Biomedical subjects

Michal Borkovec

Publications and source records attributed to Michal Borkovec.

At least 19 recordsLinked to original sources

PubMed2006

Decomposing bridging adhesion between polyelectrolyte layers into single molecule contributions.

A novel approach to analyze the force response of multiple polymer strands, which are bridged between two surfaces, is proposed. The response of single polymer strands is experimentally accessible by measuring the force upon separation of two polymer-coated surfaces with the atomic force microscope. Our approach is based on the decomposition of the stretching and desorption sequence into contributions of independently bridged chains and of the elimination of loops formed on the opposite surface during contact. This approach was applied to investigate the bridging adhesion of surfaces coated with poly(vinylamine) (PVA). The force response of single PVA molecules was described on the basis of a recently proposed model, which accounts for the discrete chain character of the polymer at higher extension forces. As exemplary results, we determined the length distributions of the individual chains and the loop number distribution of these bridging chains on the polyelectrolyte-coated surfaces. The former were compared with scaling theories of polymer adsorption.

Journal Article↗

PubMed2006

Deposition of nanosized latex particles onto silica and cellulose surfaces studied by optical reflectometry.

Deposition of positively charged nanosized latex particles onto planar silica and cellulose substrates was studied in monovalent electrolyte solutions at pH 9.5. The deposition was probed in situ with optical reflectometry in a stagnation point flow cell. The surface coverage can be estimated reliably with island film theory as well as with a homogeneous film model, as confirmed with atomic force microscopy (AFM). The deposition kinetics on the bare surface was of first order with respect to the particle concentration, whereby the deposition rate was close to the value expected for a perfect collector. The efficiency coefficient, which was defined as the ratio of the experimental and theoretical deposition rate constants, was in the range from 0.3 to 0.7. Subsequently, the surface saturated and a limiting maximum coverage was attained (i.e., blocking). These trends were in qualitative agreement with predictions of the random sequential absorption (RSA) model, where electrostatic interactions between the particles were included. It was observed, however, that the substrate strongly influenced the maximum coverage, which was substantially higher for silica than for cellulose. The major conclusion of this work was that the nature of the substrate played an important role in a saturated layer of deposited colloidal particles.

Cellulose↗

PubMed2006

Release of colloidal particles in natural porous media by monovalent and divalent cations.

We study mobilization of colloidal particles from natural porous media, such as soils and groundwater aquifers. Extensive laboratory scale column experiments of particle release from four different subsurface materials are presented. The important characteristics of the release process are (i) its non-exponential kinetics, (ii) the finite supply of colloidal particles and (iii) the strong dependence of the release kinetic on the nature of the adsorbed cations. Particle release depends most sensitively on the relative saturation of the medium with divalent cations. We propose a mathematic model, which captures all these aspects quantitatively, and can be used to describe the coupling between transport of major cations and the release of colloidal particles. The present experimental investigations as well as the developed modeling framework represent an important step towards the understanding of colloid-facilitated transport phenomena in natural porous media.

Cations↗

PubMed2006

Ionization equilibria and conformational transitions in polyprotic molecules and polyelectrolytes.

The coupling between proton binding and conformational degrees of freedom in polyprotic molecules and polyelectrolytes is studied theoretically. Our approach combines the classical rotational isomeric state (RIS) model developed by Flory and the site binding (SB) model used to treat proton binding equilibria. The properties of the resulting SBRIS model, which treats conformational degrees of freedom and proton binding on equal footing, are studied with statistical mechanical techniques. Quantities of interest, such as titration curves, conformational probabilities, or macroscopic binding constants, are expressed as thermal averages and are evaluated by direct enumeration of states or by transfer matrix techniques. We further demonstrate that in the SBRIS model conformational degrees of freedom can be averaged out, leading to the contracted description within the SB model. In most cases, this contraction leads to higher order interactions, which may not be present at the SBRIS level (e.g., triplet interactions). Several examples are discussed to illustrate the concepts developed. The case of succinic acid exemplifies the situation in its simplest form. The model can further rationalize the very different titration behavior of poly(acrylic acid) (PAA) and poly(methacrylic acid) (PMAA). In particular, the characteristic "jump" in the titration curve of PMAA is described quantitatively and is interpreted in terms of a conformational transition.

