Macromolecular Solutions Research Articles

Effect of chemical composition on the linear viscoelastic properties of spreadable-type processed cheese

Moisture, fat and protein content were studied as factors likely to contribute to the viscoelastic properties of spreadable-type processed cheese samples obtained from oscillatory shear tests. Elastic and viscous moduli, loss tangent, complex modulus and complex viscosity were determined in a frequency sweep test. Processed cheese samples exhibited differences in their linear viscoelastic behavior due to their different chemical composition. Two distinct groups of samples were observed; processed cheeses that behaved like concentrated macromolecular solutions and processed cheeses that exhibited dilute solution behavior. Prediction models were obtained by multiple regression analysis that describe the relationship between chemical composition of processed cheese and linear viscoelastic properties of the final product. Moisture acted as a plasticizer and fat as a lubricant both contributing to a more liquid-like behavior of the samples. In contrast, proteins reinforced the strength of the three-dimensional matrix leading to processed cheeses with more solid-like behavior.

Rheology of Barium Sulfate Suspensions and Pre-thickened Beverages Used in Diagnosis and Treatment of Dysphagia

Abstract Pre-thickened beverages and barium sulfate suspensions are used in the treatment and diagnosis of dysphagia. These liquids are labeled nectar consistency (NC), honey consistency (HC) etc. These labels are rather misleading and do not represent the actual rheological character of the liquids. We carefully investigated the rheology of these liquids to assist both in their formulation and use for dysphagic patients. Steady state flow properties, thixotropy, dynamic response, and creep recovery behavior were investigated for six beverages and two barium sulfate suspensions. All samples exhibited a shear-thinning behavior. The flow curves of all samples followed both Herschel-Bulkley and Casson models. HC barium sulfate suspension exhibited higher yield stress, σo, and higher storage modulus, G’, than their fluid food counterparts. In contrast, NC barium sulfate suspension had lower σo, and G’ than some of the liquid food counterparts. Frequency spectra of NC samples were similar to that of a macromolecular solution with both G’ and loss modulus, G’’, increasing with frequency; whereas those of HC samples were similar to that of a gel with a little dependency of G’ and G’’ over frequency. Stress sweep experiments showed that the linear viscoelastic region of fluid foods and barium sulfate suspensions extended up to 1 and 10 Pa, respectively. Thus, significant differences exist in the rheological properties of both pre-thickened and videofluoroscopy fluids currently used for diagnosis and treatment of dysphagia.

Automated search-model discovery and preparation for structure solution by molecular replacement

A novel automation pipeline for macromolecular structure solution by molecular replacement is described. There is a special emphasis on the discovery and preparation of a large number of search models, all of which can be passed to the core molecular-replacement programs. For routine molecular-replacement problems, the pipeline automates what a crystallographer might do and its value is simply one of convenience. For more difficult cases, the pipeline aims to discover the particular template structure and model edits required to produce a viable search model and may succeed in finding an efficacious combination that would be missed otherwise. The pipeline is described in detail and a number of examples are given. The examples are chosen to illustrate successes in real crystallography problems and also particular features of the pipeline. It is concluded that exploring a range of search models automatically can be valuable in many cases.

Selective interactions between macro-molecules and two-component solvents

A new method of measuring the refractive index increment of a macromolecular solution while it is at equilibrium with a composite solvent is presented. A synthetic boundary is formed in the ultracentrifuge cell between the solution and composite solvent, which was not dialyzed against the solution. The solvent components equilibrate much faster than the macromolecules, and by proper extrapolation the refractive index increment of the macromolecules is obtained. Refractive index increments of a series of solutions of bovine serum albumin in guanidinium chloride solutions were measured and the amount of preferentially bound guanidinium chloride was calculated.

