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R Merkel

Publications and source records attributed to R Merkel.

At least 19 recordsLinked to original sources
PubMed2006

Theoretical analysis of single-molecule force spectroscopy experiments: heterogeneity of chemical bonds.

We show that the standard theoretical framework in single-molecule force spectroscopy has to be extended to consistently describe the experimental findings. The basic amendment is to take into account heterogeneity of the chemical bonds via random variations of the force-dependent dissociation rates. This results in a very good agreement between theory and rupture data from several different experiments.

Biomechanical Phenomena↗
PubMed2004

Ellipsometric microscopy: developments towards biophysics.

Ellipsometric microscopy is a novel technique that combines the merits of ellipsometry and light microscopy, i.e. it allows noninvasive, label-free measurements of thin film thickness and refractive index at high lateral resolution. Ellipsometric microscopy has been successfully applied to silicon-air interfaces. However, typical biological systems require immersion in an aqueous buffer. Thus the authors have adapted the instrument for the observation of the interface between glass and water. In particular, the comparatively small differences in refractive indices between substrate and ambient media proved to be a challenge for instrument design. The first experiments with this new instrument are presented.

Journal Article↗
PubMed2002

Dynamic force spectroscopy to probe adhesion strength of living cells.

We studied the mechanical strength of the adhesion of living cells to model membranes. The latter contained a RGD lipopeptide which is a high affinity binding site for a cell adhesion molecule (integrin alpha(V)beta(3)). Cells adhered specifically to the vesicles. We used micropipette aspiration for breaking this adhesion with well defined forces. Systematic variation of the rate of force application revealed pronounced kinetic effects. The dependence of the detachment forces on the loading rate was well described by a power law (exponent approximately 0.4), in agreement with recent theoretical work.

Binding Sites↗
PubMed2000

A micromechanic study of cell polarity and plasma membrane cell body coupling in Dictyostelium.

We used micropipettes to aspirate leading and trailing edges of wild-type and mutant cells of Dictyostelium discoideum. Mutants were lacking either myosin II or talin, or both proteins simultaneously. Talin is a plasma membrane-associated protein important for the coupling between membrane and actin cortex, whereas myosin II is a cytoplasmic motor protein essential for the locomotion of Dictyostelium cells. Aspiration into the pipette occurred above a threshold pressure only. For all cells containing talin this threshold was significantly lower at the leading edge of an advancing cell as compared to its rear end, whereas we found no such difference in cells lacking talin. Wild-type and talin-deficient cells were able to retract from the pipette against an applied suction pressure. In these cells, retraction was preceded by an accumulation of myosin II in the tip of the aspirated cell lobe. Mutants lacking myosin II could not retract, even if the suction pressures were removed after aspiration. We interpreted the initial instability and the subsequent plastic deformation of the cell surface during aspiration in terms of a fracture between the cell plasma membrane and the cell body, which may involve destruction of part of the cortex. Models are presented that characterize the coupling strength between membrane and cell body by a surface energy sigma. We find sigma approximately 0.6(1.6) mJ/m(2) at the leading (trailing) edge of wild-type cells.

Animals↗
PubMed2000

Shape instability of a biomembrane driven by a local softening of the underlying actin cortex.

We present a theory showing that local shape instabilities of composite biological membranes, consisting of a lipid bilayer and an underlying actin cortex, can be triggered by a local softening of the membrane-associated cytoskeleton. A membrane containing such cortical defects can form blisters or invaginations, depending on external conditions. The theoretical predictions agree with observations provided by two sets of experiments: (i) microscopic observations of shape changes of giant vesicles with underlying shells of a thin actin network show the formation of local blisters and (ii) micropipet aspiration experiments of Dictyostelium discoideum cells in which we observed the formation of blisters in the aspirated cell part. In the latter experiments, the existence of a hole in the underlying cortex is confirmed by observation of the entrance of cell organelles into the blister. Our model may also be applied to the formation of lobopodia, fast-growing cell protrusions that play an important role in the locomotion and spreading of biological cells.

Actins↗
PubMed1999

Energy landscapes of receptor-ligand bonds explored with dynamic force spectroscopy.

