Hydrophobic domains of human tropoelastin interact in a context-dependent manner

Tropoelastin is the soluble precursor of elastin, the major component of the extracellular elastic fiber. Tropoelastin undergoes self-association via an inverse temperature transition termed coacervation, which is a crucial step in elastogenesis. Coacervation of tropoelastin takes place through multiple intermolecular interactions of its hydrophobic domains. Previous work has implicated those hydrophobic domains located near the center of the polypeptide as playing a dominant role in coacervation. Short constructs of domains 18, 20, 24, and a mutated form of domain 26 were largely disordered at 20 degrees C but displayed increased order on heating that was consistent with the formation of beta-structures. However, their conformational transitions were not sensitive to physiological temperature in contrast to the observed behavior of the native domain 26. A polypeptide consisting of domains 17-27 of tropoelastin coacervated at temperatures above 60 degrees C, whereas individually expressed hydrophobic regions were not capable of coacervation. We conclude that coacervation depends on the hydrophobicity of the molecule and, by inference, the number of hydrophobic domains. Tropoelastin mutants were constructed to contain a Pro --> Ala mutation in domain 26, separate deletions of domains 18 and 26, and a displacement of domain 26. These constructs displayed unequal capacities for coacervation, even when they contained the same number of hydrophobic regions and comparable levels of secondary structure. Thus, the capability for coacervation is determined by contributions from individual hydrophobic domains for which function should be considered in the context of their positions in the intact tropoelastin molecule.     

Probing the three-dimensional structure of human calreticulin

Calreticulin (CRT) is an abundant soluble protein of the endoplasmic reticulum lumen that functions as a molecular chaperone for nascent glycoproteins. We have probed the three-dimensional structure of human CRT using a series of biochemical and biophysical approaches in an effort to understand the molecular basis of its chaperone function. Sedimentation analysis and chemical cross-linking experiments showed that CRT is monodisperse and monomeric in solution with a molecular mass (MW) of 46 +/- 1 kDa. This MW value together with a sedimentation coefficient, s(o)(20,w), of 2.71 S yielded a frictional ratio, f/f(0), of 1.65. Assuming CRT to be a prolate ellipsoid, we calculated an apparent length of 29.8 nm and diameter of 2.44 nm consistent with an asymmetric elongated molecule. These hydrodynamic dimensions account for the apparent anomalous elution position of CRT on gel filtration columns. Far-UV circular dichroism experiments showed that CRT has a cooperative thermal denaturation transition with a midpoint temperature of 42.5 degrees C suggesting a marginally stable structure. Proteolysis experiments showed that the highly acidic segment at the C-terminus of CRT is most susceptible to digest, consistent with the absence of a well-defined polypeptide backbone structure in this region of the protein. Temperature-dependent proteolysis with thermolysin revealed a stable core region within the N- and P-domains. A stable fragment encompassing most of the P-domain was also identified in the thermolytic mixture. Collectively, our results suggest that CRT is likely to be a flexible molecule in solution which may be important for its chaperone function.     

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.     

Thermal stability of molecular forms of acetylcholinesterase from muscle microsomes

To establish if the predominant form of acetylcholinesterase in muscle microsomes (4.8S) corresponded to the monomeric or dimeric form of the enzyme we studied the sensitivity to heating of Triton X-100 solubilized extract and that of 4.8S, 10-11S and 13.5S species of the enzyme. Inactivation of soluble acetylcholinesterase began at 45-47 degrees C and was almost complete at 60 degrees C. Sedimentation analysis revealed that the partial loss of activity was due to inactivation of the 4.8S form, although by heating the 13.5S was converted into the 10S enzyme. Inactivation of the 4.8S form began at 45 degrees C, whereas the larger forms required higher temperature. The 4.8S component follows a time course of inactivation which could be fitted by a double exponential equation (when heated at 52 degrees C, almost 83% of the activity showed a short half-life). The 10-11S species was also inactivated following a two step process while the 13.5S enzyme was fairly stable at 52 degrees C. The results show that the lightest component behaves as a monomeric form of acetylcholinesterase.     