Accuracy of molecular mass determination of proteins in solution by small-angle X-ray scattering

One of the most important overall parameters, which can be derived from small-angle X-ray scattering (SAXS) experiments on macromolecular solutions is the molecular mass (MM) of the solute. In particular, for a monodisperse protein solution, MM of the solute is calculated from the extrapolated scattering intensity at zero angle I(0). Assessing MM by SAXS provides valuable information about the oligomeric state and absence of unspecific aggregation in solution. The value of MM can either be estimated by comparison with a protein standard with a known MM or by determining the absolute scattering intensity using, e.g., water scattering. In both cases, knowledge about the solute concentration and about the partial specific volume of the protein is required. By measuring 13 well characterized globular proteins with MMs ranging from 13.7 to 669 kDa we analyze the sources of possible systematic deviations and assess the accuracy of MM determination using SAXS. The data indicate that all these proteins have approximately the same `effective' value of the partial specific volume of about 0.7425 cm3 g−1. It is shown that both inter-protein and water calibration can be used for molecular mass determination by SAXS and in most cases the errors do not exceed 10%.

Small-angle scattering studies of macromolecular solutions

In recent years, major progress has been achieved in developing novel approaches to interpret small-angle scattering data from solutions of biological macromolecules in terms of three-dimensional models. These advanced methods include: ab initio low-resolution shape and domain structure determination; modelling of quaternary structure by rigid-body refinement; simultaneous analysis of multiple scattering patterns, e.g. from contrast variation in neutron scattering to study multicomponent complexes; validation of high-resolution models; and addition of missing loops and domains. The new techniques will be presented and practical applications of the methods are illustrated by recent examples. The use of additional information from other methods, joint applications of X-ray and neutron scattering, and the possibilities for assessing and validating the models constructed based on small-angle scattering data will be discussed.

Viscoelastic properties of cross-linked waxy cornstarch and κ-carrageenan mixtures under a wide range of potassium counter-ion concentrations

The small-deformation shear behavior at 25 °C of 1%–4%(mass fraction) cross-linked waxy corn starch (CWCS) and 0.5%(mass fraction) κC mixtures with and without KCl were studied. Dispersions were heated (1.5 °C/min) to 90 °C, held for 10 min, then cooled (1.5 °C/min) to 90 °C. When the volume fraction of CWCS is about 0.5 to 0.7, the rheological behavior is governed by the continuous phase, while above these values the disperse phase dominates the rheological behavior. When carrageenan is in a disordered state without KCl, the swollen granules are dispersed in a macromolecular solution. With KCl, the rigidity of the gels increases by effect of CWCS, carrageenan and salt concentrations. However, salt concentrations above 100 mmol/L lead to a marginal increase in rigidity. Results can be interpreted in terms of two types of systems: particles suspended in a macromolecular solution and composite gels of particles embedded in a network matrix when both κC and KCl were added.

Universal Concentration Dependence of the Soret Coefficient in Aqueous Systems

We present measurements of aqueous low molecular weight mixtures and aqueous macromolecular solutions. The Soret coefficient ST for ethanol, acetone, and DMSO in water was measured by an optical grating technique, and all systems showed a sign change around a molar fraction of water xwater = 0.85 ± 0.05. ST for poly(ethylene oxide), poly(N-isopropylacrylamide), and boehmite rods in ethanol/water mixtures showed also a sign change close to the same concentration. The occurring sign change concentration in the systems will be related to structural reorganizations in the solvent mixture and the Hildebrandt solubility parameter. We will also compare the experimental data within lattice calculations and simulations, which indicate that strong cross-interactions are important for a sign change of the Soret coefficient.

Focusing of macromolecules using equilibrium between centrifugal and electric force

A focusing separation model for macromolecules has been theoretically investigated. The method involves an ultracentrifugation device, which however, deploys an electric field gradient oriented longitudinally along the radial direction. When a macromolecular sample solution is centrifuged, the molecules which have different density to the surrounding solvent and a non-zero electric charge, experience a combination of centrifugal and electric forces. This forces the molecules to move to the equilibrium positions along the radius of the rotor, which are characterised by the charge over mass ratio of the molecules. Therefore a molecular sample will be separated into its constituents and bands will form, akin to electrophoresis. The bands are, however, focused at their equilibrium position. An example configuration has been examined whereby four proteins with masses between 20 and 100 kDa can be separated within a radial distance of 20 cm, for a rotor spinning at approximately 130,000 rpm and with a varying electric field between 0 and 100 V/cm.

