin labels tethered to polyelectrolyte constituents or added as spin labeled probe molecules that partition into the dense versus the equilibrium coacervate phase, permitting the extraction of information on local polymer dynamics, polymer packing and local water dynamics. We conclude with a snapshot of our current perspective on the molecular and structural bases of the low interfacial tension of complex coacervate fluids.

Encapsulation of active ingredients in polysaccharide-protein complex coacervates

Polysaccharide-protein complex coacervates are amongst the leading pair of biopolymer systems that has been used over the past decades for encapsulation of numerous active ingredients. Complex coacervation of polysaccharides and proteins has received increasing research interest for the practical application in encapsulation industry since the pioneering work of complex coacervation by Bungenburg de Jong and co-workers on the system of gelatin-acacia, a protein-polysaccharide system. Because of the versatility and numerous potential applications of these systems essentially in the fields of food, pharmaceutical, cosmetics and agriculture, there has been intense interest in recent years for both fundamental and applied studies. Precisely, the designing of the micronscale and nanoscale capsules for encapsulation and control over their properties for practical applications garners renewed interest. This review discusses on the overview of polysaccharide-protein complex coacervates and their use for the encapsulation of diverse active ingredients, designing and controlling of the capsules for delivery systems and developments in the area.

Hierarchical Internal Structures in Gelatin-Bovine Serum Albumin/β-Lactoglobulin Gels and Coacervates

Herein, we report the comparative study of gels and complex coacervates of bovine serum albumin (BSA) and beta-lactoglobulin (β-Lg) with gelatin close to their common pI. Surface patch binding produced a range of new soft matter phases (interpolymer complexes) such as opaque coacervates (charge neutralized complexes) and transparent gels (overcharged complexes). We emphasize on the comparative study of the microstructure of coacervates and gels formed at different mixing ratios using small angle scattering (SANS) data. It was found that phase states were entirely defined by the mixing ratio r = [GB]:[β-Lg or BSA]. Thermo-viscoelastic profiles of aforesaid samples revealed a smaller storage modulus and lower melting temperature for coacervates compared to gels. Thermally activated samples generated additional phases that were also probed by SANS and rheology. Thus, it is established that intermolecular association between globular proteins and a random coil polypeptide can generate various soft matter states that may facilitate harvesting of novel biomaterials.

Charge heterogeneity induced binding and phase stability in β-lacto-globulin–gelatin B gels and coacervates at their common pI

An understanding of the interactions between gelatin B (GB) and β-lacto-globulin (β-Lg) mainly arising from surface selective patch binding occurring at their common pI (≈5.0 ± 0.5) in the absence of added salt.

A review of the preparation and application of flavour and essential oils microcapsules based on complex coacervation technology

This paper briefly introduces the preparation and application of flavour and essential oils microcapsules based on complex coacervation technology. The conventional encapsulating agents of oppositely charged proteins and polysaccharides that are used for microencapsulation of flavours and essential oils are reviewed along with the recent advances in complex coacervation methods. Proteins extracted from animal-derived products (gelatin, whey proteins, silk fibroin) and from vegetables (soy proteins, pea proteins), and polysaccharides such as gum Arabic, pectin, chitosan, agar, alginate, carrageenan and sodium carboxymethyl cellulose are described in depth. In recent decades, flavour and essential oils microcapsules have found numerous potential practical applications in food, textiles, agriculturals and pharmaceuticals. In this paper, the different coating materials and their application are discussed in detail. Consequently, the information obtained allows criteria to be established for selecting a method for the preparation of microcapsules according to their advantages, limitations and behaviours as carriers of flavours and essential oils.

Protein/polysaccharide complexes and coacervates in food systems

Since the pioneering work of Bungenberg de Jong and co-workers on gelatin-acacia gum complex coacervation in the 1920-40s, protein/polysaccharide complexes and coacervates have received increasing research interest in order to broaden the possible food applications. This review focuses on the main research streams followed in this field during the last 12 years regarding: i) the parameters influencing the formation of complexes and coacervates in protein-polysaccharide systems; ii) the characterization of the kinetics of phase separation and multi-scale structure of the complexes and coacervates; and iii) the investigation of the functional properties of complexes and coacervates in food applications. This latter section encompasses various technological aspects, namely: the viscosifying and gelling ability, the foaming and emulsifying ability and finally, the stabilization and release of bioactives or sensitive compounds.

