Biomedical subjects

Taco Nicolai

Publications and source records attributed to Taco Nicolai.

14 recordsLinked to original sources

PubMed2006

Phase separation and percolation of reversibly aggregating spheres with a square-well attraction potential.

Reversible aggregation of spheres is simulated using a novel method in which clusters of bound spheres diffuse collectively with a diffusion coefficient proportional to their radius. It is shown that the equilibrium state is the same as with other simulation techniques, but with the present method more realistic kinetics are obtained. The behavior as a function of volume fraction and interaction strength was tested for two different attraction ranges. The binodal and the percolation threshold were determined. The cluster structure and size distribution close to the percolation threshold were found to be consistent with the percolation model. Close to the binodal phase separation occurred through the growth of spherical dense domains, while for deep quenches a system spanning network is formed that coarsens with a rate that decreases with increasing attraction. We found no indication for arrest of the coarsening.

Computer Simulation↗

PubMed2006

Coupling between polysaccharide gelation and micro-phase separation of globular protein clusters.

The effect of gelation of the polysaccharide phase on the phase separation was investigated for mixtures of anionic polysaccharide (kappa-carrageenan) and globular protein (beta-lactoglobulin) clusters at pH 7 well above the iso-electric point. Gelation of kappa-carrageenan was induced by cooling in the presence of KCl. In the liquid state the protein clusters phase-separate into relatively dense micro-domains. When the polysaccharide phase gelled during cooling, the turbidity of the systems decreased dramatically. Light scattering experiments showed that the density of the micro-domains decreased, while microscopy showed that the number and size was not strongly modified. It is concluded that smaller protein clusters leave the micro-domains when kappa-carrageenan gels. The effect could be reversed by reheating the samples and thus melting the gel and was observed for repeated heating and cooling cycles. The effect of gelation on phase separation decreases with increasing polysaccharide concentration and with ageing of the liquid mixture. The latter is caused by the formation of bonds between the protein clusters in the micro-domains that slowly reinforce with time.

Carrageenan↗

PubMed2006

Effect of the cluster size on the micro phase separation in mixtures of beta-lactoglobulin clusters and kappa-carrageenan.

The phase separation of globular protein clusters formed by heat-denatured beta-lactoglobulin (beta-lg) in mixtures with the polysaccharide kappa-carrageenan (kappa-car) has been studied at pH 7 and 20 degrees C. The effect of the protein cluster size on the phase separation was investigated by preparing clusters with radii between 20 nm and 1 mum. The formation of protein rich microdomains led to an increase of the turbidity starting at a minimum kappa-car concentration that decreased with increasing cluster size, but was only weakly dependent on the protein concentration. The size and number of microdomains do not depend much on the cluster size, but their density decreases with increasing cluster size leading to a lower turbidity.

Carrageenan↗

PubMed2005

Strain hardening and fracture of heat-set fractal globular protein gels.

Non-linear mechanical behavior at large shear deformation was been investigated for heat-set beta-lactoglobulin gels at pH 7 and 0.1 M NaCl using both oscillatory shear and shear flow. These gels have a self-similar structure at length scales smaller than the correlation length of the gel with fractal dimension d(f)=2. Strain hardening is observed that can be well described using the model proposed by Gisler et al. [T.C. Gisler, R.C. Ball, D.A. Weitz, Phys. Rev. Let. 82 (1999) 1064] for fractal colloidal gels. The increase of the shear modulus normalized by the low strain value (G(0)) is independent of G(0). For weak gels the elasticity increases up to a factor of ten, while for strong gels the increase is very small. At higher deformation irreversible fracture occurs, which leads eventually to macroscopic failure of the gel. For weak gels formed at low concentrations the deformation at failure is about 2, independent of the shear modulus. For strong gels fracture occurs at approximately constant stress (2 x 10(3) Pa).

Fractals↗

PubMed2005

Dynamic mechanical properties of suspensions of micellar casein particles.

