Brownian motions of flexible polymer chains

Locust Bean Gum, a Vegetable Hydrocolloid with Industrial and Biopharmaceutical Applications

Locust bean gum (LBG), a vegetable galactomannan extracted from carob tree seeds, is extensively used in the food industry as a thickening agent (E410). Its molecular conformation in aqueous solutions determines its solubility and rheological performance. LBG is an interesting polysaccharide also because of its synergistic behavior with other biopolymers (xanthan gum, carrageenan, etc.). In addition, this hydrocolloid is easily modified by derivatization or crosslinking. These LBG-related products, besides their applications in the food industry, can be used as encapsulation and drug delivery devices, packaging materials, batteries, and catalyst supports, among other biopharmaceutical and industrial uses. As the new derivatized or crosslinked polymers based on LBG are mainly biodegradable and non-toxic, the use of this polysaccharide (by itself or combined with other biopolymers) will contribute to generating greener products, considering the origin of raw materials used, the modification procedures selected and the final destination of the products.

Viscoelasticity Acts as a Marker for Tumor Extracellular Matrix Characteristics

Biological materials such as extracellular matrix scaffolds, cancer cells, and tissues are often assumed to respond elastically for simplicity; the viscoelastic response is quite commonly ignored. Extracellular matrix mechanics including the viscoelasticity has turned out to be a key feature of cellular behavior and the entire shape and function of healthy and diseased tissues, such as cancer. The interference of cells with their local microenvironment and the interaction among different cell types relies both on the mechanical phenotype of each involved element. However, there is still not yet clearly understood how viscoelasticity alters the functional phenotype of the tumor extracellular matrix environment. Especially the biophysical technologies are still under ongoing improvement and further development. In addition, the effect of matrix mechanics in the progression of cancer is the subject of discussion. Hence, the topic of this review is especially attractive to collect the existing endeavors to characterize the viscoelastic features of tumor extracellular matrices and to briefly highlight the present frontiers in cancer progression and escape of cancers from therapy. Finally, this review article illustrates the importance of the tumor extracellular matrix mechano-phenotype, including the phenomenon viscoelasticity in identifying, characterizing, and treating specific cancer types.

Photoreactive nanogels as versatile polymer networks with tunable in situ drug release kinetics

A simple, yet powerful approach to synthesize photoreactive nanogel networks <5 nm that can swell between ~3 and ~200 times their initial radius with control over the size and surface charge via a solution polymerization reaction protocol was demonstrated. Nanogels with hydrodynamic radii from 0.9 nm to 3.2 nm and surface charges from -6.4 mV to -16.5 mV with dramatically different abilities to swell were synthesized by altering the solvent ratio before synthesis. Additionally, the control over the release kinetics of a small molecule over a period of 30 days was demonstrated by the methacrylate functionalization of the nanogels post-synthesis and the subsequent photo-aggregation of the nanogels. Thepotential to control the release of small molecule drugs via the concentration of photoreactive groups and the photo-induced aggregation of the nanogels offers the unique ability to tailor the in situ release kinetics of the delivery network.

A new nonlinear viscoelastic model and mathematical solution of solids for improving prediction accuracy

We developed an innovative material nonlinear viscoelastic model with physical mechanism and mathematical solution to improve existing ones. The relaxation modulus transits from the glassy stage to the rubbery stage through a time-dependent viscosity in a continuous spectrum considering the nonlinear strain hardening. Experimental results of differential solid materials including asphalt concrete, agarose gel, vaginal tissue, polymer, agar, bone, spider silk, and hydrogel demonstrate that the developed model is superior to generalized Maxwell model or Prony series for more accurate prediction outside of the range for data fitting while using much less model parameters. Numerical simulation results indicate that the new model has improved accuracy. It is stable numerically, and does not reduce computation speed. Therefore, the model may be used to simulate a broad range of viscoelastic solids for predicting experimental data and responses with improved accuracy.

A mathematical model for fitting and predicting relaxation modulus and simulating viscoelastic responses

We propose a mathematical model for relaxation modulus and its numerical solution. The model formula is extended from sigmoidal function considering nonlinear strain hardening. Its physical meaning can be interpreted by a macroscale elastic network-viscous medium model with only five model parameters in a simpler format than the molecular-chain-based polymer models to represent general solid materials. We also developed a finite-element (FE) framework and robust numerical algorithm to implement this model for simulating responses under both static and dynamic loadings. We validated the model through both experimental data and numerical simulations on a variety of materials including asphalt concrete, polymer, spider silk, hydrogel, agar and bone. By satisfying the second law of thermodynamics in the form of Calusius-Duhem inequality, the model is able to simulate creep and sinusoidal deformation as well as energy dissipation. Compared to the Prony series, the widely used model with a large number of model parameters, the proposed model has improved accuracy in fitting experimental data and prediction stability outside of the experimental range with competitive numerical stability and computation speed. We also present simulation results of nonlinear stress-strain relationships of spider silk and hydrogels, and dynamic responses of a multilayer structure.