Atomic force microscopy (AFM) has been used to measure the strength of bonds between biological receptor molecules and their ligands. But for weak noncovalent bonds, a dynamic spectrum of bond strengths is predicted as the loading rate is altered, with the measured strength being governed by the prominent barriers traversed in the energy landscape along the force-driven bond-dissociation pathway. In other words, the pioneering early AFM measurements represent only a single point in a continuous spectrum of bond strengths, because theory predicts that these will depend on the rate at which the load is applied. Here we report the strength spectra for the bonds between streptavidin (or avidin) and biotins-the prototype of receptor-ligand interactions used in earlier AFM studies, and which have been modelled by molecular dynamics. We have probed bond formation over six orders of magnitude in loading rate, and find that the bond survival time diminished from about 1 min to 0.001 s with increasing loading rate over this range. The bond strength, meanwhile, increased from about 5 pN to 170 pN. Thus, although they are among the strongest noncovalent linkages in biology (affinity of 10(13) to 10(15) M(-1)), these bonds in fact appear strong or weak depending on how fast they are loaded. We are also able to relate the activation barriers derived from our strength spectra to the shape of the energy landscape derived from simulations of the biotin-avidin complex.

Biotin↗
PubMed1999

Desmin filaments studied by quasi-elastic light scattering.

We studied polymers of desmin, a muscle-specific type III intermediate filament protein, using quasi-elastic light scattering. Desmin was purified from chicken gizzard. Polymerization was induced either by 2 mM MgCl(2) or 150 mM NaCl. The polymer solutions were in the semidilute regime. We concluded that the persistence length of the filaments is between 0.1 and 1 microm. In all cases, we found a hydrodynamic diameter of desmin filaments of 16-18 nm. The filament dynamics exhibits a characteristic frequency in the sense that correlation functions measured on one sample but at different scattering vectors collapse onto a single master curve when time is normalized by the experimentally determined initial decay rate.

Animals↗
PubMed1998

Micropipet-based pico force transducer: in depth analysis and experimental verification.

Measurements of forces in the piconewton range are very important for the study of molecular adhesion and mechanics. Recently, a micropipet-based force transducer for this type of experiment was presented (E. Evans, K. Ritchie, and R. Merkel, 1995, Biophys. J., 68:2580-2587). In the present article we give a detailed mechanical analysis of this transducer, including nonlinear effects. An analytical expression for the transducer stiffness at small elongations is given. Using magnetic tweezers (F. Ziemann, J. Rädler, and E. Sackmann, 1994, Biophys. J., 66:2210-2216), we were able to determine the force displacement relation of this transducer experimentally. Forces from approximately 10 pN to 500 pN were applied. Theoretical predictions and experimental results coincide remarkably well.

Biomechanical Phenomena↗
PubMed1997

Diffusion measurement of fluorescence-labeled amphiphilic molecules with a standard fluorescence microscope.

The lateral diffusion of fluorescence-labeled amphiphilic tracer molecules dissolved within a two-dimensional matrix of lipids was measured by continuous illumination of an elongated rectangular region. The resulting spatial concentration profile of unbleached tracer molecules was observed with a standard epifluorescence microscope and analyzed with digital image-processing techniques. These concentration profiles are governed by the mobility of the tracers, their rate of photolysis, and the geometry of the illuminated area. For the case of a long and narrow illuminated stripe, a mathematical analysis of the process is given. After prolonged exposure, the concentration profile can be approximated by a simple analytical function. This fact was used to measure the quotient of the rate of photolysis, and the diffusion constant of the fluorescent probe. With an additional measurement of the rate of photolysis, the mobility of the tracer was determined. As prototype experiments we studied the temperature dependence of the lateral diffusion of N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)-dipalmitoylphosphatidyl++ + ethanolamine in glass-supported bilayers of L-alpha-dimyristoylphosphatidylcholine. Because of its simple experimental setup, this technique represents a very useful method of determining the lateral diffusion of fluorescence-labeled membrane molecules.

Chemical Phenomena↗
PubMed1995

Sensitive force technique to probe molecular adhesion and structural linkages at biological interfaces.

Adhesion and cytoskeletal structure are intimately related in biological cell function. Even with the vast amount of biological and biochemical data that exist, little is known at the molecular level about physical mechanisms involved in attachments between cells or about consequences of adhesion on the material structure. To expose physical actions at soft biological interfaces, we have combined an ultrasensitive transducer and reflection interference microscopy to image submicroscopic displacements of probe contact with a test surface under minuscule forces. The transducer is a cell-size membrane capsule pressurized by micropipette suction where displacement normal to the membrane under tension is proportional to the applied force. Pressure control of the tension tunes the sensitivity in operation over four orders of magnitude through a range of force from 0.01 pN up to the strength of covalent bonds (approximately 1000 pN)! As the surface probe, a microscopic bead is biochemically glued to the transducer with a densely-bound ligand that is indifferent to the test surface. Movements of the probe under applied force are resolved down to an accuracy of approximately 5 nm from the interference fringe pattern created by light reflected from the bead. With this arrangement, we show that local mechanical compliance of a cell surface can be measured at a displacement resolution set by structural fluctuations. When desired, a second ligand is bound sparsely to the probe for focal adhesion to specific receptors in the test surface. We demonstrate that monitoring fluctuations in probe position at low transducer stiffness enhances detection of molecular adhesion and activation of cytoskeletal structure. Subsequent loading of an attachment tests mechanical response of the receptor-substrate linkage throughout the force-driven process of detachment.

