Intermolecular complexation and phase separation in aqueous solutions of oppositely charged biopolymers

Turbidity measurements performed at 450nm were used to follow the process of complex formation, and phase separation in gelatin-agar aqueous solutions. Acid (Type-A) and alkali (Type-B) processed gelatin (polyampholyte) and agar (anionic polyelectrolyte) solutions, both having concentration of 0.1% (w/v) were mixed in various proportions, and the mixture was titrated (with 0.01 M HCl or NaOH) to initiate associative complexation that led to coacervation. The titration profiles clearly established observable transitions in terms of the solution pH corresponding to the first occurrence of turbidity (pH(C), formation of soluble complexes), and a point of turbidity maximum (pH(phi), formation of insoluble complexes). Decreasing the pH beyond pH(phi) drove the system towards precipitation. The values of pH(C) and pH(phi) characterized the initiation of the formation of intermolecular charge neutralized soluble aggregates, and the subsequent formation of microscopic coacervate droplets. These aggregates were characterized by dynamic light scattering. It was found that Type-A and -B gelatin samples formed soluble intermolecular complexes (and coacervates) with agar molecules through electrostatic and patch-binding interactions, respectively.     

Domain 26 of tropoelastin plays a dominant role in association by coacervation

The temperature-dependent association of tropoelastin molecules through coacervation is an essential step in their assembly leading to elastogenesis. The relative contributions of C-terminal hydrophobic domains in coacervation were assessed. Truncated tropoelastins were constructed with N termini positioned variably downstream of domain 25. The purified proteins were assessed for their ability to coacervate. Disruption to domain 26 had a substantial effect and abolished coacervation. Circular dichroism spectroscopy of an isolated peptide comprising domain 26 showed that it undergoes a structural transition to a state of increased order with increasing temperature. Protease mapping demonstrated that domain 26 is flanked by surface sites and is likely to be in an exposed position on the surface of the tropoelastin molecule. These results suggest that the hydrophobic domain 26 is positioned to play a dominant role in the intermolecular interactions that occur during coacervation.     

Self-healing fish gelatin/sodium montmorillonite biohybrid coacervates: structural and rheological characterization

Complex coacervation driven by associative electrostatic interactions was studied in mixtures of exfoliated sodium-montmorillonite (Na(+)-MMT) nanoplatelets and fish gelatin, at a specific mixing ratio and room temperature. Structural and viscoelastic properties of the coacervate phase were investigated as a function of pH by means of different complementary techniques. Independent of the technique used, the results consistently showed that there is an optimum pH value at which the coacervate phase shows the tightest structure with highest elasticity. The solid-like coacervates showed an obvious shear-thinning behavior and network fracture but immediately recovered back into their original elastic character upon removal of the shear strain. The nonlinear mechanical response characterized by single step stress relaxation experiments revealed the same trend for the yield stress and isochronal shear modulus of the coacervates as a function of pH with a maximum at pH 3.0 and lower values at 2.5 and 3.5 pHs, followed by a very sharp drop at pH 4.0. Finally, small-angle X-ray scattering (SAXS) data confirmed that at pHs lower than 4.0 the coacervate phases were dense and structured with a characteristic length scale (ξ(SAXS)) of ~7-9 nm. Comparing the ξ(SAXS) with rheological characteristic length (ξ(rheol)) estimated from low-frequency linear viscoelastic data and network theory, it was concluded that both the strength of the electrostatic interactions and the conformation of the gelatin chains before and during of the coacervation process are responsible for the structure and rigidity of the coacervates.     