Time-Resolved Linear Dichroism Measurements of Carbonmonoxy Myoglobin as a Probe of the Microviscosity in Crowded Environments

The distribution of viscosities in living cells is heterogeneous because of the different sizes and natures of macromolecular components. When thinking about protein folding/function processes in such an environment, the relevant (micro)viscosity at the micrometer length scale is necessarily distinguished from the bulk (macro)viscosity. The concentration dependencies of microviscosities are determined by a number of factors, such as electrostatic interactions, van der Waals forces, and excluded volume effects. To explore such factors, the rotational diffusion time of myoglobin in the presence of varying concentrations of macromolecules that differ in molecular weight (dextran 6000, 10 000, and 70 000), shape (dextran versus Ficoll), size, and surface charge is measured with time-resolved linear dichroism spectroscopy. The results of these studies offer simple empirically determined linear and exponential functions useful for predicting microviscosities as a function of concentration for these macromolecular crowders that are typically used to study crowding effects on protein folding. To understand how relevant these microviscosity measurements are to intracellular environments, the TRLD results are discussed in the context of studies that measure viscosity in cells.

Effect of 7S/11S ratio on the network structure of heat-induced soy protein gels: a study of probe release

The effect of 7S/11S ratio on the soy gel network will be uncovered by probe diffusion kinetics.

Release behavior of non-network proteins and its relationship to the structure of heat-induced soy protein gels

Heat-induced soy protein gels were prepared by heating protein solutions at 12%, 15% ,or 18% for 0.5, 1.0, or 2.0 h. The release of non-network proteins from gel slices was conducted in 10 mM pH 7.0 sodium phosphate buffer. SDS-PAGE and diagonal electrophoresis demonstrated that the released proteins consisted of undenatured AB subunits and denatured proteins including monomers of A polypeptides, disulfide bond linked dimers, trimers, and polymers of A polypeptides, and an unidentified 15 kDa protein. SEC-HPLC analysis of non-network proteins revealed three major protein peaks, with molecular weights of approximately 253.9, 44.8, and 9.7 kDa. The experimental data showed that the time-dependent release of the three fractions from soy protein gels fit Fick's second law. An increasing protein concentration or heating time resulted in a decrease in diffusion coefficients of non-network proteins. A power law expression was used to describe the relationship between non-network protein diffusion coefficient and molecular weight, for which the exponent (α) shifted to higher value with an increase in protein concentration or heating time, indicating that a more compact gel structure was formed.

Diffusion and partitioning of macromolecules in casein microgels: evidence for size-dependent attractive interactions in a dense protein system

Understanding the mechanisms that determine the diffusion and interaction of macromolecules (such as proteins and polysaccharides) that disperse through dense media is an important fundamental issue in the development of innovative technological and medical applications. In the current work, the partitioning and diffusion of macromolecules of different sizes (from 4 to 10 nm in diameter) and shapes (linear or spherical) within dispersions of casein micelles (a protein microgel) is studied. The coefficients for diffusion and partition are measured using FRAP (fluorescence recovery after photobleaching) and analyzed with respect to the structural characteristics of the microgel determined by the use of TEM (transmission electron microscopy) tomography. The results show that the casein microgel displays a nonspecific attractive interaction for all macromolecules studied. When the macromolecular probes are spherical, this affinity is clearly size-dependent, with stronger attraction for the larger probes. The current data show that electrostatic effects cannot account for such an attraction. Rather, nonspecific hydration molecular forces appear to explain these results. These findings show how weak nonspecific forces affect the diffusion and partitioning of proteins and polysaccharides in a dense protein environment. These results could be useful to better understand the mechanisms of diffusion and partitioning in other media such as cells and tissues. Furthermore, there arises the possibility of using the casein micelle as a size-selective molecular device.

Probe mobility in native phosphocaseinate suspensions and in a concentrated rennet gel: effects of probe flexibility and size

Pulsed field gradient nuclear magnetic resonance and proton nuclear magnetic resonance relaxometry were used to study the self-diffusion coefficients and molecular dynamics of linear (PEGs) and spherical probes (dendrimers) in native phosphocaseinate suspensions and in a concentrated rennet gel. It was shown that both the size and the shape of the diffusing molecules and the matrix topography affected the diffusion and relaxation rates. In suspensions, both translational and rotational diffusion decreased with increasing casein concentrations due to increased restriction in the freedom of motion. Rotational diffusion was, however, less hindered than translational diffusion. After coagulation, translational diffusion increased but rotational diffusion decreased. Analysis of the T₂ relaxation times obtained for probes of different sizes distinguished the free short-chain relaxation formed from a few monomeric units from (i) the relaxation of protons attached to long polymer chains and (ii) the short-chain relaxation attached to a rigid dendrimer core.

