Search PubMed⌕ Search

Biomedical subjects

P Schurtenberger

Publications and source records attributed to P Schurtenberger.

At least 19 recordsLinked to original sources
PubMed2005

Small-angle neutron scattering on a core-shell colloidal system: a contrast-variation study.

Small-angle neutron scattering (SANS) measurements are reported on a sterically stabilized, core-shell colloidal system using contrast variation. Aqueous dispersions of polystyrene particles bearing grafted poly(ethylene glycol) (PEG) have been studied over a large range of particle concentrations and two different solvent conditions for the PEG polymer. SANS data are analyzed quantitatively by modeling the particles as core-shell colloids. In a good solvent and under particle contrast conditions, an effective hard-sphere interaction captures excluded-volume interactions up to high concentrations. Contrast variation, through isotopic substitution of both the core and solvent, expedite a detailed study of the PEG layer, both in the dilute limit and as a function of the particle concentration. Upon diminishing the solvent quality, subtle changes in the PEG layer translate into attractions among particles of moderate magnitude.

Journal Article↗
PubMed2004

Photonic properties of strongly correlated colloidal liquids.

The optical and structural properties of dense colloidal suspensions in the presence of long-range electrostatic repulsion are determined from both light and small-angle neutron scattering experiments. Short-range structural order induces an enhancement of the scattering strength while at the same time the total transmission shows strong wavelength dependence, reminiscent of a photonic crystal. Interestingly, the interplay between diffusive scattering and local order leads to negative values of the scattering anisotropy parameter. The tunable optical properties of these liquids furthermore suggest potential applications such as transparency switches or filters.

Journal Article↗
PubMed2004

Structure of peptide solutions: a light scattering and numerical study.

We investigated the interactions between protein molecules in solution, in particular for low salt concentrations and thus strong electrostatic interactions where a treatment based on the second virial coefficient is not sufficient. Static and dynamic light scattering experiments on solutions containing the peptide human calcitonin (hCT) were combined with calculations based on the Ornstein-Zernike equation with the hypernetted chain (HNC) closure and computer simulations within the primitive electrolyte model. The simulation illustrates the distribution of proteins in solution and the formation of (transient) protein aggregates. It furthermore allows us to predict the physical stability of hCT solutions in dependence of ionic strength, pH and hCT concentration.

Calcitonin↗
PubMed2003

Particle dynamics in concentrated colloidal suspensions.

We have studied the dynamical and structural properties of dense charge stabilized colloidal suspensions by a combination of small-angle neutron scattering (SANS), 3D dynamic light scattering (3DDLS) and diffusing wave spectroscopy (DWS). SANS and 3DDLS extend static and dynamic light scattering techniques to the regime of opaque samples while DWS additionally provides information on short length scales, typically from 1-50 nm. This offers an increased range of accessible length and time scales perfectly suited for the (non-invasive) investigation of highly concentrated suspensions. Different systems have been examined ranging from hard sphere like suspensions over strongly repulsive charged spheres to colloidal gels. We furthermore present an extended theoretical frame for DWS to characterize the internal dynamics of turbid gels made from nanosized colloidal particles.

Journal Article↗
PubMed2002

Structure, dynamics, and optical properties of concentrated milk suspensions: an analogy to hard-sphere liquids.

A systematic study of the effects of volume fraction increment on the optical properties, the structure, and the dynamics of the casein micelles and fat droplets in milk was performed using diffusing wave spectroscopy. Four types of milk were investigated, NIDO full fat milk, fat-free milk, whey and fat-free milk, and finally lactose and fat-free milk. Independent measurements to calculate the dependence of the viscosity and the index of refraction of the milk serum and casein micelles as a function of the volume fraction were also performed. We compare the experimentally determined quantities photon transport mean free path (l*) and self-diffusion coefficient D(s) with the predictions from theoretical calculations using classical colloidal models such as a hard-sphere fluid. We demonstrate that all types of milk with and without fat content behave, structurally, like colloidal hard-sphere systems up to volume fractions well over 45%. In the case of dynamic measurements, both lactose- and fat-free and whey- and fat-free milk behave also like hard-sphere systems whereas fat-free milk and fat-containing NIDO milk deviate slightly at volume fractions over 35%. Finally, a comparative measurement and theoretical calculation of the casein micelle's size was performed.

Animals↗
PubMed2002

Diffusing wave spectroscopy and small-angle neutron scattering from concentrated colloidal suspensions.

We have studied the properties of dense colloidal suspensions with a combination of small-angle neutron scattering (SANS) and diffusing wave spectroscopy (DWS). Contrary to single light scattering, DWS provides dynamic information on length scales, from 1 to 100 nm, comparable to SANS. This offers a unique range of accessible length and time scales perfectly suited for the (noninvasive) investigation of highly concentrated systems. By this we obtain valuable information about the structural properties and the short-time diffusion of electrostatically stabilized, but strongly screened, hard-sphere-like colloidal suspensions with volume fractions up to 30%. We furthermore discuss the consequences of local structural ordering on the optical properties, such as optical density and polarization. Quantitative agreement is found when comparing transmission measurements (optical density) with parameter-free numerical calculations based on the structural characterization from SANS.