Journal Article↗

PubMed2006

Probing the validity of the Derjaguin approximation for heterogeneous colloidal particles.

The Derjaguin approximation states that the interaction force between two curved surfaces is proportional to their effective radius, whereby the inverse effective radius is the arithmetic mean of the inverse curvature radii of the surfaces involved. The present study investigates the validity of this approximation with an atomic force microscope (AFM) by measuring interaction forces between colloidal particles of different sizes, but of identical composition. Forces were measured between silica particles of 2.0, 4.8 and 6.8 microm in diameter in KCl electrolyte solution with and without adsorbed poly(amido amine) (PAMAM) dendrimers. The Derjaguin approximation could be confirmed at all distances investigated, including those comparable with the characteristic length scales of the surface roughness or the surface charge heterogeneities. For the conditions investigated, the Derjaguin approximation turns out to be surprisingly robust.

Journal Article↗

PubMed2006

Simple thermodynamics for unravelling sophisticated self-assembly processes.

During the past 15 years, coordination chemistry has rapidly developed toward multicomponent assemblies involving several ligands and metal ions, which are connected via intra- or intermolecular processes. The fascinating structural aspect of these complexation reactions has been early recognized for the design of sophisticated (supra)molecular architectures with novel topologies and functions, while the concomitant energetic part only recently emerged as a potential tools for controlling and programming self-assemblies. In this Perspective, we focus on the modelling of the free energy changes accompanying self-assembly processes. Starting with the original protein-ligand model borrowed from biology, which describes complicated multicomponent assemblies, we present (i) its adaptation to coordination chemistry and (ii) its significance for addressing cooperativity as an extra energy cost resulting from intercomponent interactions. An additional entropic concept arising from the separation of intra- and intermolecular complexation processes is then discussed, together with its explicit consideration for modeling multicomponent complexation reactions. Finally, both aspects (i.e. cooperativity and intra-/intermolecular connections) are combined in the extended site binding model, which is able to dissect free energy changes occurring in sophisticated metal-ligand assemblies with a minimum set of microscopic parameters. Applications to experimental complexation reactions of increasing complexity are systematically discussed, and illustrate the potential and limitations of each model.

Kinetics↗

PubMed2006

Heteroaggregation in binary mixtures of oppositely charged colloidal particles.

Heteroaggregation (or heterocoagulation) rate constants have been measured in mixtures of well-characterized colloidal particles of opposite charge with multiangle static and dynamic light scattering. This technique permits routine measurements of absolute heteroaggregation rate constants, also in the presence of homoaggregation. Particularly with multiangle dynamic light scattering, one is able to estimate absolute heteroaggregation rate constants accurately in the fast aggregation regime for the first time. Heteroaggregation rate constants have also been measured over a wide range of parameters, for example, ionic strength and different surface charge densities. Amidine latex particles, sulfate latex particles, and silica particles have been used for these experiments, and they were well characterized with respect to their charging and homoaggregation behavior. It was shown that heteroaggregation rate constants of oppositely charged particles increase slowly with decreasing ionic strength, and provided the surface charge is sufficiently large, the rate constant is largely independent of the surface charge. These trends can be well described with DLVO theory without adjustable parameters.