Denaturation of thymidylate synthetase from amethopterin-resistant Lactobacillus casei.

The effects of various concentrations of urea and guanidine hydrochloride on enzyme activity and on subunit association were determined. Incubation of thymidylate synthetase with buffered solutions of 3M to 3.5M guanidine hydrochloride or 5 M to 6 M urea resulted in the loss of about 90% of the enzyme activity. Under these denaturing conditions a red shift of the fluorescence emission maximum from 340 nm to 351 nm was observed together with a significant decrease in the relative fluorescence intensity of the protein. Studies at both 4 degrees C and 25 degrees C indicated that the enzyme was in the dimer form in 2 M guanidine hydrochloride but was dissociated into monomers in concentrations of this denaturant of 3 M and above. Although only monomeric species were evident at 4 degrees C in 6 M urea, at 25 25 degrees C this denaturant caused protein aggregation which increased with decreasing phosphate buffer concentration. Enzyme (5 mg/ml) in 0.5 M potassium phosphate buffer, pH 6.8, containing 4 M guanidine hydrochloride gave a minimum S20, w value of 1.22S at 25 degrees C. Sedimentation behavior of the native enzyme in the range of 5 to 20 mg/ml was only slightly concentration-dependent (4.28 S to 4.86 S) but extensive aggregation occurred above 20 mg/ml.     

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.     

Fibrillins, fibulins, and matrix-associated glycoprotein modulate the kinetics and morphology of in vitro self-assembly of a recombinant elastin-like polypeptide

Elastin is the polymeric protein responsible for the properties of extensibility and elastic recoil of the extracellular matrix in a variety of tissues. Although proper assembly of the elastic matrix is crucial for its durability, the process by which this assembly takes place is not well-understood. Recent data suggest the complex interaction of tropoelastin, the monomeric form of elastin, with a number of other elastic matrix-associated proteins, including fibrillins, fibulins, and matrix-associated glycoprotein (MAGP), is important to achieve the proper architecture of the elastic matrix. At the same time, it is becoming clear that self-assembly properties intrinsic to tropoelastin itself, reflected in a temperature-induced phase separation known as coacervation, are also important in this assembly process. In this study, using a well-characterized elastin-like polypeptide that mimics the self-assembly properties of full-length tropoelastin, the process of self-assembly is deconstructed into "coacervation" and "maturation" stages that can be distinguished kinetically by different parameters. Members of the fibrillin, fibulin, and MAGP families of proteins are shown to profoundly affect both the kinetics of self-assembly and the morphology of the maturing coacervate, restricting the growth of coacervate droplets and, in some cases, causing clustering of droplets into fibrillar structures.     

Co-reconstitution of phospholamban mutants with the Ca-ATPase reveals dependence of inhibitory function on phospholamban structure

Phospholamban (PLB), a 52-amino acid integral membrane protein, regulates the Ca-ATPase (calcium pump) in cardiac sarcoplasmic reticulum through PLB phosphorylation mediated by beta-adrenergic stimulation. Based on site-directed mutagenesis and coexpression with Ca-ATPase (SERCA2a) in Sf21 insect cells or in HEK 293 cells, and on spin label detection of PLB oligomeric state in lipid bilayers, it has been proposed that the monomeric form of PLB is the inhibitory species, and depolymerization of PLB is essential for its regulatory function. Here we have studied the relationship between PLB oligomeric state and function by in vitro co-reconstitution of PLB and its mutants with purified Ca-ATPase. We compared wild type-PLB (wt-PLB), which is primarily a pentamer on SDS-polyacrylamide gel electrophoresis (PAGE) at 25 degrees C, with two of its mutants, C41L-PLB and L37A-PLB, that are primarily tetramer and monomer, respectively. We found that the monomeric mutant L37A-PLB is a more potent inhibitor than wt-PLB, supporting the previous proposal that PLB monomer is the inhibitory species. On the other hand, C41L-PLB, which has a monomeric fraction comparable to that of wt-PLB on SDS-PAGE at 25 degrees C, has no inhibitory activity when assayed at 25 degrees C. However, at 37 degrees C, a 3-fold increase in the monomeric fraction of C41L-PLB on SDS-PAGE resulted in inhibitory activity comparable to that of wt-PLB. Upon increasing the temperature from 25 to 37 degrees C, no change in fraction monomer or inhibitory activity for wt-PLB and L37A-PLB was observed. Based on these results, the extent of inhibition of Ca-ATPase by PLB or its mutants appears to depend not only on the propensity of PLB to dissociate into monomers but also on the relative potency of the particular PLB monomer when interacting with the Ca-ATPase.     