Sorption of water vapor, hydration, and viscosity of carboxymethylhydroxypropyl guar, diutan, and xanthan gums, and their molecular association with and without salts (NaCl, CaCl2, HCOOK, CH3COONa, (NH4)2SO4 and MgSO4) in aqueous solution

Gums are routinely used in food industry, pharmacy and oil recovery process. In these uses, the hydrocolloids very often encounter interactions with salts at moderate to high temperature. Since they are normally employed in the form of solution and gel, their viscous or fluidity properties need detailed investigation. In the present work, properties such as water vapor adsorption of finely powdered carboxymethylhydroxypropyl derivatized guar (CMHPG) as well as xanthan (Xn) and diutan (Dn) gums, their hydration in solution, their viscosity behaviors, and salt effects on fluidity have been studied. The concentration domains for the existence of free and associated molecules in the studied solutions have been assessed from the viscosity results. The gums have been found to bind a fair amount of water from the vapor phase with them. In solution, they can interact and arrest a large amount of water in their folded configuration. Intrinsic viscosities of the gums in aqueous medium declined in the presence of salts. The activation energies for their viscous flow were moderate and comparable, and were dependent on their concentrations. From the power law relation and viscosity master curve behavior mostly two critical association states of the macromolecular dispersions were envisaged.

Rheological characterization of poly(ethylene oxide) solutions of different molecular weights

The rheological properties of aqueous solutions of poly(ethylene oxide) (PEO) of different molecular weights (1x10(5), 4x10(5), 1x10(6) and 4x10(6) g mol(-1)) and concentrations were investigated using shear viscosity and dynamic rheological measurements. It was found that the aqueous solutions of PEO do not exhibit a yield stress and that, above a critical shear rate, all PEO solutions exhibit shear-thinning behavior, well described by the Cross model, except for the solutions made by the lowest molecular weight (1x10(5) g mol(-1)) which were almost Newtonian. The parameters of the Cross model, namely the zero-shear rate viscosity and reciprocal of the time constant, allowed the determination of the critical concentrations c(*) and c(**) (respectively, the transition to semi-dilute network solution and concentrated solution). At concentrations higher than c(**) and below a critical shear rate, solutions made of PEO of high molecular weight exhibited a clearly shear-thickening behavior at very low shear rates. In addition, the dynamic tests showed that PEO solutions exhibit concentration-dependent viscoelastic properties, with a dominant viscous behavior at PEO concentrations lower than c(**) and a dominant elastic behavior at PEO concentrations greater than c(**).

Liposomes: technologies and analytical applications

Liposomes are structurally and functionally some of the most versatile supramolecular assemblies in existence. Since the beginning of active research on lipid vesicles in 1965, the field has progressed enormously and applications are well established in several areas, such as drug and gene delivery. In the analytical sciences, liposomes serve a dual purpose: Either they are analytes, typically in quality-assessment procedures of liposome preparations, or they are functional components in a variety of new analytical systems. Liposome immunoassays, for example, benefit greatly from the amplification provided by encapsulated markers, and nanotube-interconnected liposome networks have emerged as ultrasmall-scale analytical devices. This review provides information about new developments in some of the most actively researched liposome-related topics.

Impact of casein gel microstructure on self-diffusion coefficient of molecular probes measured by 1H PFG-NMR

The translational dynamics of poly(ethylene glycol) (PEG) polymers with molecular weights (Mw) varying from 6x10(2) to 5x10(5) were investigated by pulsed field gradient NMR in casein suspensions and in gels induced by acidification, enzyme action, and a combination of both. For molecules with Mw<or=1020, the diffusion was only dependent on the casein concentration whatever the molecular weight of the probe or the sample studied. However, for PEG with Mw>or=8000, there was strong dependence of diffusion on PEG size and on the casein network structure as revealed by scanning electron microscopy images. The diffusion coefficients of the two largest PEGs were increased after coagulation by amounts that depended on the internal structure of the gel. In addition, the 527,000 g/mol PEG was found to deviate from Gaussian diffusion behavior to greater or lesser extents according to the casein concentration and the sample microstructure. The results are discussed in terms of network rearrangements.