The model Lysozyme-PSSNa system for electrostatic complexation: Similarities and differences with complex coacervation

We review, based on structural information, the mechanisms involved when putting in contact two nano-objects of opposite electrical charge, in the case of one negatively charged polyion, and a compact charged one. The central case is mixtures of PSS, a strong flexible polyanion (the salt of a strong acid, and with high linear charge density), and Lysozyme, a globular protein with a global positive charge. A wide accurate and consistent set of information in different situations is available on the structure at local scales (5-1000Å), due to the possibility of matching, the reproducibility of the system, its well-defined electrostatics features, and the well-defined structures obtained. We have related these structures to the observations at macroscopic scale of the phase behavior, and to the expected mechanisms of coacervation. On the one hand, PSS/Lysozyme mixtures show accurately many of what is expected in PEL/protein complexation, and phase separation, as reviewed by de Kruif: under certain conditions some well-defined complexes are formed before any phase separation, they are close to neutral; even in excess of one species, complexes are only modestly charged (surface charges in PEL excess). Neutral cores are attracting each other, to form larger objects responsible for large turbidity. They should lead the system to phase separation; this is observed in the more dilute samples, while in more concentrated ones the lack of separation in turbid samples is explained by locking effects between fractal aggregates. On the other hand, although some of the features just listed are the same required for coacervation, this phase transition is not really obtained. The phase separation has all the macroscopic aspects of a fluid (undifferentiated liquid/gas phase) - solid transition, not of a fluid-fluid (liquid-liquid) one, which would correspond to real coacervation). The origin of this can be found in the interaction potential between primary complexes formed (globules), which agrees qualitatively with a potential shape of the type repulsive long range attractive very short range. Finally we have considered two other systems with accurate structural information, to see whether other situations can be found. For Pectin, the same situation as PSS can be found, as well as other states, without solid precipitation, but possibly with incomplete coacervation, corresponding to differences in the globular structure. It is understandable that these systems show smoother interaction potential between the complexes (globules) likely to produce liquid-liquid transition. Finally, we briefly recall new results on Hyaluronan/Lysozyme, which present clear signs of coacervation in two liquid phases, and at the same time the existence of non-globular complexes, of specific geometry (thin rods) before any phase separation. These mixtures fulfill many of the requirements for complex coacervation, while other theories should also be checked like the one of Shklovskii et al.

Thermodynamic characterization of acacia gum-beta-lactoglobulin complex coacervation

The interactions of beta-lactoglobulin (BLG) with total acacia gum (TAG) in aqueous solutions have been investigated at pH 4.2 and 25 degrees C. Isothermal titration calorimetry (ITC) has been used to determine the type and magnitude of the energies involved in the complexation process of TAG to BLG. Dynamic light scattering (DLS), electrophoretic mobility (mu(E)), turbidity measurements (tau), and optical microscopy were used as complementary methods on the titration mode to better understand the sum of complicated phenomena at the origin of thermodynamic behavior. Two different binding steps were detected. Thermodynamic parameters indicate a first exothermic step with an association constant K(a1) of (48.4 +/- 3.6) x 10(7) M(-1) that appeared to be mostly enthalpy-driven. A positive heat capacity change was obtained corresponding at the signature for electrostatic interactions. The second binding step, 45 times less affinity (K(a2) = (1.1 +/- 0.1) x 10(7) M(-1)), was largely endothermic and more entropy-driven with a negative value of heat capacity change, indicative of a hydrophobic contribution to the binding process. The population distribution of the different species in solution and their sizes were determined through DLS. Dispersion turbidity of particles markedly increased and reached a maximum at a 0.015 TAG/BLG molar ratio. Largely more numerous coacervates appeared at this molar ratio (0.015) and two different kinds of morphologies were noticed for the large coacervates. Above the TAG/BLG molar ratio of 0.015, dispersions turbidity decreased, which might be due to an excess of negative charges onto particles as revealed by electrophoretic mobility measurements. The results presented in this study should provide information about the thermodynamic mechanisms of TAG/BLG binding processes and will facilitate the application of the formed supramolecular assemblies as functional ingredients in food and nonfood systems.