Journal Article↗

PubMed2006

scyllo-inositol pentakisphosphate as an analogue of myo-inositol 1,3,4,5,6-pentakisphosphate: chemical synthesis, physicochemistry and biological applications.

myo-Inositol 1,3,4,5,6-pentakisphosphate (Ins(1,3,4,5,6)P(5)), an inositol polyphosphate of emerging significance in cellular signalling, and its C-2 epimer scyllo-inositol pentakisphosphate (scyllo-InsP(5)) were synthesised from the same myo-inositol-based precursor. Potentiometric and NMR titrations show that both pentakisphosphates undergo a conformational ring-flip at higher pH, beginning at pH 8 for scyllo-InsP(5) and pH 9 for Ins(1,3,4,5,6)P(5). Over the physiological pH range, however, the conformation of the inositol rings and the microprotonation patterns of the phosphate groups in Ins(1,3,4,5,6)P(5) and scyllo-InsP(5) are similar. Thus, scyllo-InsP(5) should be a useful tool for identifying biologically relevant actions of Ins(1,3,4,5,6)P(5), mediated by specific binding sites, and distinguishing them from nonspecific electrostatic effects. We also demonstrate that, although scyllo-InsP(5) and Ins(1,3,4,5,6)P(5) are both hydrolysed by multiple inositol polyphosphate phosphatase (MINPP), scyllo-InsP(5) is not dephosphorylated by PTEN or phosphorylated by Ins(1,3,4,5,6)P(5) 2-kinases. This finding both reinforces the value of scyllo-InsP(5) as a biological control and shows that the axial 2-OH group of Ins(1,3,4,5,6)P(5) plays a part in substrate recognition by PTEN and the Ins(1,3,4,5,6)P(5) 2-kinases.

Inositol Phosphates↗

PubMed2005

Interaction forces and molecular adhesion between pre-adsorbed poly(ethylene imine) layers.

Interaction forces between pre-adsorbed layers of branched poly(ethylene imine) (PEI) of different molecular mass were studied with the colloidal probe technique, which is based on atomic force microscopy (AFM). During approach, the long-ranged forces between the surfaces are repulsive due to overlap of diffuse layers down to distances of a few nanometers, whereby regulation of the surface charge is observed. The ionic strength dependence of the observed diffuse layer potentials can be rationalized with a surface charge of 2.3 mC/m2. The forces remain repulsive down to contact, likely due to electro-steric interactions between the PEI layers. These electro-steric forces have a range of a few nanometers and appear to be superposed to the force originating from the overlap of diffuse layers. During retraction of the surfaces, erratic attractive forces are observed due to molecular adhesion events (i.e., bridging adhesion). The frequency of the molecular adhesion events increases with increasing the ionic strength. The force response of the PEI segments is dominated by rubber-like extension profiles. Strong adhesion forces are observed for low molecular mass PEI at short distances directly after separation, while for high molecular mass weaker adhesion forces at larger distances are more common. The work of adhesion was estimated by integrating the retraction force profiles, and it was found to increase with the ionic strength.

Journal Article↗

PubMed2005

A simple thermodynamic model for quantitatively addressing cooperativity in multicomponent self-assembly processes--Part 2: Extension to multimetallic helicates possessing different binding sites.

The extended site-binding model, which explicitly separates intramolecular interactions (i.e., intermetallic and interligand) from the successive binding of metal ions to polytopic receptors, is used for unravelling the self-assembly of trimetallic double-stranded Cu(I) and triple-stranded Eu(III) helicates. A thorough analysis of the available stability constants systematically shows that negatively cooperative processes operate, in strong contrast with previous reports invoking either statistical behaviours or positive cooperativity. Our results also highlight the need for combining successive generations of complexes with common binding units, but with increasing metallic nuclearities, for rationalizing and programming multicomponent supramolecular assemblies.

Journal Article↗

PubMed2005

A simple thermodynamic model for quantitatively addressing cooperativity in multicomponent self-assembly processes--part 1: Theoretical concepts and application to monometallic coordination complexes and bimetallic helicates possessing identical binding sites.

