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Biomedical subjects

C A Helm

Publications and source records attributed to C A Helm.

10 recordsLinked to original sources
PubMed2004

Nanoparticle composition of a ferrofluid and its effects on the magnetic properties.

Experiments were carried out on a water-based ferrofluid (gamma-Fe2O3 with carboxydextran shell) using photon correlation spectroscopy (PCS), atomic force microscopy, and magnetic nanoparticle relaxation measurements. The experiments were designed with the aim to relate the Néel signals that are in theory generated by large single core particles with nanoscopic properties, that is, particle size, particle size distribution, shell properties, and aggregation. For this purpose, the ferrofluid was fractionated by magnetic fractionation and size exclusion chromatography. Nanoparticles adsorbed onto positively charged substrates form a two-dimensional monolayer. Their mean core diameters are in the range of 6 to about 20 nm, and particles above 10 nm are mostly aggregates. The hydrodynamic particle diameters are between 13 and 80 nm. The core diameter of the smallest fraction is confirmed by X-ray reflectometry; the surface coverage is controlled by bulk diffusion. Comparison with the hydrodynamic radius yields a shell thickness of 3.8 nm. Considering the shell thickness to be constant for all particles, it was possible to calculate the apparent particle diameter in the original ferrofluid from the PCS signals of all fractions. As expected, the small cores yielded no Néel relaxation signals in freeze-dried samples; however, the fractions containing mostly aggregates yielded Néel relaxation signals.

Journal Article↗
PubMed1999

Wetting of mesoscopic soft cylinders: structure and layering transitions.

The wetting of soft mesoscopic long-chain particles is studied. As a model system, a cylindrical brush with poly(vinyl)pyridine side chains on the water surface is characterized by isotherms and x-ray reflectivity. The forces from the two planar interfaces and the intra- and interparticle interactions are all of comparable magnitude. Two layering transitions occur, one from the monolayer to the double layer, the next to a homogeneous multilayer. The hard wall from which layering starts is the smooth polymer/air interface. Indeed, they particles in the top layer of both the double- and the multilayer have their cylinder axis parallel to the surface and are laterally compressed. In contrast, the polymer/water interface is diffuse due to brush swelling. Generally, the long-chain particles adjacent to the respective interfaces do not maintain their circular diameters. The thickness of the monolayer can be varied by a factor 3.5, up to 53 A. An additional phase transition occurs within the monolayer, which is attributed to a change of the side chains from a flattened to a compressed state at constant volume. Atmoic force microscope images of the monolayer transferred onto a solid indicate local cylinder alignment.

Journal Article↗
PubMed1993

Role of calcium in the adhesion and fusion of bilayers.

The interaction forces and fusion mechanisms of mixed zwitterionic-anionic phospholipid bilayers were measured with the surface forces apparatus. The bilayers were 3:1 mixtures of either dimyristoylphosphatidylcholine and dimyristoylphosphatidylglycerol (DMPC/DMPG) or dilauroylphosphatidylcholine and dilauroylphosphatidylglycerol (DLPC/DLPG), and experiments were carried out in NaCl solutions with and without CaCl2. In NaCl solutions, the forces between either mixed bilayer system were consistent with the DLVO (Derjaguin-Landau-Verwey-Overbeek) theory of repulsive electrostatic and attractive van der Waals forces, and fusion did not occur. At high pH (> 6) and in high (20 mM) NaCl concentrations, a short-range hydration force extending about 13 A was evident, indicative of Na+ binding to the surfaces. In the presence of this large hydration repulsion, the interbilayer adhesion was abolished. When CaCl2 was added to the bathing solutions in the presence or absence of NaCl, the bilayers phase separate into small domains, coinciding with the occurrence of a large, long-range attractive force. Fusion occurred readily between the more fluid domains. The phase separations and fusion events could be directly visualized by observing the shapes of the optical fringes used to measure the surface separation and the change in surface profiles with time. The ease of fusion between mixed bilayers in the presence of calcium correlated closely with the strength of the long-range attractive force. This force is attributed to the additional hydrophobic force between domains or domain boundaries due to the exposure of excess hydrophobic groups resulting from the Ca(2+)-induced condensation of the PG- headgroups.(ABSTRACT TRUNCATED AT 250 WORDS)

Calcium↗
PubMed1992

Role of hydrophobic forces in bilayer adhesion and fusion.