On the interpretation of ternary diffusion measurements in low-molecular weight fluids by dynamic light scattering

Dynamic light scattering (DLS) allows for efficient measurement of physical transport properties in fluids. It is now a well established technique for determination of mutual mass diffusion coefficients in binary mixtures and multicomponent macromolecular solutions. More recently, ternary systems of low-molecular weight compounds have been analyzed by DLS where it was found that only one of the two mass diffusion modes can be observed. In this work, a theoretical interpretation of these findings is provided. For this purpose, a simple theory is developed that describes the observed signals. Thereby, it can be shown that both diffusion modes contribute to the signal but one mode is strongly enhanced for situations typically encountered in DLS experiments. The developed theory thus clarifies the relationship between ternary DLS measurements and other diffusion experiments. It provides the practical basis for quantitative analysis of DLS experiments in ternary low-molecular weight fluids.

Atomically Detailed Simulations of Concentrated Protein Solutions: The Effects of Salt, pH, Point Mutations, and Protein Concentration in Simulations of 1000-Molecule Systems

An ability to accurately simulate the dynamic behavior of concentrated macromolecular solutions would be of considerable utility in studies of a wide range of biological systems. With this goal in mind, a Brownian dynamics (BD) simulation method is reported here that allows systems to be modeled that comprise in excess of 1000 protein molecules, all of which are treated in atomic detail. Intermolecular forces are described in the method using an energy function that incorporates electrostatic and hydrophobic interactions and that is calibrated to reproduce experimental thermodynamic information with a single adjustable parameter. Using the method, BD simulations have been performed over a wide range of pH and ionic strengths for three proteins: hen egg white lysozyme (HEWL), chymotrypsinogen, and T4 lysozyme. The simulations reproduce experimental trends in second virial coefficients (B(22)) and translational diffusion coefficients, correctly capture changes in B(22) values due to single amino acid substitutions, and reveal a new explanation for the difficulties reported previously in the literature in reproducing B(22) values for protein solutions of very low ionic strength. In addition, a strong correlation is found between a residue's probability of being involved in a protein-protein contact in the simulations and its probability of being involved in an experimental crystal contact. Finally, exploratory simulations of HEWL indicate that the simulation model also gives a promising description of behavior at very high protein concentrations (approximately 250 g/L), suggesting that it may provide a suitable computational framework for modeling the complex behavior exhibited by macromolecules in cellular conditions.

The effect of the presence of globular proteins and elongated polymers on enzyme activity

We have studied the effect of a crowded (macromolecular) solution on reaction rates of the decarboxylating enzymes urease, pyruvate decarboxylase and glutamate decarboxylase. A variety of crowding agents were used including haemoglobin, lysozyme, various dextrans and polyethylene glycol. Enzyme reaction rates of all three enzymes show two different types of effect that separate the globular proteins from the polysaccharides/polymers. Increasing concentration of globular proteins caused a dramatic rise in enzyme activity up to 30% crowding concentration then the activity decreased, whereas the polymers caused a concentration dependent decrease in activity. The viscosities of the globular proteins were low compared to the polymers. The increased activity with proteins may be due to the decreased amount of free water, which leads to higher effective concentration of substrates, or to an increased oligomeric state by self-association. The lower activities of all enzymes with all agents at high concentrations may be explained by a decrease in rates of diffusion. An increase in protein crowding (decrease in cell water content) may contribute to pathological conditions including cataract and Alzheimer's disease.

Enhancement of membrane permeability by gas-sparging in submerged hollow fibre ultrafiltration of macromolecular solutions: Role of module design

Permeability in membrane filtration processes suffers from two major limiting factors: concentration polarization and membrane fouling. Gas-sparging, which involves bubbling of a gas in close proximity of a membrane, is known to minimise both of these. Gas-sparged membrane filtration is carried out either by pressurising a gas-sparged feed side or by using suction to draw the permeate through a membrane from the un-pressurised, gas-sparged feed side. The first approach is mainly used in ultrafiltration processes. The second approach which is easier to implement and is widely used in microfiltration processes. This paper discusses the enhancement of permeability by gas-sparging in suction-driven, submerged hollow fibre ultrafiltration using two different membrane module types. These modules were prepared using hollow fibre membranes having nominal MWCO of 150 kDa and were used to ultrafilter polysaccharide solutions. Depending on the operating conditions and on the module design, gas-sparging enhanced effective hydraulic permeability by as much as 115%. The extent of membrane fouling was also significantly lower in the gas-sparged mode. The effectiveness of gas-sparging was found to be greater with one membrane module type, clearly highlighting the effect of module design on process efficiency.