Characterization of in vitro interactions between a truncated TonB protein from Escherichia coli and the outer membrane receptors FhuA and FepA

High-affinity iron uptake in gram-negative bacteria depends upon TonB, a protein which couples the proton motive force in the cytoplasmic membrane to iron chelate receptors in the outer membrane. To advance studies on TonB structure and function, we expressed a recombinant form of Escherichia coli TonB lacking the N-terminal cytoplasmic membrane anchor. This protein (H(6)-'TonB; M(r), 24,880) was isolated in a soluble fraction of lysed cells and was purified by virtue of a hexahistidine tag located at its N terminus. Sedimentation experiments indicated that the H(6)-'TonB preparation was almost monodisperse and the protein was essentially monomeric. The value found for the Stokes radius (3.8 nm) is in good agreement with the value calculated by size exclusion chromatography. The frictional ratio (2.0) suggested that H(6)-'TonB adopts a highly asymmetrical form with an axial ratio of 15. H(6)-'TonB captured both the ferrichrome-iron receptor FhuA and the ferric enterobactin receptor FepA from detergent-solubilized outer membranes in vitro. Capture was enhanced by preincubation of the receptors with their cognate ligands. Cross-linking assays with the purified proteins in vitro demonstrated that there was preferential interaction between TonB and ligand-loaded FhuA. Purified H(6)-'TonB was found to be stable and thus shows promise for high-resolution structural studies.     

Aggregation of granulocyte colony stimulating factor under physiological conditions: characterization and thermodynamic inhibition

We have investigated the aggregation of recombinant human granulocyte colony stimulating factor (rhGCSF), a protein that rapidly aggregates and precipitates at pH 6.9 and 37 degrees C. We observed that native monomeric rhGCSF reversibly forms a dimer under physiological conditions and that this dimeric species does not participate in the irreversible aggregation process. Sucrose, a thermodynamic stabilizer, inhibits the aggregation of rhGCSF. We postulate that sucrose acts by reducing the concentration of structurally expanded species, consistent with the hypothesis that preferential exclusion favors most compact species in the native state ensemble. Thermodynamic stability data from unfolding curves and hydrogen-deuterium exchange experimental results support the above hypothesis. Thus, the strategy of stabilizing the native state of the protein under physiological conditions using thermodynamic stabilizers, especially ligands binding with high affinity to the native state, is expected to protect against protein aggregation occurring under such nonperturbing solution conditions.     

Hydrodynamic examination of the dimeric cytoplasmic domain of the human erythrocyte anion transporter, band 3.

Solution studies of the cytoplasmic domain (molecular mass approximately 40kDa) of band 3, the anion exchanger from human erythrocyte membranes, previously suggested a dimeric molecule on the basis of the relative techniques of calibrated gel filtration and calibrated preparative ultracentrifugation. This dimeric behavior is firmly established on an absolute basis by a combination of calibrated gel chromatography and absolute ultracentrifugation techniques. Sedimentation velocity in the analytical ultracentrifuge combined with calibrated gel chromatography give a molecular mass M of (77 +/- 4) kDa, a value confirmed by low-speed sedimentation equilibrium. Velocity sedimentation in the analytical ultracentrifuge gave a single sedimenting species with an s o 20,w of (3.74 +/- 0.07)S. Sedimentation equilibrium analysis was also used to establish the strength of the binding via the dissociation constant Kd, with a value from direct fitting of the concentration distribution curves of (2.8 +/- 0.5) microM, confirmed by a value of approximately 3 microM obtained from fitting a plot of molecular weight Mw,app versus cell loading concentration. Hydrodynamic calculations based on the classical translational frictional ratio showed that the protein was highly asymmetric, with an axial ratio of approximately 10:1, consistent with observations from electron microscopy.     