Interaction of alpha-gliadin with polyanions: design considerations for sequestrants used in supportive treatment of celiac disease

Copolymers of sodium 4-styrene sulfonate (SS) and hydroxyethyl methacrylate (HEMA) were investigated as sequestrants of alpha-gliadin, a gluten protein, for the treatment of gluten intolerance. The interactions of alpha-gliadin with poly(SS) and poly(HEMA-co-SS) with 9 and 26 mol% SS content were studied at gastric (1.2) and intestinal (6.8) pH using circular dichroism and measurements of turbidity, dynamic light scattering and zeta potential. The interactions and their influence on alpha-gliadin secondary and aggregated structures depended mainly on the ratio of polymer negative and protein positive charges at pH 1.2, and on polymer SS content at polymer concentrations providing in excess of negative charges at either pH. Poly(SS) could not form complex particles with alpha-gliadin in a sufficient excess of negative charges. Copolymerization with HEMA enhanced the formation of complex particles. Poly(HEMA-co-SS) with intermediate SS content was found to be the most effective sequestrant for alpha-gliadin. This study provides insight into design considerations for polymer sequestrants used in the supportive treatment of celiac disease.

Multiple scale reorganization of electrostatic complexes of poly(styrenesulfonate) and lysozyme

We report on a SANS investigation into the potential for these structural reorganization of complexes composed of lysozyme and small PSS chains of opposite charge if the physicochemical conditions of the solutions are changed after their formation. Mixtures of solutions of lysozyme and PSS with high matter content and with an introduced charge ratio [-]/[+](intro) close to the electrostatic stoichiometry lead to suspensions that are macroscopically stable. They are composed at local scale of dense globular primary complexes of radius approximately 100 A; at a higher scale they are organized fractally with a dimension 2.1. We first show that the dilution of the solution of complexes, all other physicochemical parameters remaining constant, induces a macroscopic destabilization of the solutions but does not modify the structure of the complexes at submicronic scales. This suggests that the colloidal stability of the complexes can be explained by the interlocking of the fractal aggregates in a network at high concentration: dilution does not break the local aggregate structure, but it does destroy the network. We show, second, that the addition of salt does not change the almost frozen inner structure of the cores of the primary complexes, although it does encourage growth of the complexes; these coalesce into larger complexes as salt has partially screened the electrostatic repulsions between two primary complexes. These larger primary complexes remain aggregated with a fractal dimension of 2.1. Third, we show that the addition of PSS chains up to [-]/[+](intro) approximately 20, after the formation of the primary complex with a [-]/[+](intro) close to 1, only slightly changes the inner structure of the primary complexes. Moreover, in contrast to the synthesis achieved in the one-step mixing procedure where the proteins are unfolded for a range of [-]/[+](intro), the native conformation of the proteins is preserved inside the frozen core.

Isothermal microcalorimetry to investigate non specific interactions in biophysical chemistry

Isothermal titration microcalorimetry (ITC) is mostly used to investigate the thermodynamics of “specific” host-guest interactions in biology as well as in supramolecular chemistry. The aim of this review is to demonstrate that ITC can also provide useful information about non-specific interactions, like electrostatic or hydrophobic interactions. More attention will be given in the use of ITC to investigate polyelectrolyte-polyelectrolyte (in particular DNA-polycation), polyelectrolyte-protein as well as protein-lipid interactions. We will emphasize that in most cases these “non specific” interactions, as their definition will indicate, are favoured or even driven by an increase in the entropy of the system. The origin of this entropy increase will be discussed for some particular systems. We will also show that in many cases entropy-enthalpy compensation phenomena occur.