Biophysics↗
PubMed2001

Diffusing-wave spectroscopy of nonergodic media.

We introduce an elegant method that allows the application of diffusing-wave spectroscopy (DWS) to nonergodic, solidlike samples. The method is based on the idea that light transmitted through a sandwich of two turbid cells can be considered ergodic even though only the second cell is ergodic. If absorption and/or leakage of light take place at the interface between the cells, we establish a so-called "multiplication rule," which relates the intensity autocorrelation function of light transmitted through the double-cell sandwich to the autocorrelation functions of individual cells by a simple multiplication. To test the proposed method, we perform a series of DWS experiments using colloidal gels as model nonergodic media. Our experimental data are consistent with the theoretical predictions, allowing quantitative characterization of nonergodic media and demonstrating the validity of the proposed technique.

Journal Article↗
PubMed2000

Sol-Gel transition of concentrated colloidal suspensions

Diffusing-wave spectroscopy (DWS) was used to follow the sol-gel transition of concentrated colloidal suspensions. We present a new technique based on a sandwich of two scattering cells aimed to overcome the problem of nonergodicity in DWS of solidlike systems. Using this technique we obtain quantitative information about the microscopic dynamics all the way from an aggregating suspension to the final gel, thereby covering the whole sol-gel transition. At the gel point a dramatic change of the particle dynamics from diffusion to a subdiffusive arrested motion is observed. A critical-power-law behavior is found for the time evolution of the maximum mean square displacement delta(2) probed by a single particle in the gel.

Journal Article↗
PubMed2000

New controlled environment vitrification system for cryo-transmission electron microscopy: design and application to surfactant solutions.

A newly designed controlled environment vitrification system for cryo-transmission electron microscopy of vitrified samples (cryo TEM) is presented. It significantly reduces the lag-time from blotting to vitrification by means of an optimized blotting procedure. Furthermore, a modified transfer system allows rapid transfer of the vitrified sample into the microscope and reduces the equilibration time. The practicable throughput of samples is thereby markedly increased. This new equipment is used to determine the overall size of individual polymer-like mixed lecithin-bile salt micelles, which very critically depends on the composition of the sample. The quantitative agreement with the results from independent light scattering experiments on the same sample supports the validity and performance of the presented sample preparation and handling procedures. In addition, these experiments demonstrate that cryo TEM experiments not only provide direct structural information, but furthermore permit a quantitative determination of micellar properties.

Bile Acids and Salts↗
PubMed2000

Size, flexibility, and scattering functions of semiflexible polyelectrolytes with excluded volume effects: monte carlo simulations and neutron scattering experiments

We present a systematic Monte Carlo study of the scattering function S(q) of semiflexible polyelectrolytes at infinite dilution, in solutions with different concentrations of added salt. In the spirit of a theoretical description of polyelectrolytes in terms of the equivalent parameters, namely, persistence length and excluded volume interactions, we used a modified wormlike chain model, in which the monomers are represented by charged hard spheres placed at distance a. The electrostatic interactions are approximated by a Debye-Huckel potential. We show that the scattering function is quantitatively described by that of uncharged wormlike chains with excluded volume effects provided that an electrostatic contribution is added to the persistence length. In addition we have studied the expansion of the radius of gyration and of the end-to-end distance. The results are in agreement with the picture outlined in the Odijk-Skolnick-Fixman theory, in which the behavior of charged polymers is described only in terms of increasing local rigidity and excluded volume effects. Moreover, the Monte Carlo data are found to be in very good agreement with experimental scattering measurements with equilibrium polyelectrolytes, i.e., giant wormlike micelles formed in mixtures of nonionic and ionic surfactants in dilute aqueous solution, with added salt.

Journal Article↗
PubMed1999

Thermodynamics of a nonionic sponge phase.

Different suggestions for the mechanism governing the narrow stability of the L(3) (sponge) phase have led to a series of debates in recent years. There have been several models developed to describe such a mechanism via thermodynamics. To date, experimental data are insufficient to test present theories. In this study, we revisit the sponge phase with two series of thermodynamic data performed on the well-characterized C(12)E(5)-n-decane-H(2)O system. These thermodynamic data sets stem from phase equilibrium and static light scattering experiments designed to link system-specific parameters such as the temperature dependence of the spontaneous curvature H(o) and the two bending moduli kappa and (-)kappa, which have only been loosely connected in earlier experiments. The use of a well-characterized system is important in that it allows usage of molecular descriptors from earlier studies to reduce fit parameters. Another advantage for using this system is that its phase behavior is analogous to a two-component system which, from an experimental standpoint, is more practical to perform accurate measurements and, from a theoretical standpoint, more simple to model. In the present investigation, we use these tools to quantitatively test parameters obtained by different experimental techniques and assumptions inherited in theoretical models designed to interpret them.