Small micellar casein particles, so-called submicelles, were obtained by removing colloidal calcium phosphate from native casein by adding sodium polyphosphate. Aqueous submicelle suspensions were characterized using light scattering and rheology as a function of concentration and temperature. The casein submicelles behave like soft spheres that jam at a critical concentration (C(c)) of about 100 g L(-1). The viscosity does not diverge at C(c), but increases sharply, similarly to that of multiarm star polymers. C(c) increases weakly with increasing temperature, which leads to a strong decrease of the viscosity close to and above C(c). Concentrated submicelle suspensions show strong shear-thinning above a critical shear rate and the shear stress becomes independent of the shear rate. The critical shear rates at different temperatures and concentrations are inversely proportional to the zero-shear viscosity. At much higher shear rates, the shear stress fluctuates strongly in time indicating inhomogeneous flow. The frequency dependence of casein submicelle suspensions is characterized by elastic behavior at high frequencies (concentrations) and viscous behavior at low frequencies (concentrations).

Calcium Phosphates↗

PubMed2005

Aggregation and gelation of micellar casein particles.

Micellar casein particles (submicelles) are formed by removing calcium phosphate from native casein. The submicelles aggregate and eventually form a gel with a rate that increases strongly with increasing temperature and casein concentration. At low casein concentrations the gel is very weak and collapses under its own weight so that a precipitate is formed. The structure of the aggregates is studied using light scattering and cryo-electron microscopy. It is found that the aggregates have a self-similar structure with fractal dimension 2. The viscoelastic properties of the gel are studied by frequency scans of the loss and storage moduli during the gelation process. The bonds between the submicelles probably involve calcium phosphate complexes.

Calcium Phosphates↗

PubMed2005

Revised state diagram of Laponite dispersions.

We propose a state diagram of charged disk-like mineral particle (Laponite) dispersions as a function of the Laponite concentration (C) and the concentration of added salt (C(s)), based on simple observation and light-scattering measurements. At low C or high C(s) the dispersions separate into two domains due to sedimentation of Laponite aggregates, while at high C and low C(s) they form homogeneous gels that do not flow upon tube reversal. The aggregation rate and the structure factor of the Laponite dispersions is determined with light scattering as a function of C and C(s). We discuss in detail the controversy on the origin of gelation of Laponite dispersions in the absence of added salt. We argue that aggregation rather than glass formation causes gelation.

Journal Article↗

PubMed2004

Influence of pyrophosphate or polyethylene oxide on the aggregation and gelation of aqueous laponite dispersions.

The influence of pyrophosphate or polyethylene oxide (PEO) on the aggregation and gelation of dispersions of model disklike clay particles (Laponite) is studied using light scattering and rheology. Pyrophosphate adsorbs onto the positively charged rim and inhibits bond formation between the rim and the negatively charged faces of the particles. At low concentrations of pyrophosphate the aggregation of Laponite is only retarded, without significant modification of the structure of the aggregates and gels. The decrease of the aggregation rate can be explained by an increase of the energy barrier to the formation of bonds in proportion to the pyrophosphate concentration. Addition of a large amount of pyrophosphate leads to the breakup of Laponite aggregates and gels. PEO adsorbs onto the Laponite particles and inhibits aggregation by steric hindrance. The reduction of the aggregation rate depends on the molar mass and is maximal at about 1000 g/mol. Higher molar mass PEO bridges between the particles and leads to the formation of clusters or a weak gel immediately after mixture.

Journal Article↗

PubMed2004

Influence of the ionic strength on the heat-induced aggregation of the globular protein beta-lactoglobulin at pH 7.

The influence of the ionic strength on the structure of beta-lactoglobulin aggregates formed after heating at pH 7 has been studied using static and dynamic light scattering. The native protein depletion has been monitored using size exclusion chromatography. Above a critical association concentration (CAC) well-defined clusters are formed containing about 100 monomers. The CAC increases with decreasing ionic strength. The so-called primary aggregates associate to form self similar semi-flexible aggregates with a large scale structure that is only weakly dependent on the ionic strength. The local density of the aggregates increases with increasing ionic strength. At a critical gel concentration, Cg, the size of the aggregates diverges. Cg decreases from 100 g/l without added salt to 1 g/l at 0.4M NaCl. For C > Cg the system gels except at high ionic strength close to Cg where the gels collapse under gravity and a precipitate is formed.

