Biomedical subjects

J O Rädler

Publications and source records attributed to J O Rädler.

14 recordsLinked to original sources

PubMed2006

Wormlike lipid/DNA micelles in a non-polar solvent.

The phase behavior of DOPE/DOTAP-DNA complexes in phase-separated oil(dodecane)/water mixtures was explored using Small Angle X-Ray Scattering (SAXS) and Fluorescence Correlation Spectroscopy (FCS). Inverse micelles of DNA with cationic-lipid coating were found in the oil phase. Varying the ratio between cationic and neutral lipids a transition from wormlike to spherical structures is observed for both long ( approximately 75000bp) and short (30-1246bp) DNA. In contrast to lipid/DNA complexes in the water phase, there is no indication of condensed liquid-crystalline structures in the non-polar phase. In fact, FCS measurements on short DNA oligomers complexed with cationic lipid in alkane give clear evidence for monomeric inverse micelles of DNA. Dilution series revealed a critical lower concentration of lipids and DNA for observing lipid/DNA micelles.

Alkanes↗

PubMed2006

Intramolecular dynamics of linear macromolecules by fluorescence correlation spectroscopy.

A theoretical description of the dynamics of DNA molecules and actin filaments in solution as measured experimentally by fluorescence correlation spectroscopy is provided and compared to recent experimental results. Particular attention is paid to the contribution of the intramolecular dynamics to the fluorescence correlation function. Using a semiflexible chain model, a theoretical expression is presented for the fluorescence correlation spectroscopy correlation function. The dependence of this function on various model parameters, such as chain length, persistence length, and fluorescence label density, is discussed. Our investigations show that the intramolecular dynamics provides a significant contribution or even dominates the correlation function as soon as the longest intramolecular relaxation time significantly exceeds the shortest experimentally accessible time. Correspondingly, the shape of the correlation function changes considerably. Approximate analytical expressions are provided, which are in qualitative agreement with the exact theoretical solutions as well as experimental results, for both DNA and actin filaments. Our approach is in agreement with the predictions of the Zimm model, in the limit of very flexible polymers, as well as the predictions of semiflexible polymer models with respect to the intramolecular dynamics in solution.

Actin Cytoskeleton↗

PubMed2005

Structural investigations of DNA-polycation complexes.

The internal structure of DNA-polycation complexes is investigated by synchrotron small-angle X-ray scattering (SAXS). Hexagonal packing of DNA is observed for DNA complexed with poly-L-lysine (PL), poly-L-arginine (PA), spermine (Sp), and linear and branched polyethyleneimine (lPEI and bPEI, respectively). Variations in the internal spacings and degree of long-range ordering are dependent on both polycation type and concentration of added salt. With increasing concentration of monovalent salt, a discontinuous phase transition is observed from compact to loose bundles and finally to an isotropic network phase. This salt-induced melting transition was found to be universal for all polyplexes studied and is in quantitative agreement with a simple free energy model based solely on electrostatic and entropic contributions. Using the osmotic stress method, bulk modulus (K) is measured for PL-DNA and PA-DNA polyplexes at various salt concentrations. With increasing osmotic force, we show that the salt-induced melting transition is shifted and compression in the loose bundle regime is in qualitative agreement with our model.

Computer Simulation↗

PubMed2005

Chains, dimers, and sandwiches: melting behavior of DNA nanoassemblies.

We have studied, in experiment and theory, the melting behavior of DNA nanoassemblies of perylenediimide-bis-oligonucleotides (PON), in which two short oligonucleotide chains (16- or 24-mers) are attached to the extremities of a spacer molecule. By varying both the sequence orientation (exchange of 3' and 5' ends neighboring the spacer) and the oligomer composition of the solutions, nanoassemblies of different complexity can be generated. Our results show a subtle dependence of the melting behavior on the supramolecular arrangement of the DNA-based assemblies.

Computer Simulation↗

PubMed2004

Flow profiling of a surface-acoustic-wave nanopump.

The flow profile in a capillary gap and the pumping efficiency of an acoustic micropump employing surface acoustic waves is investigated both experimentally and theoretically. Ultrasonic surface waves on a piezoelectric substrate strongly couple to a thin liquid layer and generate a quadrupolar streaming pattern within the fluid. We use fluorescence correlation spectroscopy and fluorescence microscopy as complementary tools to investigate the resulting flow profile. The velocity was found to depend on the applied power approximately linearly and to decrease with the inverse third power of the distance from the ultrasound generator on the chip. The found properties reveal acoustic streaming as a promising tool for the controlled agitation during microarray hybridization.