Fluorescence Investigations into Complex Coacervation between Polyvinylimidazole and Sodium Alginate

Electrostatic interactions between the imidazole-based cationic homopolymer, polyvinylimidazole (PVIm), and anionic polysaccharide, sodium alginate, lead to the formation of colloidal aggregates known as complex coacervates in the pH range 4-6.5. PVIm was labeled with the fluorescent reporter pyrene to investigate the coacervation-induced changes in and around PVIm chains. While the pyrene-tagged PVIm had blue fluorescence in water, the coacervate phase exhibited an additional broad band around 492 nm (green) due to formation of pyrene excimers. Fluorescence spectroscopic investigations point toward aggregation of PVIm chains and desolvation upon coacervation. Highly anisotropic fluorescence emission indicates tight packing of the polymer chains in the coacervate. Confocal microscopy of fluorescein-labeled alginate and rhodamine-labeled PVIm shows coacervates as dense aggregates with uniform distribution of the polymers. Fluorescence spectroscopy offers sensitive and easy investigation into polyelectrolyte interactions.

Interfacial structure and stability of food emulsions as affected by protein-polysaccharide interactions

The exploitation of protein-polysaccharide interactions offers opportunities for the design of new ingredients and interfacial structures with applications in the food and pharmaceutical industries. Association of protein and polysaccharide molecules may occur chemically through covalent bonds or physically through electrostatic interactions. Theoretical and experimental studies indicate that various molecular and thermodynamic factors can be adjusted to optimize the effectiveness of covalent conjugates and electrostatic complexes in the stabilization of interfaces, gels and emulsions. Maillard-type protein-polysaccharide conjugates have excellent emulsifying and steric stabilizing properties, especially under conditions where the protein alone is poorly soluble. Charged polysaccharides form soluble complexes or coacervates with proteins depending on pH, ionic strength, and biopolymer charge distribution. The structure and stabilizing properties of the mixed protein + polysaccharide layer depends on the sequence of adsorption of the biopolymers to the interface. There is good potential for use of interfacial protein-polysaccharide complexes in the nanoscale engineering of delivery vehicles for nutrient encapsulation and in the protection of adsorbed proteins and emulsified lipids against enzymatic breakdown during digestion.

Effects of salt concentration on formation and dissociation of beta-lactoglobulin/pectin complexes

The formation and dissociation of beta-lactoglobulin/pectin complexes at various sodium chloride concentrations (CNaCl) have been studied by turbidimetric titration. An increase of CNaCl up to 0.1 M shifts the critical pHphi1, which designates the formation of beta-lactoglobulin/pectin coacervates, to higher pH values, whereas further increase of CNaCl from 0.1 to 0.8 M decreases pHphi1 values. These salt effects can be explained in terms of a salt-enhanced effect at lower salt concentrations or a salt-reduced effect at higher salt concentrations, respectively. On the other hand, the value of pHphi2, which corresponds to the dissociation of beta-lactoglobulin/pectin coacervates, tends to have smaller pH values when CNaCl increases from 0.1 to 0.3 M. No observable pHphi2 values are found at CNaCl higher than 0.3 M. The disappearance of pHphi2 is mainly attributed to the strong self-aggregation capability of beta-lactoglobulin at higher CNaCl. The aggregation of beta-lactoglobulin at high CNaCl is reversible, as suggested by the atomic force microscopy results.

Surface patch binding induced intermolecular complexation and phase separation in aqueous solutions of similarly charged gelatin-chitosan molecules

The formation of selective surface patch binding induced complex coacervates between polyions, chitosan (cationic polyelectrolyte), and alkali-processed gelatin (polyampholyte), both carrying similar net charge, was investigated for two volumetric mixing ratios: r = [chitosan]/[gelatin] = 1:5 and 1:10. Formation of soluble intermolecular complexes between gelatin and chitosan molecules was observed in a narrow range of pH, though these biopolymers had the same kind of net charge, which was evidenced from electrophoretic measurement. This clearly established the role played by selective surface patch binding driven interactions. The temperature sweep measurements conducted on these coacervate samples through rheology and differential scanning calorimetry (DSC) studies yielded two characteristic melting temperatures located at approximately 68 +/- 3 degrees C and 82 +/- 3 degrees C. In the flow mode, the shear viscosity (eta) of the coacervate samples was found to scale with (power-law model) applied shear rate (gamma*) as eta(gamma*) approximately (gamma*)(-k); this yielded k = 0.76 +/- 0.2 (1 s(-1) < gamma* < 100 s(-1)), indicating non-Newtonian behavior. The static structure factor (I(q)) deduced from small angle neutron scattering (SANS) data in the low q (q is the scattering wavevector) (0.018 A(-1) < q < 0.072 A(-1)) region was fitted to the Debye-Bueche regime, I(q) approximately 1/(1 + zeta(2)q(2))2 that yielded a size of zeta approximately 215 +/- 20 A (for r = 1:10) and zeta approximately 260 +/- 20 A (for r = 1:5) samples, implying change in the size of inhomogeneities present with mixing ratio. In the intermediate q region, called the Ornstein-Zernike regime, I(q) approximately 1/(1 + xi(2)q(2)) gave a correlation length of xi approximately 10.0 +/- 2.0 A independent of the mixing ratio. The results taken together imply the existence of a weakly interconnected and heterogeneous network structure inside the coacervate phase separated by domains of polymer-poor regions.