Hot Temperature↗

PubMed2002

Relation between aggregation and phase separation: three-dimensional Monte Carlo simulations.

We study phase separation of particles in solution using Monte Carlo simulations of reversible aggregation on a cubic lattice. Two stages of the phase separation can be clearly distinguished: initial random aggregation and subsequent densification. Step one leads to a distribution of fractal aggregates close to the binodal and to a temporary gel for large attractive interaction. Step two leads to isolated spherical dense domains close to the binodal and branched wormlike strands for large attractive interactions. The transition between the two types of structure is gradual and there is no clear feature that shows the existence of a spinodal. The first stage of the phase separation is metastable very close to the binodal or at very large interaction energy. In the latter case, the second step can be viewed as an aging process of the gel formed in the first step.

Biophysical Phenomena↗

PubMed2001

Light Scattering Study of Turbid Heat-Set Globular Protein Gels Using Cross-Correlation Dynamic Light Scattering.

The structure factor of aqueous solutions of the globular protein beta-lactoglobulin was determined as a function of heating time at 76 degrees C. We show how the effect of multiple scattering on the scattered light intensity can be effectively corrected using cross-correlation dynamic light scattering even if the transmission is only 1%. The structure factor of aggregated and gelled proteins can be described by the Ornstein-Zernike equation. The system is characterized by a correlation length that increases with heating time and stabilizes some time after the gel is formed. The correlation length of the protein gels decreases with decreasing concentration. Measurements after progressive dilution of a sample close to the gel point showed that the protein aggregates are initially interpenetrated and disinterpenetrate upon dilution. Copyright 2001 Academic Press.

Journal Article↗

PubMedDate not supplied

Influence of the NaCl or CaCl2 concentration on the structure of heat-set bovine serum albumin gels at pH 7.

The structure of heat-set systems of the globular protein bovine serum albumin (BSA) was investigated at pH 7 in different salt conditions (NaCl or CaCl(2)) using light scattering. Cross-correlation dynamic light scattering was used to correct for multiple scattering from turbid samples. After heat treatment, aggregates are formed whose size increases as the protein concentration increases. Beyond a critical concentration that decreases with increasing salt concentration, gels are formed. The heterogeneity and the reduced turbidity of the gels were found to increase with increasing salt concentration and to decrease with increasing protein concentration. The structure of the gels is determined by the strength of the repulsive electrostatic interactions between the aggregated proteins. The results obtained in NaCl are similar to those reported in previous studies for other globular proteins. CaCl(2) was found to be much more efficient in reducing electrostatic interactions than NaCl at the same ionic strength.

Calcium Chloride↗

PubMedDate not supplied

Influence of chain length and polymer concentration on the gelation of (amidated) low-methoxyl pectin induced by calcium.

The gelation of low-methoxyl pectin (LMP) induced by addition of Ca2+ was studied by measuring the storage modulus as a function of temperature during cooling. Samples with different molar masses were prepared by mechanical degradation. The effect of the molar mass and the pectin concentration on the gelation properties was investigated. The effect of partial amidation was studied by comparing LMP and partially amidated LMP with the same molar mass and degree of methylation. The results are compared to those from a model developed for Ca2+-induced pectin gelation, and good agreement is found except at low concentrations and low molar masses where the gels are weaker than predicted. At low concentrations intrachain bonding weakens the gel, while the presence of small pectin chains weakens the gel because it neutralizes binding sites on larger chains.

Amides↗

PubMedDate not supplied

Jamming and gelation of dense beta-casein micelle suspensions.

The rheology of dense suspensions of beta-casein micelles is investigated at pH 6. For a given temperature, the viscosity increases dramatically at a critical concentration (Cc) of about 100 g/L due to jamming of the micelles. For a given concentration close to and above Cc, the viscosity of dense suspensions decreases strongly with increasing temperature because Cc increases. The suspensions show weak shear thickening followed by strong shear thinning. At lower pH, that is, closer to the isoelectric point, spontaneous gelation is observed, which is favored by lowering the temperature and addition of sodium polyphosphate. The gelation process is studied at pH 5.5 by rheology and light scattering.

Animals↗