Animals↗
PubMed1993

Intersecting polymers in lipid bilayers: cliques, static order parameters and lateral diffusion.

We have modelled a macrolipid polymer composed of lipid molecules (monomers) embedded in a lipid bilayer or monolayer and polymerized via their polar groups. Because of fluctuations perpendicular to the plane of the bilayer, the polar region occupied by the polymer chain possesses sufficient space so that the polymer might exhibit 'self-intersection' if its conformational state is projected onto the plane of the bilayer/monolayer. We represent the plane of the bilayer/monolayer by a triangular lattice. Each site can be occupied by a monomer or be empty (and thus occupied by one of the unpolymerizable lipids which make up the bilayer/monolayer). A macrolipid is represented by a sequence of N monomers connected by N-1 bonds. Bonds may be either short (connecting nearest neighbour monomers) or long (between second neighbour monomers), in accord with the average properties of the spacers between the polymerized lipids. We have carried out computer simulation of this system using the Carmesin-Kremer bond stretching algorithm. Although no two monomers can occupy the same site, bonds may cross each other. We analyzed the dependence of and approximately N2vc and + approximately N2 sigma c, where Nsc and Nmc are the number of bond-crossings in the same macrolipid ('self-crossing') or in two different macrolipids ('mutual-crossing'). For single macrolipids, we confirmed that vc = 3/4 and have found that sigma c approximately 0.52, which we consider supports that sigma c = 1/2. For the dense case with monomer concentration, c = 0.72, we found that vc = 1/2 and that sigma c approximately 0.52 supports that sigma c = 1/2. In the semi-dilute regime (c = 0.2) we found crossover behaviour, although sigma c = 1/2. The total number of bond crossings thus scale like N, independent of concentration. We studied the connectivity of the system by calculating the weight averaged cluster, or 'clique', size. Cliques are defined as being composed of all macrolipids which exhibit at least one crossing bond with one other member of the clique. We found that while the average clique contains about two macrolipids at low concentrations, the clique size approaches the maximum possible value at high concentrations if the macrolipids are sufficiently long. In the latter case a transition appears to occur as the macrolipid length increases. This transition occurs at length = 40 when c = 0.72. These observations should have experimental consequences for the viscoelastic properties of the system.(ABSTRACT TRUNCATED AT 400 WORDS)

Computer Simulation↗
PubMed1981

Adipocyte size and number in dietary obesity resistant and susceptible rats.

Fat depot cellularity was assessed in overfed dietary obesity resistant S 5B/Pl (S) and susceptible Osborne-Mendel (OM) rats. Cell number and lipid per cell were determined for three fat depots in both 24- and 105-day-old rats. Between these two ages, fat cell number doubled in inguinal fat depots of S rats fed high- or low-fat diets and OM rats fed a low-fat diet, but quadrupled in OM rats fed a high-fat diet. The size of adipocytes in this depot was influenced by strain but not by diet. Compared to normal weight S rats, 24-day-old overfed S rats had twice as many adipocytes in the perirenal-retroperitoneal fat depot. Overfed OM rats had 3 times perirenal-retroperitoneal fat depot. Overfed OM rats had 3 times as many. In OM rats fed the high-fat diet, there was a 16-fold increase in adipocyte number between 24 and 105 days of age. Overfeeding caused a slight increase in perirenal-retroperitoneal adipocyte size in 24-day-old rats but had little influence on cell size in 105-day-old rats.

Adipose Tissue↗
PubMed1976

[Adenosine degradation in human erythrocytes in the presence of dilazep].

1. In the molar end concentration of 1 X 10(-6), 1,4-bis-[3-(3,4,5-trimethoxybenzoyloxy)-propyl]-perhydro-1,4-diazepine (dilazep) has no effect on crystalline adenosine desaminase. 2. In the same concentration dilazep does not affect adenosine desaminase activity of human hemolysing erythrocytes. 3. Under the influence of dilazep the methylene blue activated respiration of human erythrocytes is inhibited adenosine serving as substrate. a) The final molar concentration of 1 X 10(-7) leads to minor inhibition of the respiration. b) After transitory activation, final molar concentrations of 1 X 10(-6) and 1 X 10(-5) also lead to inhibition, which is complete after 4 h and in which the concentration of 1 X 10(-6) has the most pronounced effect. 4. Basing on the experimental results obtained it is not yet possible to classify the mode of activity of dilazep.

Adenosine↗