Effect of thermal and chemical denaturants on Thermoanaerobacter ethanolicus secondary-alcohol dehydrogenase stability and activity

Thermoanaerobacter ethanolicus 39E secondary-alcohol dehydrogenase (2 degrees ADH) was optimally active near 90 degrees C displaying thermostability half-lives of 1.2 days, 1.7 h, 19 min, 9.0 min, and 1.3 min at 80 degrees C, 90 degrees C, 92 degrees C, 95 degrees C, and 99 degrees C, respectively. Enzyme activity loss upon heating (90-100 degrees C) was accompanied by precipitation, but the soluble enzyme remaining after partial inactivation retained complete activity. Enzyme thermoinactivation was modeled by a pseudo-first order rate equation suggesting that the rate determining step was unimolecular with respect to protein and thermoinactivation preceded aggregation. The apparent 2 degrees ADH melting temperature (T(m)) occurred at approximately 115 degrees C, 20 degrees C higher than the temperature for maximal activity, suggesting that it is completely folded in its active temperature range. Thermodynamic calculations indicated that the active folded structure of the 2 degrees ADH is stabilized by a relatively small Gibbs energy (triangle upG(stab.)(double dagger) = 110 kJ mol(-1)). 2 degrees ADH catalytic activities at 37 degrees C to 75 degrees C, were 2-fold enhanced by guanidine hydrochloride (GuHCl) concentrations between 120 mM and 190 mM. These results demonstrate the extreme resistance of this thermophilic 2 degrees ADH to thermal or chemical denaturation; and suggest increased temperature or GuHCl levels seem to enhance protein fixability and activity.     

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.     

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.     

A characterization of the lumenal region of human tapasin reveals the presence of two structural domains

Tapasin is a type I membrane glycoprotein involved with other accessory proteins in the assembly of class I MHC-beta(2)m-peptide complexes in the endoplasmic reticulum. We have probed the three-dimensional structure of the lumenal region of human tapasin (residues 1-392) tagged with a (His)(6) sequence at its C-terminus using biochemical and biophysical techniques. The far-UV circular dichroism spectrum revealed that tapasin possesses well-defined secondary structural elements corresponding predominantly to beta-sheets. A thermal denaturation curve recorded at 216 nm showed a midpoint transition centered at approximately 45 degrees C. Sedimentation analysis showed that tapasin is monomeric in solution with a sedimentation coefficient, S degrees (20,w), of 2.68 S. This value of S degrees (20,w) combined with the value of the molar mass obtained by MALDI mass spectrometry (44.2 kDa) yielded a frictional ratio, f/f(0), of 1.47. Assuming tapasin is a prolate ellipsoid, we calculated an apparent length of 22.5 nm and a diameter of 2.62 nm, consistent with an elongated molecular shape. Controlled proteolysis using various enzymes revealed that a narrow region of tapasin near residue 90 is highly susceptible to digestion, resulting in two fragments that are resistant to further cleavage. The identity of these fragments was determined by amino acid sequencing and MALDI mass spectrometry and revealed a 9 kDa N-terminal fragment and a 34 kDa C-terminal fragment. Collectively, these results suggest that tapasin is comprised of two core domains of different sizes loosely linked by a flexible region.     

Improving solubility of Shewanella oneidensis MR-1 and Clostridium thermocellum JW-20 proteins expressed into Esherichia coli