Study on coacervation of the repeat pentapeptide model of tropoelastin: effect of cations

The effects of various cations on coacervation of the polypentapeptide (Val-Pro-Gly-Val-Gly)n, a sequential model of tropoelastin, were studied by following the turbidity at 300 nm and the fluorescence intensity due to the interaction between the polypentapeptide and a hydrophobic probe (4-benzoylamido-4'-aminostilbene-2,2'-disulfonic acid). In the absence of cations, as the polypentapeptide concentration was increased, the turbidity curves shifted to lower temperatures and became much steeper, while the fluorescence intensity increased markedly. This suggests that a conformational change is induced by the association of the polypentapeptide molecules and indicates that coacervation occurs predominantly by intermolecular hydrophobic association. In the presence of Mg2+, the temperature profile of the coacervation curve of the polypentapeptide obtained by turbidity measurement moved to higher temperature and the fluorescence intensity decreased significantly. This suggests that Mg2+ inhibits the hydrophobic interaction of the polypentapeptide molecules on coacervation but does not induce conformational change on association of the molecules. In the presence of Ca2+, Na+, and K+, the temperature profile of the coacervation curve of the polypentapeptide was not affected appreciably, but the fluorescence intensity was decreased by these cations in the order of Na+, K+, and Ca2+. Circular dichroism spectra of the polypentapeptide in 80% trifluoroethanol in the presence of cations showed a less ordered state of the polypentapeptide. This less ordered state implies inhibition by the cations of the hydrophobic interaction between the molecules.     

Synthesis of polymeric models of elastin: Polyhexapeptides and an insoluble,hybrid cross-linked polypeptide

The polymer of the hexapeptide sequence, Val-Ala-Pro-Gly-Glu-Gly, was synthesized and demostrated of exhibit a reversible, pH-dependent coacervation in low-pH aqueous solution. In addition, the synthesis of an insoluble, hybrid, cross-linked polypeptide matrix is described. The cross-linking was achieved in the coacervate state during flow orientation of the polymers. The chemical means of covalent cross-linking was intermolecular primary amide bond formation between the lysyl side chains in one polypentapeptide unit and the glutamyl side chains in another polyhexapeptide unit. The carboxyl activating reagent was a water-soluble carbodiimide. The key intermediates in the syntheses, the hexamers and their high polymers, were analyzed by carbon-13 magnetic resonance to verify the correctness of synthesis and to obtain information on conformation.     

FTIR 2D correlation spectroscopy of α₁ and α₂ fractions of an alkali-pretreated gelatin

An alkali-pretreated gelatin (pI~4.9) was fractionated by means of alcohol coacervation and semi-preparative gel chromatography. The thermal responses of the isolated α fractions, the coacervate and the total gelatin were investigated by 2D-correlation FTIR spectroscopy in the amide I band region (1600-1700 cm?1). The gelation temperature was the same for all examined samples (24.5°C) while the melting temperature of the α? fraction was lower (30°C) than that of the other samples (32.5°C). The 2D COS plots indicate that on cooling (gelation) the core sequence of conformational changes is the same for all samples. On heating, however, the α? fraction deviates from the α?-containing samples and shows an earlier disappearance of the triple helix signal in the event sequence. The lower melting temperature (less thermostable gelatin gel) of the α? fraction thus results from a different conformational cascade of the α? chains upon melting. In all samples the initial conformational changes take place in the β-turns, providing further evidence for the models proposed previously.     

Physicochemical and structural characterization of a polyionic matrix of interest in biotechnology, in the pharmaceutical and biomedical fields

This paper reports on the swelling degree and the rheological and structural characteristics of a hydrogel composed by chitosan and xanthan. The latter is a polyionic hydrogel obtained by complexation between the both polysaccharides. The swelling degree has been found to be influenced by the time of coacervation, the pH of the solution of chitosan used to form the hydrogel and the pH of the swelling solution. The molecular weight and the degree of acetylation of the chitosan also influence the swelling degree of this matrix. The connectivity between chitosan and xanthan affects the swelling degree of this matrix. A rheological study has been carried out in order to understand the formation of the coacervate and of the subsequent hydrogel. The evolution of the storage modulus with time during the coacervation has allowed to optimize the time of coacervation required for a subsequently hydrogel, with desirable swelling degree. The kinetics has shown that (a) the coacervate is formed in two distinct steps and (b) the storage modulus of the hydrogel reaches a stable plateau. The values of the storage modulus have been correlated with the swelling degree. The microscopic characterization has shown the presence of a porous network with a fibrillar structure. To complete the characterization studies fine powder of this hydrogel has been used to determine the surface, perimeter, Feret diameter and sphericity factor distribution of dry and hydrated (swollen) particles.     