A thermodynamic model has been developed for quantitatively estimating cooperativity in supramolecular polymetallic [M(m)L(n)] assemblies, as the combination of two simple indexes measuring intermetallic (I(c)MM) and interligand (I(c)LL) interactions. The usual microscopic intermolecular metal-ligand affinities (f(i)(M,L)) and intermetallic interaction parameters (uMM), adapted to the description of successive intermolecular binding of metal ions to a preorganized receptor, are completed with interligand interactions (uLL) and effective concentrations (c(eff)), accounting for the explicit free energy associated with the aggregation of the ligands forming the receptor. Application to standard monometallic pseudo-octahedral complexes [M(L)(n)(H2O)(6 - n)] (M = Co, Ni, Hf, L = ammonia, fluoride, imidazole, n = 1-6) systematically shows negative cooperativity (uLL < 1), which can be modulated by the electronic structures, charges, and sizes of the entering ligands and of the metal ions. Extension to the self-assembly of more sophisticated bimetallic helicates possessing identical binding sites is discussed, together with the origin of the positively cooperative formation of [Eu2(L3)3].

Journal Article↗

PubMed2005

Colloid facilitated transport of strongly sorbing contaminants in natural porous media: mathematical modeling and laboratory column experiments.

Mobile colloidal particles may act as carriers of strongly sorbing contaminants in subsurface materials. Such colloid-facilitated transport can be induced by changes in salinity, similar to freshwater intrusion to a contaminated aquifer saturated with saltwater, or groundwater penetration into a contaminated site saturated with a dumpsite leachate. This process is studied for noncalcareous soil material with laboratory column experiments with sodium and calcium as major cations and with lead as a strongly sorbing model contaminant. The measured breakthrough curves of these elements were described with a mathematical transport model, which invokes release and deposition kinetics of the colloids, together with adsorption and desorption of the relevant ions to the solid matrix as well as to the suspended colloids. In particular, the specific coupling between colloid and solute transport is considered. The crux of a successful description of such colloidal transport processes is to capture the inhibition of the particle release by adsorbed divalent ions properly and explicitly to considerthe dependence of colloid release on the solution chemistry and the chemical conditions at the solid-liquid interface. Experiments and modeling address colloid-facilitated transport of lead out of a contaminated zone and through a noncontaminated zone, including effects of flow velocity and length of the noncontaminated zone. We finally show that colloid-facilitated transport can be suppressed by the injection of a suitably chosen solution of a calcium salt.

Adsorption↗

PubMed2005

Light-scattering form factors of asymmetric particle dimers from heteroaggregation experiments.

Measurements of form factors of asymmetric particle dimers composed of oppositely charged polystyrene latex particles are presented. These measurements are based on time-resolved static and dynamic light scattering on dilute aggregating aqueous suspensions. The experimental form factors are compared with independent calculations based on the superposition T-matrix method and Rayleigh-Debye-Gans (RDG) approximation. While the RDG approximation is found to be reliable only up to particle diameters of about 250 nm, the superposition T-matrix method is very accurate for all types of dimers investigated. The present results show clearly the appropriateness of the superposition T-matrix method to estimate the optical properties of colloidal particles in the micrometer range reliably.

Journal Article↗

PubMed2005

Effects of heat treatment on the aggregation and charging of Stöber-type silica.

Colloidal silica is known to be stable at high salt concentrations and low pH, where silica is basically uncharged. This observation is in qualitative disagreement with the theory of Derjaguin, Landau, Verwey, and Overbeek (DLVO), which predicts rapid aggregation (or coagulation) under these conditions. This study reports a very different behaviour for Stöber-type silica heated at 800 degrees C, as these particles follow DLVO theory quantitatively. Unheated samples behave approximatively according to DLVO theory, but they show systematic deviations, in particular, featuring higher stability at low pH. The heat treatment also substantially modifies the charging properties, as heated particles show titratable surface charge densities in the range expected for the water-silica interface, while much higher charge densities are observed for the unheated samples. The electrophoretic mobilities, on the other hand, are hardly influenced by the heat treatment. We suspect that the suspension stability of the unheated particles is influenced by the presence of a hairy-layer of polysilicilic acid chains on the surface.