Impact of Glucose in Peritoneal Dialysis: Saint or Sinner?

Peritoneal dialysis (PD) solutions using glucose as osmotic agent have been used for more than two decades as effective treatment for patients with end-stage renal disease. Although alternative osmotic agents such as amino acids and macromolecular solutions, including polypeptides and glucose polymers, are now available, glucose is still the most widely used osmotic agent in PD. It has been shown to be safe, effective, readily metabolized, and inexpensive. On the other hand, it is widely assumed that exposure of the peritoneal membrane to high glucose concentrations contributes to both structural and functional changes in the dialyzed peritoneal membrane. As in diabetes, glucose, either directly or indirectly through the generation of glucose degradation products or the formation of advanced glycation end products, may contribute to peritoneal membrane failure. Although efforts to reduce glucose toxicity have been made for years, only a few suggestions, such as dual-bag systems with bicarbonate as buffer system, have found broader acceptance. Recently, some interesting new approaches to the problem of glucose-related toxicity have been made, but further investigations will be necessary before they can be used clinically. This review will focus on adverse effects of glucose in PD solutions and summarize different aspects of glucotoxicity and potential therapeutic interventions.

Unconventionally Modeling Analysis of Permeate Flux for Ultrafiltration in Hollow-Fiber Modules

Membrane ultrafiltration of macromolecular solutions is usually analyzed with the gel polarization model, the osmotic pressure model, and the resistance-in-series model. The disadvantages of these conventional models are, respectively, the limited range of application, the mathematical difficulty in treatment and the incorrect definition of the growth of the concentration polarization resistance. In the present work, however, the permeate flux was analyzed with the exponential model, hyperbolic-tangent model, and error-function model. The development of these models is based on three essential ultrafiltration behaviors, and the exponential-function and hyperbolic-tangent models correlate well with the experimental results for polyvinyl alcohol (PVA) aqueous solution.

Automatic structure determination based on the single-wavelength anomalous diffraction technique away from an absorption edge

The phasing of macromolecular structures based on the use of the single-wavelength anomalous diffraction method has recently enjoyed a revival. Here, additional evidence is provided that the method may be successfully applied at wavelengths remote from the absorption edge of interest and that it is in principle applicable to a large number of systems. This opens up the possibility of rapid and reliable automatic de novo structure determination using simple experimental configurations with no need for wavelength tunability or absorption-edge scanning. The method should therefore be exploitable at most synchrotron beamlines. The effects of data completeness and multiplicity on the quality of the phases obtained are discussed as are the prospects for the automation of macromolecular structure solution based on the experimental protocols described.

Comparative studies on solution characteristics of mannuronan epimerized by C-5 epimerases

Both dilute and concentrated solutions of bacterial Mannuronan (MANNA) and its epimerized products by AlgE1 at 5 and 24 h, named MANNAEp1t5h and MANNAEp1t24h, respectively, and AlgE4 (MANNAEp4) have been studied as a function of variables such as polymer concentration and ionic strength (NaCl) in order to investigate the macromolecular solution properties of these innovative polyuronic acids having the same charge density but different composition and sequence of β- d-mannuronic acid (-M-), α- l-guluronic acid (-G-) or MG-blocks. Measurements of intrinsic viscosity [ η] as a function of ionic strength, I, by capillary viscometry has led to an estimate of the Smidsrød-Haug parameter B, an index useful to characterize the stiffness of polymeric chains. The results are largely consistent with much of the published data relative to chain extension and conformational freedom around the torsional angles of the glycosidic linkages occurring in alginates. Steady shear rheometry provided information about the coil-overlapping parameter c*, which marks the transition from dilute to concentrated solution. The slopes of the double logarithmic plots of η sp vs. c[ η] both at low and high degrees of coil overlap suggest that all samples solutions behave like linear polymer entangled network systems. The value of c* is strictly influenced by the stiffness of the chains, and hence by the primary structure. Dynamic shear rheometry shows that the frequency dependence of dynamic viscosity is only partially superimposable to the shear rate dependence of viscosity. Such behaviour may be ascribed to the presence of semiflexible polymeric coils in a non-totally destructured entangled state. By solvent/non-solvent (H 2O/isopropanol) fractionation carried out on mannuronan, a set of samples with different average molecular weights and narrow polydispersities were obtained. Triple detection GPC allowed the evalutation of the Mark–Houwink–Sakurada parameters as well as of the characteristic ratio C ∞ for one of the fractionated MANNA samples. The chain persistence length was estimated by the wormlike chain model.