Characterization of Fish Gelatin at Nanoscale Using Atomic Force Microscopy

Atomic force microscopy (AFM) was used as a meaningful tool to characterize the nanostructure of gelatin from catfish (Ictalurus punctatus) skin. The gelatins extracted with pretreatments including acid pretreatment, alkaline pretreatment, and alkaline followed by acid pretreatment (optimized extraction conditions). The resulting gelatins were imaged using AFM and their nanostructure was studied. The AFM images showed that gelatin extracted with acid pretreatment had a coacervate structure while with alkaline pretreatment there were separate aggregates. Spherical aggregates and annular pores were observed in AFM images of gelatin with the optimized extraction conditions. AFM imaging of gelatin with a relative high concentration (0.5%) was successfully done and the results help researchers to understand gelatin structures at the nanoscale.     

Transient tropoelastin nanoparticles are early-stage intermediates in the coacervation of human tropoelastin whose aggregation is facilitated by heparan sulfate and heparin decasaccharides

Tropoelastin assembly is a key step in the formation of elastin. We consider how nanoscale intracellular assemblies of tropoelastin can congregate in an extracellular environment to give microscale aggregates. We describe novel 200–300 nm spherical particles that serve as intermediates in the formation of the coacervate. Their aggregation gives 800 nm to 1 µm species. This process is facilitated by heparan sulfate and dermatan sulfate interactions which effectively lower the critical concentration to facilitate this transition. This coacervation process was examined using a panel of heparin chains of various lengths and showed greatest efficacy for the decasaccharide, followed by the octasaccharide, while the hexasaccharide displayed the shortest efficacious length. We propose that these oligosaccharide interactions enable the charge-mediated aggregation of positively charged tropoelastin. This biochemistry models glycosaminoglycan interactions on the cell surface during elastogenesis which is characterized by the clustering of nascent tropoelastin aggregates to form micron-sized spherules.     

Formation of α-synuclein aggregates in aqueous ethylammonium nitrate solutions

The effect of adding ethylammonium nitrate (EAN), which is an ionic liquid (IL), on the aggregate formation of α-synuclein (α-Syn) in aqueous solution has been investigated. FTIR and Raman spectroscopy were used to investigate changes in the secondary structure of α-Syn and in the states of water molecules and EAN. The results presented here show that the addition of EAN to α-Syn causes the formation of an intermolecular β-sheet structure in the following manner: native disordered state → polyproline II (PPII)-helix → intermolecular β-sheet (α-Syn amyloid-like aggregates: α-SynA). Although cations and anions of EAN play roles in masking the charged side chains and PPII-helix-forming ability involved in the formation of α-SynA, water molecules are not directly related to its formation. We conclude that EAN-induced α-Syn amyloid-like aggregates form at hydrophobic associations in the middle of the molecules after masking the charged side chains at the N- and C-terminals of α-Syn.     