Journal Article↗

PubMed2005

Aggregation and charging of colloidal silica particles: effect of particle size.

We studied systematically aqueous suspensions of amorphous well-characterized silica particles by potentiometric titration, electrophoretic mobility, and time-resolved light scattering. Their charging behavior and aggregation rate constants were measured as a function of pH and ionic strength in KCl electrolytes for three types of particles of approximately 30, 50, and 80 nm in diameter. The charging behavior was consistent with the basic Stern model; the silica particles carry a negative charge, and its magnitude gradually increases with increasing pH and ionic strength. On the other hand, their early-stage aggregation (or coagulation) behavior is complex. The aggregation of the largest particles shows features resembling predictions of the Derjaguin, Landau, Verwey, and Overbeek (DLVO) theory. On one hand, the rate constant decreases sharply with increasing pH at low ionic strengths and attains fast aggregation conditions at high ionic strengths. On the other hand, we observe a characteristic slowing down of the aggregation at low pH and high ionic strengths. This feature becomes very pronounced for the medium and the small particles, leading to a complete stabilization at low pH for the latter. Stabilization is also observed at higher pH for the medium and the small particles. From these aggregation measurements we infer the existence of an additional repulsive force. Its origin is tentatively explained by postulating hairy layers of consisting of poly(silicilic acid) chains on the particle surface.

Journal Article↗

PubMed2005

Super-stoichiometric charge neutralization in particle-polyelectrolyte systems.

The adsorption of poly(vinylamine) (PVA) on poly(styrene sulfate) latex particles is studied, and its consequences on the charging behavior and suspension stability are investigated. The adsorption process is assessed by batch depletion experiments and time-resolved electrophoretic mobility measurements. The adsorption of PVA appears to be basically irreversible. The rate of adsorption decreases with decreasing polymer dose. At low polymer dose, the polymer coverage corresponds to the amount of the polyelectrolyte added, while at high polymer dose, the polymer coverage saturates the surface. Stability ratios are determined by dynamic light scattering, and strongly depend on the polymer dose and salt level. The aggregation is rapid near the isoelectric point (IEP), and it slows down when moving away from it. The charge neutralization is highly nonstoichiometric with charging ratios (CR) larger than unity, meaning that several charges on an adsorbed polyelectrolyte chain are necessary to neutralize a single charge on the particle surface. By comparing the IEP for particles and polyelectrolytes of different charge densities, we find a strong dependence of the CR on the mismatch between the average distances between individual charges on the surface and on the polyelectrolyte. A simple model is proposed to explain this trend.

Journal Article↗

PubMed2005

Inframolecular protonation process of norbadione A: influence of the ionic environment and stereochemical consequences.

The microscopic protonation mechanism, at an inframolecular level, of norbadione A, a pigment extracted from mushrooms and known to complex cesium cations, was determined by using 1H NMR titrations and the cluster expansion method. This study revealed a pH dependent Z to E isomer switch that occurs in both pulvinic moieties. As a consequence, norbadione A can exist in solution in four stereomeric forms (E-E, E-Z, Z-E, and Z-Z), which can be of interest in the development of molecular-level devices. In the presence of 0.15 M NaCl, the calculated microconstants showed an unusual apparent cooperativity between the enol groups, which results from the release of the sodium cations upon protonation of norbadione A.

4-Butyrolactone↗

PubMed2004

Statistical mechanical approach to competitive binding of metal ions to multi-center receptors.

A microscopic site binding model to treat binding of several metal ions to multi-center receptors is proposed. The model introduces the appropriate parameterization in terms of microscopic complexation constants and metal-metal pair interaction energies. The model is solved with statistical mechanical techniques, including direct enumeration or transfer matrices. We obtain microscopic and macroscopic complexation constants, microstate probabilities, and binding isotherms for chain-like receptors, including the long-chain limit. Various examples to illustrate the usefulness of the model are given.

Binding Sites↗