PFG-NMR diffusometry: A tool for investigating the structure and dynamics of noncommercial purified pig gastric mucin in a wide range of concentrations

For the first time, Pulsed Field Gradient-Nuclear Magnetic Resonance, a powerful noninvasive tool for studying the dynamics and structure of complex gels, has been used to measure diffusion of probe molecules in aqueous solutions/gels of noncommercial purified pig gastric mucin (PGM), in a concentration range up to 5 wt %. Complementary data were obtained from rheology measurements. The combination of techniques revealed a strong pH dependency of the structure of the PGM samples while changes in concentration, ionic strength, and temperature appeared to induce less pronounced alterations. Viscosity was found to vary in a nonmonotonous way with pH, with the more viscous solutions found at intermediate pH. We propose that this finding is due to a reduced charge density at lower pH, which is expected to continuously increase the relative importance of hydrophobic associations. The results suggest a loose network of expanded fully charged PGM molecules with considerable mobility at neutral pH (pH 7.4). At intermediate pH (pH 4), a three-dimensional expanded network is favored. At pH 1, the charge density is low and microphase separation occurs since hydrophobic associations prevail. This leads to the formation of clusters concentrated in PGM molecules separated by regions depleted in PGM. The results obtained increase our knowledge about the gastric mucosal layer, which in vivo contains mucin in the same concentration range as that of the samples investigated here.

Effects of ionic strength and denaturation time on polyethyleneglycol self-diffusion in whey protein solutions and gels visualized by nuclear magnetic resonance

Pulsed field gradient NMR spectroscopy was used to determine the poly(ethylene glycol) (PEG) self-diffusion coefficient (D(PEG)) as a function of NaCl concentration (C(NaCl)) and denaturation time (t(D)) in whey protein solutions and gels. D(PEG) in the gel decreased with increasing C(NaCl) concentrations and increased with increasing t(D); the increase ceased for all PEGs when the gel was fixed. This increase was more pronounced for the 82250 g/mol PEG than the 1080 g/mol PEG. Moreover, the diffusion coefficient of nonaggregated whey protein was measured and an increase for longer t(D) was also observed. Scanning electron microscopy images and (1)H spectra demonstrated that D(PEG) were related to the structure changes and to the percentage of beta-lactoglobulin denaturation.

DWS Microrheology of a Linear Polysaccharide

Diffusing wave spectroscopy has been used to measure the rheological behavior of pullulan (M(w) = 1 x 10(5)) aqueous solutions up to concentration of 40 g/dL. It was found that these solutions were mainly viscous, with the loss modulus G'' higher than the elastic modulus G'. The plot of the specific viscosity eta(sp) as a function of pullulan concentration showed two critical concentrations c = 4 g/dL and c = 15 g/dL. For c < c, eta( sp) approximately c(1.25+/-0.05); for c < c < c, eta( sp) approximately c(2+/-0.05); and for c > c, eta( sp) approximately c(4.5+/-0.5). These results are in very good agreement with those reported in the literature.

Probing structural heterogeneities and fluctuations of nucleic acids and denatured proteins

We study protein and nucleic acid structure and dynamics using single-molecule FRET and alternating-laser excitation. Freely diffusing molecules are sorted into subpopulations based on single-molecule signals detected within 100 micros to 1 ms. Distance distributions caused by fluctuations faster than 100 micros are studied within these subpopulations by using time-correlated single-photon counting. Measured distance distributions for dsDNA can be accounted for by considering fluorophore linkers and fluorophore rotational diffusion, except that we find smaller fluctuations for internally labeled dsDNA than DNA with one of the fluorophores positioned at a terminal site. We find that the electrostatic portion of the persistence length of short single-stranded poly(dT) varies approximately as the ionic strength (I) to the -1/2 power (I(-1/2)), and that the average contribution to the contour length per base is 0.40-0.45 nm. We study unfolded chymotrypsin inhibitor 2 (CI2) and unfolded acyl-CoA binding protein (ACBP) even under conditions where folded and unfolded subpopulations coexist (contributions from folded proteins are excluded by using alternating-laser excitation). At lower denaturant concentrations, unfolded CI2 and ACBP are more compact and display larger fluctuations than at higher denaturant concentrations where only unfolded proteins are present. The experimentally measured fluctuations are larger than the fluctuations predicted from a Gaussian chain model or a wormlike chain model. We propose that the larger fluctuations may indicate transient residual structure in the unfolded state.

The GB1 amyloid fibril: recruitment of the peripheral beta-strands of the domain swapped dimer into the polymeric interface

Three-dimensional domain swapping has been evoked as a mechanism for oligomerization of proteins. Here, we show for the immunoglobulin-binding domain B1 of streptococcal protein G (GB1) that fibril formation is observed readily for variants that exist as domain-swapped dimers. No fibril was formed by a revertant that exhibits the stable wild-type GB1 fold or a mutant comprising a highly destabilized, fluctuating ensemble of conformers. Structural features of the GB1 amyloid fibril were characterized by cysteine disulfide cross-linking. Residues in the outer edge beta-strands of the domain-swapped dimer readily form intermolecular disulfide bonds prior to and during fibril formation. On the basis of these data, a structural model for the assembly of domain-swapped dimers into a polymeric structure of the GB1 fibril is proposed.