Composition and structure of whey protein/gum arabic coacervates

Complex coacervation in whey protein/gum arabic (WP/GA) mixtures was studied as a function of three main key parameters: pH, initial protein to polysaccharide mixing ratio (Pr:Ps)(ini), and ionic strength. Previous studies had already revealed under which conditions a coacervate phase was obtained. This study is aimed at understanding how these parameters influence the phase separation kinetics, the coacervate composition, and the internal coacervate structure. At a defined (Pr:Ps)(ini), an optimum pH of complex coacervation was found (pH(opt)), at which the strength of electrostatic interaction was maximum. For (Pr:Ps)(ini) = 2:1, the phase separation occurred the fastest and the final coacervate volume was the largest at pH(opt) = 4.0. The composition of the coacervate phase was determined after 48 h of phase separation and revealed that, at pH(opt), the coacervate phase was the most concentrated. Varying the (Pr:Ps)(ini) shifted the pH(opt) to higher values when (Pr:Ps)(ini) was increased and to lower values when (Pr:Ps)(ini) was decreased. This phenomenon was due to the level of charge compensation of the WP/GA complexes. Finally, the structure of the coacervate phase was studied with small-angle X-ray scattering (SAXS). SAXS data confirmed that at pH(opt) the coacervate phase was dense and structured. Model calculations revealed that the structure factor of WP induced a peak at Q = 0.7 nm(-1), illustrating that the coacervate phase was more structured, inducing the stronger correlation length of WP molecules. When the pH was changed to more acidic values, the correlation peak faded away, due to a more open structure of the coacervate. A shoulder in the scattering pattern of the coacervates was visible at small Q. This peak was attributed to the presence of residual charges on the GA. The peak intensity was reduced when the strength of interaction was increased, highlighting a greater charge compensation of the polyelectrolyte. Finally, increasing the ionic strength led to a less concentrated, a more heterogeneous, and a less structured coacervate phase, induced by the screening of the electrostatic interactions.     

Rheological interfacial properties of plant protein-arabic gum coacervates at the oil-water interface

This study concerns the interfacial properties of the plant proteins-arabic gum coacervates, which are involved in encapsulation processes based on complex coacervation. The results make it possible to deduce the prerequisite characteristics of the protein, which are involved in the coacervate interfacial properties. The influence of pH and concentration on protein interfacial properties was also studied so as to enable us to predict the best conditions to achieve encapsulation. It has been established that, to obtain a good encapsulation yield, the coacervate must show high surface-active properties and its adsorption on the oil droplets must be favored compared to the free protein adsorption. On the other hand, mechanical properties of the interfacial film made of the coacervate, appear to be a key parameter, as reflected by the dilational viscoelasticity measurements. When compared to the properties of the proteins films, an increase of the rigidity of the interfacial film was shown with the coacervates. It was also observed that viscoelastic properties of the coacervate film were strongly reduced, as well as the associated relaxation times. In acidic conditions, the coacervates containing alpha-gliadin are characterized by an interfacial viscoelastic behavior. This behavior reflects the softness of the interfacial film. This viscoelasticity allows also the formation of a continuous layer around the oil droplets to be encapsulated. Drop tensiometry is shown to be a method that could allow the most adapted protein to be selected and the conditions of the coacervation process to be optimized with regard to concentration and pH.     

Effects of arginine on kinetics of protein aggregation studied by dynamic laser light scattering and tubidimetry techniques

Prevention of undesirable protein aggregation is an extremely important strategy in protein science, medicine, and biotechnology. Arginine is one of the most widely used low molecular weight solution additives effective in suppressing aggregation, assisting refolding of aggregated proteins, and enhancing the solubility of aggregation-prone unfolded molecules in vitro. However, the mechanism of suppression of protein aggregation by arginine is not well understood. To address the mechanism, two model systems have been investigated: protection of alcohol dehydrogenase (ADH) and insulin from heat- and dithiothreitol-induced aggregation, respectively, in the presence of arginine. Using dynamic light scattering (DLS) technique, we have demonstrated the concentration-dependent suppression of light scattering intensity of both ADH and insulin aggregates upon addition of arginine to the incubation medium, a significant effect being revealed in the physiological concentration range of arginine (1-10 mM). DLS studies showed that arginine shifted the populations of nanoparticles with higher hydrodynamic radii to the lower ones, suggesting that the preventive effect of arginine on the protein aggregation process arises because it suppresses intermolecular interactions among aggregation-prone molecules. The results of turbidity measurements were also shown to be consistent with these findings.     