With the aim of gaining more insight into the forces and molecular mechanisms associated with bilayer adhesion and fusion, the surface forces apparatus (SFA) was used for measuring the forces and deformations of interacting supported lipid bilayers. Concerning adhesion, we find that the adhesion between two bilayers can be progressively increased by up to two orders of magnitude if they are stressed to expose more hydrophobic groups. Concerning fusion, we find that the most important force leading to direct fusion is the hydrophobic attraction acting between the (exposed) hydrophobic interiors of bilayers; however, the occurrence of fusion is not simply related to the strength of the attractive interbilayer forces but also to the internal bilayer stresses (intrabilayer forces). For all the bilayer systems studied, a single basic fusion mechanism was found in which the bilayers do not "overcome" their short-range repulsive steric-hydration forces. Instead, local bilayer deformations allow these repulsive forces to be "bypassed" via a mechanism that is like a first-order phase transition, with a sudden instability occurring at some critical surface separation. Some very slow relaxation processes were observed for fluid bilayers in adhesive contact, suggestive of constrained lipid diffusion within the contact zone.

Adhesiveness↗
PubMed1991

Measurement of ligand-receptor interactions.

One distinguishing feature of "life" is that the physical forces between biological molecules and membrane surfaces are often highly specific, in contrast to nonspecific interactions such as van der Waals, hydrophobic, and electrostatic (Coulombic) forces. We have used the surface-forces-apparatus technique to study the specific "lock and key" or "ligand-receptor" interaction between two model biomembrane surfaces in aqueous solution. The membranes were lipid bilayers supported on mica surfaces; one carrying streptavidin receptors, the other exposing biotin ligand groups. We found that, although no unusual or specific interaction occurs between two avidin or two biotin surfaces, an avidin and a biotin surface exhibit a very strong, very short-range (less than 1 nm) attraction and that the binding mechanism involves equally specific molecular rearrangements. The results also show that highly specific biological interactions such as are involved in immunological recognition and cell-cell contacts may be studied at the molecular level and in real time by the surface-forces-apparatus technique.

Aluminum Silicates↗
PubMed1991

Phases of phosphatidyl ethanolamine monolayers studied by synchrotron x-ray scattering.

For the first time, phospholid monolayers at the air/water interface have been studied by x-ray diffraction and reflection all along the isotherm from the laterally isotropic fluid (the so-called LE phase) to the ordered phases. The model used to analyze the data, and the accuracy of the parameters deduced, were tested by comparing the results obtained with two lipids having the same head group but different chain lengths. Compression of the fluid phase leads predominantly to a change of thickness of the hydrophobic moiety, much less of its density, with the head group extension remaining constant. The main transition involves a considerable increase (approximately 10%) of the electron density in the hydrophobic region, a dehydration of the head group and a positional ordering of the aliphatic tails, albeit with low coherence lengths (approximately 10 spacings). On further compression of the film, the ordered phase undergoes a continuous transition. This is characterized by an increase in positional ordering, a discontinuous decrease in lateral compressibility, a decrease in chain tilt angle with respect to the surface normal towards zero and probably also a head group dehydration and ordering.

Liposomes↗
PubMed1991

Atomic force microscopy of hydrated phosphatidylethanolamine bilayers.

We present images of the polar or headgroup regions of bilayers of dimyristoyl-phosphatidylethanolamine (DMPE), deposited by Langmuir-Blodgett deposition onto mica substrates at high surface pressures and imaged under water at room temperature with the optical lever atomic force microscope. The lattice structure of DMPE is visualized with sufficient resolution that the location of individual headgroups can be determined. The forces are sufficiently small that the same area can be repeatedly imaged with a minimum of damage. The DMPE molecules in the bilayer appear to have relatively good long-range orientational order, but rather short-range and poor positional order. These results are in good agreement with x-ray measurements of unsupported lipid monolayers on the water surface, and with electron diffraction of adsorbed monolayers.

Biophysical Phenomena↗
PubMed1989

Molecular mechanisms and forces involved in the adhesion and fusion of amphiphilic bilayers.

The surface forces apparatus technique was used for measuring the adhesion, deformation, and fusion of bilayers supported on mica surfaces in aqueous solutions. The most important force leading to the direct fusion of bilayers is the hydrophobic interaction, although the occurrence of fusion is not simply related to the force law between bilayers. Bilayers do not need to "overcome" some repulsive force barrier, such as hydration, before they can fuse. Instead, once bilayer surfaces come within about 1 nanometer of each other, local deformations and molecular rearrangements allow them to "bypass" these forces.

Chemical Phenomena↗
PubMed1987

Phospholipid monolayers between fluid and solid states.

Monolayers of the phospholipid dimyristoyl phosphatidic acid on the surface of water have been studied by a combination of the new techniques of synchrotron x-ray diffraction and fluorescence microscopy with classical surface pressure data. The pressure vs. area isotherm changes slope at the surface pressures pi c and pi s. The optical technique demonstrates that between pi c and pi s the fluid phase coexists with a denser "gel" phase. Electron diffraction data have shown that the gel phase has bond orientational order over tens of micrometers. However, the x-ray data demonstrate that positional correlations extend only over tens of angstroms. Thus, the gel phase is not crystalline. Above pi s a solid phase is formed with a positional correlation range that is eight times longer for the chemically purest films.

Glycerophospholipids↗