Low solubility of proteins overexpressed in E. coli is a frequent problem in high-throughput structural genomics. To improve solubility of proteins from mesophilic Shewanella oneidensis MR-1 and thermophilic Clostridium thermocellum JW20, an approach was attempted that included a fusion of the target protein to a maltose-binding protein (MBP) and a decrease of induction temperature. The MBP was selected as the most efficient solubilizing carrier when compared to a glutathione S-transferase and a Nus A protein. A tobacco etch virus (TEV) protease recognition site was introduced between fused proteins using a double polymerase-chain reaction and four primers. In this way, 79 S. oneidensis proteins have been expressed in one case with an N-terminal 30-residue tag and in another case as a fusion protein with MBP. A foreign tag might significantly affect the properties of the target polypeptide. At 37 degrees C and 18 degrees C induction temperatures, only 5 and 17 tagged proteins were soluble, respectively. In fusion with MBP 4, 34, and 38 proteins were soluble upon induction at 37 degrees, 28 degrees, and 18 degrees C, respectively. The MBP is assumed to increase stability and solubility of a target protein by changing both the mechanism and the cooperativity of folding/unfolding. The 66 C. thermocellum proteins were expressed as fusion proteins with MBP. Induction at 37 degrees, 28 degrees, and 18 degrees C produced 34, 57, and 60 soluble proteins, respectively. The higher solubility of C. thermocellum proteins in comparison with the S. oneidensis proteins under similar conditions of induction correlates with the thermophilicity of the host. The two-factor Wilkinson-Harrison statistical model was used to identify soluble and insoluble proteins. Theoretical and experimental data showed good agreement for S. oneidensis proteins; however, the model failed to identify soluble/insoluble Clostridium proteins. A suggestion has been made that the Wilkinson-Harrison model is not applicable to C. thermocellum proteins because it did not account for the peculiarities of protein sequences from thermophiles.     

Analytical ultracentrifugation studies of translin: analysis of protein-DNA interactions using a single-stranded fluorogenic oligonucleotide.

Translin is a recently identified nucleic acid binding protein that appears to be involved in the recognition of conserved sequences found at many chromosomal breakpoints. Previous reports indicate that, based on gel filtration analysis and electron microscopy of protein-DNA complexes, translin forms an octameric structure that binds the DNA. In this study, we further examine the possibility of self-association of translin and its interactions with DNA by analytical ultracentrifugation. Sedimentation velocity analysis of translin indicates that the predominant species sediments with a sedimentation coefficient of 8.5 S and has a frictional ratio, f/f(omicron), of 1.35; these data are consistent with the presence of an octamer with an ellipsoidal configuration; a small amount of a component with significantly higher mass is also present. Equilibrium sedimentation studies of translin at three different protein concentrations also indicate that the predominant species present is an octamer with a minor fraction of aggregated species. Neither monomer nor dimer was detected. Sedimentation equilibrium studies of translin with an FITC-labeled single-stranded oligonucleotide were performed to examine the interaction. A novel analysis method has been developed to analyze protein-nucleic acid interactions based on global fitting of scans of 280 and 490 nm to appropriate mathematical models. Utilizing this method, it was determined that the DNA binding species of translin is an octamer binding a single-stranded oligonucleotide with a DeltaG degrees value of -9.49 +/- 0.12 kcal/mol, corresponding to a dissociation constant, K(d), of 84 +/- 17 nM. On the basis of this evidence and electron microscopy, it is envisioned that translin forms an annular structure of eight subunits, hydrodynamically an oblate ellipsoid, which binds DNA at chromosomal breakpoints.     

The hydrophobic domain 26 of human tropoelastin is unstructured in solution

Elastin is the protein responsible for the elastic properties of vertebrate tissue. Very little is currently known about the structure of elastin or of its soluble precursor tropoelastin. We have used high-resolution solution NMR methods to probe the conformational preferences of a conserved hydrophobic region in tropoelastin, domain 26 (D26). Using a combination of homonuclear, 15N-separated and triple resonance experiments, we have obtained essentially full chemical shift assignments for D26 at 278K. An analysis of secondary chemical shift changes, as well as NOE and 15N relaxation data, leads us to conclude that this domain is essentially unstructured in solution and does not interact with intact tropoelastin. D26 does not display exposed hydrophobic clusters, as expected for a fully unfolded protein and commensurate with an absence of flexible structural motifs, as identified by lack of binding of the fluorescent probe 4,4'-dianilino-1,1'-binaphthyl-5,5'-disulfonic acid. Sedimentation equilibrium data establish that this domain is strictly monomeric in solution. NMR spectra recorded at 278 and 308K indicate that no significant structural changes occur for this domain over the temperature range 278-308K, in contrast to the characteristic coacervation behavior that is observed for the full-length protein.     

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.