Structural changes and facilitated association of tropoelastin

Circular dichroism studies of tropoelastin secondary structure show 4+/-1% alpha-helix in aqueous solutions. This is in contrast to the substantially higher amounts (up to 23+/-7%) of alpha-helix predicted by computer algorithms, which propose that regions of alpha-helix are limited to the alanine-rich cross-linking domains. Through the addition of trifluoroethanol, the amount of alpha-helix increased to 17+/-1%, equivalent to that expected on the basis of primary structure. The physiological ability of the protein to coacervate and the critical concentration of monomer required for coacervation were unaffected by levels of alpha-helix. However, the temperature required for coacervation decreased linearly with increasing alpha-helical structure, which correlates with the participation of alpha-helices in association. We propose that the alanine-rich cross-linking domains exist as nascent helices in tropoelastin in aqueous solution. We further suggest a novel mechanism for coacervation whereby formation of alpha-helices and subsequent helical side chain interactions limit the conformational flexibility of the polypeptide, to facilitate associations between hydrophobic domains during elastogenesis.     

Coacervate-like microspheres from lysine-rich proteinoid

Microspheres form isothermally from lysine-rich proteinoid when the ionic strength of the solution is increased with NaCl or other salts. Studies with different monovalent anions and with polymers of different amino acid composition indicate that charge neutralization and hydrophobic bonding contribute to microsphere formation. The particles also form in sea water, especially if heated or made slightly alkaline. The microspheres differ from those made from acidic proteinoid but resemble coacervate dtoplets in some ways (isothermal formation, limited stability, stabilization by quinone, uptake of dyes). Because the constituent lysine-rich proteinoid is of simulated prebiotic origin, the study is interpreted to add emphasis to and suggest an evolutionary continuity for coacervation phenomena.     

Formation and Functional Attributes of Canola Protein Isolate—Gum Arabic Electrostatic Complexes

The formation of electrostatic complexes within mixtures of canola protein isolates (CPI) and gum Arabic (GA) was investigated by turbidity during an acid pH titration (7.00–1.50) as a function of mixing ratio (1:1 to 8:1 CPI: GA), and the resulting functional properties (e.g., flow behavior, solubility, foaming and emulsification) of formed complexes were studied. Complexation typically follows two pH-dependent structure forming events associated with the formation of soluble (pH_c) and insoluble complexes (pH_?1). Both pH_c and pH_?1, was found to shift to higher pHs with increasing mixing ratio until reaching a plateau at a 4:1 CPI-GA ratio. Maximum coacervation occurred at pH 4.20 at a ratio of 2:1 CPI-GA, prior to complete dissolution at pH 2.20. The coacervate phase was pseudoplastic in nature, with some evidence of elastic-like behavior associated with a weakly interconnected network or entangled polymer solution. Solubility of CPI and CPI-GA was found to be pH-dependent with minimum solubility occurring at pH 4.00 and 3.00, respectively. Foaming and emulsifying properties of CPI-GA remained unaffected relative to CPI alone, except foaming capacity which was reduced for the mixed system.     

Complex coacervation-controlled release from monoolein cubic phase containing silk fibroin and alginate

pH-dependent release from monoolein (MO) cubic phase was obtained by taking advantage of complex coacervation between hydrophobically modified alginate (HmAL) and hydrophobically modified silk fibroin (HmSF) in the water channels. The degree of coacervation was investigated at pH 3.0 by a light scattering method and the maximum coacervation was observed when the ratio of HmAL to HmSF was 1:15. The degree of coacervation dramatically decreased (from 581.2 to 5.2 nm in size and from 267.9 to 12.3 nm in Kcps) when the pH of medium increased from 3.0 to 5.0. The % release in 100 h of FITC-dextran increased from 2.42 to 7.20% when pH of release medium increased from 3.0 to 9.0. Under acidic conditions, coacervate will block the water channels of cubic phase, suppressing the release. As the pH of release medium increases, the coacervate will dissolve, resulting in a higher release. The cubic phase could be exploited as a pH-sensitive carrier for the oral delivery of an acid-labile drug.     