Acoustics↗

PubMed2003

Dynamics of large semiflexible chains probed by fluorescence correlation spectroscopy.

Fluorescence correlation spectroscopy was used to probe the dynamics of lambda-phage DNA in aqueous solution labeled with the randomly intercalating dye TOTO. The linear macromolecules (i). carry more than one chromophore and (ii). are larger than the waist of the focal volume. The correlation function decays significantly faster than expected for a stiff globule of corresponding size but is in good agreement with the dynamic model of semiflexible chains including hydrodynamic interactions. As the chromophore density is lowered the correlation time decreases in accordance with this model.

Bacteriophage lambda↗

PubMed2003

Flow profile near a wall measured by double-focus fluorescence cross-correlation.

We present an experimental approach to flow profiling within femtoliter sample volumes, which allows the high-precision measurements at the solid interface. The method is based on the spatial cross-correlation of the fluorescence response from labeled tracer particles (latex nanospheres or single dye molecules). Two excitation volumes, separated by a few micrometers, are created by two laser foci under a confocal microscope. The velocity of tracer particles is measured in a channel about 100 microm wide within a typical accuracy of 0.1%, and the positions of the walls are estimated independently of any hydrodynamic data. The underlying theory for the optical method is given for an arbitrary velocity profile, explicitly presenting the numerical convolutions necessary for a quantitative analysis. It is illustrated by using the Poiseuille flow of a Newtonian liquid with slip as an example. Our analysis yields a large apparent fluid velocity at the wall, which is mostly due to the impact of the colloidal (electrostatic) forces. This colloidal lift is crucially important in accelerating the transport processes of molecules and nanoparticles in microfluidic devices.

Journal Article↗

PubMed2001

Interface dynamics of lipid membrane spreading on solid surfaces.

As a model system for two-dimensional interface dynamics, we study the wetting front of a lipid membrane moving over a solid substrate that is structured with regularly spaced pinning centers. By analyzing the contour of the front, we derive the normal growth rate and the relaxation coefficient. Both exhibit a 1/t(1/2) time dependence. Moreover, the friction coefficient and the line tension can be determined. Randomly distributed pinning centers cause a fractal contour line, whereas on surfaces that are artificially roughened, self-affine contour lines are observed. The latter exhibit an anomalous roughness exponent of zeta = 0.81+/-0.05.

Algorithms↗

PubMed2001

Microelectrophoresis of a bilayer-coated silica bead in an optical trap: application to enzymology.

We describe an apparatus that combines microelectrophoresis and laser trap technologies to monitor the activity of phosphoinositide-specific phospholipase C-delta1 (PLC-delta) on a single bilayer-coated silica bead with a time resolution of approximately 1 s. A 1-microm-diameter bead was coated with a phospholipid bilayer composed of electrically neutral phosphatidylcholine (PC) and negatively charged phosphatidylinositol 4,5-bisphosphate (2% PIP2) and captured in a laser trap. When an AC field was applied (160 Hz, 20 V/cm), the electrophoretic force produced a displacement of the bead, Delta(x), from its equilibrium position in the trap; Delta(x), which was measured using a fast quadrant diode detector, is proportional to the zeta potential and thus to the number of PIP2 molecules on the outer leaflet (initially, approximately 10(5)). When a solution containing PLC-delta flows past the bead, the enzyme adsorbs to the surface and hydrolyzes PIP2 to form the neutral lipid diacylglycerol. We observed a nonexponential decay of PIP2 on the bead with time that is consistent with a model based on the known structural properties of PLC-delta.

Calcium↗

PubMed2000

Lipid-DNA and lipid-polyelectrolyte mesophases: structure and exchange kinetics.

Cationic lipid-DNA complexes are used as gene transfer vehicles in molecular biology and potentially in human gene therapy. In recent synchrotron X-ray scattering studies the molecular structure of such self-assembling aggregates was elucidated. A rich polymorphism of lamellar, hexagonal, lamellar-columnar and micellar mesophases was found. In this article we describe composite phases of cationic lipid mixed with hyaluronic acid and dextran sulfate which likewise form intercalated lamellar complexes. Heterogeneous phases of lipid/dextran sulfate mixed with lipid/DNA exhibit macroscopic phase separation. When dextran sulfate is added to preformed cationic lipid DNA complexes the latter are dissolved in favor of the lipid-polyelectrolyte phases. We investigated the kinetics of the DNA replacement by dextran sulfate. The experiments are intended to mimic the interaction of cationic lipid gene delivery complexes with highly charged extracellular matrix components.