Addendum

In the paper “CRANK: New Methods for Automated Macromolecular Crystal Structure Solution” by Ness et al. (2004, Structure 12, 1753–1761), crystallographic programs were mentioned, but not included in the reference list. These programs were ARP/wARP (Perrakis et al. 1999Perrakis A. Morris R. Lamzin V.S. Automated protein model building combined with iterative structure refinement.Nature Struct. Biol. 1999; 6: 458-463Crossref PubMed Scopus (2563) Google Scholar), REFMAC (Murshudov et al. 1997Murshudov G.N. Vagin A.A. Dodson E.J. Refinement of macromolecular structures by the maximum-likelihood method.Acta Crystallogr. 1997; D 53: 240-255Google Scholar), DM (Cowtan 1994Cowtan K. dm an automated procedure for phase improvement by density modification.Joint CCP4 and ESF-EACBM Newsletter on Protein Crystallography. 1994; 31: 34-38Google Scholar), SHELXE (Sheldrick 2002Sheldrick G.M. Macromolecular phasing with SHELXE.Z. Kristallogr. 2002; 217: 644-650Crossref Scopus (360) Google Scholar), and SOLOMON (Abrahams and Leslie 1996Abrahams J.P. Leslie A.G.W. Methods used in the structure determination of bovine mitochondrial F1 ATPase.Acta Crystallogr. 1996; D 52: 30-42Google Scholar).

Mass transfer in microfiltration with laminar and turbulent flow of macromolecular solutions

An experimental study was made on the microfiltration of macromolecular solutions of guar and xanthan gums. The transmembrane flux was investigated using a laboratory unit equipped with a tubular ceramic membrane with a pore size of 0.6 μm. Experiments were conducted with transmembrane pressures of 300 kPa, 400 kPa and 500 kPa and with tangential flow rates corresponding to the laminar and turbulent flow, respectively, in the Reynolds generalized number interval: 440–1000 (guar gum solution) and 3800–12000 (xanthan gum solution). Two gum solutions showed a polarization layer during microfiltration and the solutions’ rheological behavior remained stable throughout the operation of the membrane and experimental pumping system. The analysis dimensionless is presented for correlations in the form: ( J / u ) 1 / 3 in function of Δ P tm/ ρJ 2, and the results related to the Sherwood number were found to be compatible with the literature.

POOR NUTRITIONAL STATUS AND INFLAMMATION: Metabolic Consequences of Peritoneal Dialysis

ABSTRACTOptimization of the peritoneal dialysis (PD) prescription includes attempts to normalize the patient's blood pressure and extracellular volume. To do so, one must utilize crystalloid or colloid osmotic agents to achieve ultrafiltration. These osmotic agents are systemically absorbed and thus have both potential benefits and adverse effects. With glucose‐based dialysate solutions, the average patient absorbs 300–450 kcal of glucose per day on either continuous ambulatory peritoneal dialysis (CAPD) or the cycler. The amount of glucose absorbed varies based on peritoneal transport characteristics, prescription, and tonicity of fluids used. Alternative osmotic agents such as amino acids and macromolecular solutions, including polypeptides and polyglucose (icodextrin) solutions, have a different rate of systemic absorption and thus a different caloric load profile. In addition, there are protein losses that average about 10 g/day with glucose‐based solutions and glucose losses with either amino acid or icodextrin dialysate solutions. There are also potential advantages of these alternative solutions with regard to ultrafiltration. Glucose‐based solutions require the development of significant crystalloid osmotic forces, which are dissipated as glucose is absorbed systemically. In contrast, macromolecular solutions achieve ultrafiltration via differences in colloid osmotic pressure, and the absorption of these agents is of a lesser magnitude than glucose‐based solutions. Further research is needed to determine other potential risks and benefits of these alternative dialysate solutions.