Complex coacervation of silk fibroin and hyaluronic acid

This study aimed to investigate the pH-induced complexation of silk fibroin (SF) and hyaluronic acid (HA). SF-HA complex coacervation was investigated by monitoring turbidity of the SF-HA system under slow acidification. Gravimetric analysis was performed to determine the yield of complex coacervation and viscosity of the system was measured to study the formation of the complexes at different pH values. The influences of total biopolymer concentration and biopolymer weight ratio on complex coacervation were examined during the analyses. Formation of the complexes was evidenced by the minimum viscosity and the maximum turbidity observed in the system. SF-HA complexes were formed within the pH-window of 2.5-3.5 regardless of the total biopolymer concentration or biopolymer ratio. Complex coacervation of SF-HA showed a reversible behavior and coacervation could be handled even in excess amounts of the biopolymers, which pointed out a non-stoichiometric complexation.     

Functional properties of hemoglobin immobilized in coacervates prepared from gelatin A and polyanionic carbohydrates

Complex coacervation is a phenomenon of phase separation that may occur in a solution of positively and negatively charged polyions. The resulting two phases are distinguished by the total concentration of both polyions, with the concentrated phase often containing vesicular structures composed of the two polyelectrolytes. We have used this phenomenon in an attempt to-prepare a hemoglobin-based red blood cell analog. Hemoglobin-containing coacervate vesicles have been prepared from gelatin A and the polyanionic carbohydrates acacia, pectin, or dextranstilfate. Hemoglobin seems to be anchored into the vesicle walls through interaction of its polyanion binding site with the negatively charged residues on the carbohydrates. Oxygen binding by the immobilized HbA is reversible and cooperative, with p50 values at 20 degrees C of 2.8, 6, and 24 mm Hg for the acacia- (pH 7.5), pectin- (pH 6.6), and dextransulfate-(pH 6.6) derived coacervates. Kinetic studies on CO binding show that the rate of CO uptake by the coacervates (t((1/2)) = 13-27 ms at 0.5 mM CO) is similar to that of human erythrocytes.The HbA-containing coacervates slowly dissolve in isotonic salt solutions (145 mM NaCl, pH 7.4), but they can be stabilized by treatment with glutaraldehyde. Oxygen binding by HbA incorporated into the stabilized coacervates derived from dextran sulfate is very similar to oxy gen binding by human red blood cells: p50 = 26 mm Hg and n = 1.89 at 37 degrees C in isotonic salt. These results show how a novel approach, based on an old concept, has led to the preparation of immobilized HbA, with functional properties similar to those of intraerythrocytic HbA.     

Effect of dense gas CO2 on the coacervation of elastin

In this study for the first time the effect of high-pressure CO2 on the coacervation of alpha-elastin was investigated using analytical techniques including light spectroscopy and attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopic imaging and circular dichroism (CD) spectroscopy. The coacervation behavior of alpha-elastin, a protein biopolymer, was determined at temperatures below 40 degrees C and pressures lower than 180 bar. At these conditions elevated pressures did not disrupt the ability of alpha-elastin to coacervate. It was feasible to monitor the presence of amide I, II, and III bands for alpha-elastin at high-pressure CO2 using ATR-FTIR imaging. At a constant temperature the peak absorption was substantially enhanced by increasing the pressure of the system. CD analysis demonstrated the preservation of secondary structure attributes of alpha-elastin exposed to dense gas CO2 at the pressure range investigated in this study. The lower critical solution temperature of alpha-elastin was dramatically decreased from 37 to 16 degrees C when the CO2 pressure increased from 1 to 50 bar, without a significant change after that. Carbon dioxide at high pressures also impeded the reversible coacervation of alpha-elastin solution. These effects were predominantly associated with the lowered pH of the aqueous solution and maybe the interaction between CO2 and hydrophobic domains of alpha-elastin.