Animals↗

PubMed1998

An inverted hexagonal phase of cationic liposome-DNA complexes related to DNA release and delivery.

A two-dimensional columnar phase in mixtures of DNA complexed with cationic liposomes has been found in the lipid composition regime known to be significantly more efficient at transfecting mammalian cells in culture compared to the lamellar (LalphaC) structure of cationic liposome-DNA complexes. The structure, derived from synchrotron x-ray diffraction, consists of DNA coated by cationic lipid monolayers and arranged on a two-dimensional hexagonal lattice (HIIC). Two membrane-altering pathways induce the LalphaC --> HIIC transition: one where the spontaneous curvature of the lipid monolayer is driven negative, and another where the membrane bending rigidity is lowered with a new class of helper-lipids. Optical microscopy revealed that the LalphaC complexes bind stably to anionic vesicles (models of cellular membranes), whereas the more transfectant HIIC complexes are unstable and rapidly fuse and release DNA upon adhering to anionic vesicles.

Birefringence↗

PubMed1998

Collective membrane motions of high and low amplitude, studied by dynamic light scattering and micro-interferometry.

Undulations of lipid bilayers were experimentally studied for the two limiting cases of high and weak lateral tension using two well established model systems: freely suspended planar lipid bilayers, so-called black lipid membranes (BLM) for high-tension studies and large unilamellar vesicles (LUV) for measurements at weak tension. This variation in tension results in changes of undulation amplitudes from several hundred nm (LUV) down to 1 nm (BLM), thus requiring different physical methods for their detection. We have employed microinterferometric techniques (RICM) for studying the regime of weak tension and dynamic light scattering (DLS) for that of high tension. The dedicated DLS set-up allowed the measurements of undulations over a wide wave vector range of 250 < q/cm-1 < 35,000 cm-1. This enabled the observation of collective membrane modes in two regimes, the oscillating one at low q and the overdamped regime at high q. The transition between both regimes at the bifurcation point is rather abrupt and depends on the lateral tension of the bilayer, as is demonstrated by comparing the dispersion curves of pure lipid and of lipid-cholestrol BLMs over the same q-range. The DLS measurements allowed a critical test of a hydrodynamic theory of the dispersion behaviour of membrane collective modes under tension. The DLS measurements are compared with RICM results of undulatory excitations of giant vesicles weakly adhering to substrates in the 10(-6)-2.5 x 10(-7) m wavelength regime and at low frequencies (0.1-25 Hz). Experimental evidence for the strong decrease in the relaxation rate by the hydrodynamic coupling of the membrane with the wall is established.

Animals↗

PubMed1997

Structure of DNA-cationic liposome complexes: DNA intercalation in multilamellar membranes in distinct interhelical packing regimes.

Cationic liposomes complexed with DNA (CL-DNA) are promising synthetically based nonviral carriers of DNA vectors for gene therapy. The solution structure of CL-DNA complexes was probed on length scales from subnanometer to micrometer by synchrotron x-ray diffraction and optical microscopy. The addition of either linear lambda-phage or plasmid DNA to CLs resulted in an unexpected topological transition from liposomes to optically birefringent liquid-crystalline condensed globules. X-ray diffraction of the globules revealed a novel multilamellar structure with alternating lipid bilayer and DNA monolayers. The lambda-DNA chains form a one-dimensional lattice with distinct interhelical packing regimes. Remarkably, in the isoelectric point regime, the lambda-DNA interaxial spacing expands between 24.5 and 57.1 angstroms upon lipid dilution and is indicative of a long-range electrostatic-induced repulsion that is possibly enhanced by chain undulations.

Bacteriophage lambda↗

PubMed1994

A phase of liposomes with entangled tubular vesicles.

An equilibrium phase belonging to the family of bilayer liposomes in ternary mixtures of dimyristoylphosphatidylcholine (DMPC), water, and geraniol (a biological alcohol derived from oil-soluble vitamins that acts as a cosurfactant) has been identified. Electron and optical microscopy reveal the phase, labeled Ltv, to be composed of highly entangled tubular vesicles. In situ x-ray diffraction confirms that the tubule walls are multilamellar with the lipids in the chain-melted state. Macroscopic observations show that the Ltv phase coexists with the well-known L4 phase of spherical vesicles and a bulk L alpha phase. However, the defining characteristic of the Ltv phase is the Weissenberg rod climbing effect under shear, which results from its polymer-like entangled microstructure.

Acyclic Monoterpenes↗