Mesophase separation and probe dynamics in protein-polyelectrolyte coacervates

Protein-polyelectrolyte coacervates are self-assembling macroscopically monophasic biomacromolecular fluids whose unique properties arise from transient heterogeneities. The structures of coacervates formed at different conditions of pH and ionic strength from poly(dimethyldiallylammonium chloride) and bovine serum albumin (BSA), were probed using fluorescence recovery after photobleaching. Measurements of self-diffusion in coacervates were carried out using fluorescein-tagged BSA, and similarly tagged Ficoll, a non-interacting branched polysaccharide with the same size as BSA. The results are best explained by temporal and spatial heterogeneities, also inferred from static light scattering and cryo-TEM, which indicate heterogeneous scattering centers of several hundred nm. Taken together with previous dynamic light scattering and rheology studies, the results are consistent with the presence of extensive dilute domains in which are embedded partially interconnected 50-700 nm dense domains. At short length scales, protein mobility is unobstructed by these clusters. At intermediate length scales, proteins are slowed down due to tortuosity effects within the blind alleys of the dense domains, and to adsorption at dense/dilute domain interfaces. Finally, at long length scales, obstructed diffusion is alleviated by the break-up of dense domains. These findings are discussed in terms of previously suggested models for protein-polyelectrolyte coacervates. Possible explanations for the origin of mesophase separation are offered.

Composition and Rheological Properties of β-Lactoglobulin/Pectin Coacervates: Effects of Salt Concentration and Initial Protein/Polysaccharide Ratio

The composition and rheological properties of beta-lactoglobulin/pectin coacervates have shown significant correlations with sodium chloride concentration (C(NaCl)) and initial protein/polysaccharide ratio (r). An increase of C(NaCl) from 0.01 to 0.21 M at r = 5:1 leads to the increase in both beta-lactoglobulin and pectin contents in the coacervates, which can be explained in terms of salt-enhanced effect at lower salt concentrations. Further increase of C(NaCl) from 0.21 to 0.41 M decreases the proportions of these two biopolymers in the coacervates, exhibiting salt-reduced effect at higher salt concentrations. Moreover, the stronger self-aggregation of beta-lactoglobulin with increasing salt concentration gives rise to a decreasing actual protein/polysaccharide ratio in the coacervates at 0.01-0.21 M C(NaCl) and r = 5:1. An increase of r from 5:1 to 40:1 often increases the actual amount of pectin chains in beta-lactoglobulin/pectin coacervates, but it exhibits a maximum in beta-lactoglobulin content at r = 20:1. A much higher storage modulus (G') than loss modulus (G' ') for all beta-lactoglobulin/pectin coacervates suggests the formation of highly interconnected gel-like structure. The values of G' increase as C(NaCl) increases from 0.01 to 0.21 M, whereas a further increase of C(NaCl) from 0.21 to 0.41 M causes G' values to decrease to much lower values. These results further disclose the salt-enhanced effect and the salt-reduced effect at low and high salt concentrations, respectively. On the other hand, increasing r from 5:1 to 40:1 favors the formation of stronger gel-like beta-lactoglobulin/pectin coacervates, which mainly originates from the higher actual amount of pectin chains in beta-lactoglobulin/pectin coacervates at higher r values.