Journal Article↗
PubMed1999

Quantitative measurements of Ostwald ripening using time-resolved small-angle neutron scattering.

Using a unique method we present accurate quantitative measurements of the Ostwald ripening of an emulsion system. Time-resolved small-angle neutron scattering monitors the time evolution of the average radius and number density of the emulsion drops. The results qualitatively agree with the current theory of Ostwald ripening but there is a quantitative, experimentally significant, discrepancy of a factor of 1.7. We argue that these accurate experiments, performed on a well characterized system, provide a most useful basis for testing further refinements of the theory.

Journal Article↗
PubMed1998

Characterization of Turbid Colloidal Suspensions Using Light Scattering Techniques Combined with Cross-Correlation Methods.

The ability to characterize colloidal suspensions by means of dynamic light scattering is in general limited to systems with negligible contributions from multiple scattering. For larger particle sizes with high scattering contrast this immediately limits the technique to very low concentrations. A promising solution of this problem is to suppress multiple scattering in dynamic light scattering experiments using cross-correlation schemes. Based on these considerations we have constructed a so-called 3D cross-correlation experiment with which we are able to characterize extremely turbid suspensions. We have measured monomodal and bimodal suspensions of latex particles of relatively high volume fraction. The results show clearly that we are able to measure the dynamic structure factor in concentrated polydisperse suspensions with dynamic light scattering. Combining static and dynamic light scattering measurements for characterizing turbid suspensions the single scattering particle form factor and also the static structure factor can be evaluated. We demonstrate that the implementation of a 3D cross-correlation scheme is a powerful method in suppressing multiple scattering contributions in light scattering experiments and opens a wide field of characterization of colloidal dispersions with high turbidity without having to resort to high dilution. Copyright 1998 Academic Press.

Journal Article↗
PubMed1998

The solution structure of functionally active human proliferating cell nuclear antigen determined by small-angle neutron scattering.

The function of proliferating cell nuclear antigen (PCNA) in DNA replication and repair is to form a sliding clamp with replication factor C (RF-C) tethering DNA polymerase delta or epsilon to DNA. In addition, PCNA has been found to interact directly with various proteins involved in cell cycle regulation. The crystal structure of yeast PCNA shows that the protein forms a homotrimeric ring lining a hole through which double-stranded DNA can thread, thus forming a moving platform for DNA synthesis. Human and yeast PCNA are highly conserved at a structural and functional level. We determined the solution structure of functionally active human PCNA by small-angle neutron scattering. Our measurements strongly support a trimeric ring-like structure of functionally active PCNA in solution, and the data are in good agreement with model calculations based on the crystal structure from yeast PCNA. The human PCNA used in the small-angle neutron scattering experiments was active before and after the measurements in a RF-C independent and a RF-C dependent assay suggesting that the trimeric structure is the in vivo functional form.

Buffers↗
PubMed1997

Measurement of Absolute Coagulation Rate Constants for Colloidal Particles: Comparison of Single and Multiparticle Light Scattering Techniques

The absolute coagulation rate constants of monodisperse spherical colloids in aqueous suspension were measured at the early stage of the coagulation processes by using two different techniques: single particle light scattering (SPLS) and simultaneous static and dynamic light scattering (SLS + DLS). Single particle light scattering determines the cluster-size distribution directly by counting the number of different clusters during the coagulation process and therefore can be used to test the kinetic growth model used for obtaining the coagulation rate constants. The simultaneous static and dynamic light-scattering method is an in-situ experiment, where the average cluster size is estimated by simultaneous static and dynamic light-scattering measurements at different angles on a multiangle fiber-optics-based setup. A combined evaluation of the static and dynamic light-scattering data permits the determination of the absolute rate constants without the explicit use of light scattering form factors and hydrodynamic properties for dimers. In this paper, we compare results obtained with both techniques. Good agreement was found for the absolute coagulation rate constants of two different latex particle suspensions. Furthermore, we show that the two techniques complement one another and the limitations of the first are overcome by the second and vice versa. Copyright 1997Academic Press

Journal Article↗
PubMed1993

Changes in structure of alpha 2-macroglobulin upon reaction with trypsin as assessed by light scattering and differential scanning calorimetry.

Employing a combination of static and dynamic light scattering, as well as differential scanning calorimetry (DSC), the structural changes which appear in alpha 2-macroglobulin (alpha 2M) upon trypsin binding have been further characterized. Light-scattering measurements suggest that a 15% reduction in both the hydrodynamic radius and radius of gyration occurs when two molecules of trypsin complex to alpha 2M. Approx. 85% of this trypsin-induced compaction results from the binding of the first proteinase. A complementary result was obtained from DSC measurements in which the major fraction of the trypsin-induced conversion of alpha 2M to a single more thermally stable form results from interaction with the first proteinase molecule. These observations support a functionally asymmetric model of trypsin binding to alpha 2M in which the significant reduction in size of the complex is primarily due to the initial interaction of alpha 2M with a single proteinase molecule.

Calorimetry, Differential Scanning↗