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Biomedical subjects

C F Schmidt

Publications and source records attributed to C F Schmidt.

At least 19 recordsLinked to original sources
PubMed2006

Correlated fluctuations of microparticles in viscoelastic solutions: quantitative measurement of material properties by microrheology in the presence of optical traps.

The Brownian motions of microscopic particles in viscous or viscoelastic fluids can be used to measure rheological properties. This is the basis of recently developed one- and two-particle microrheology techniques. For increased temporal and spatial resolution, some microrheology techniques employ optical traps, which introduce additional forces on the particles. We have systematically studied the effect that confinement of particles by optical traps has on their auto- and cross-correlated fluctuations. We show that trapping causes anticorrelations in the motion of two particles at low frequencies. We demonstrate how these anticorrelations depend on trap strength and the shear modulus of viscoelastic media. We present a method to account for the effects of optical traps, which permits the quantitative measurement of viscoelastic properties in one- and two-particle microrheology over an extended frequency range in a variety of viscous and viscoelastic media.

Journal Article↗
PubMed2006

Nanoindentation studies of full and empty viral capsids and the effects of capsid protein mutations on elasticity and strength.

The elastic properties of capsids of the cowpea chlorotic mottle virus have been examined at pH 4.8 by nanoindentation measurements with an atomic force microscope. Studies have been carried out on WT capsids, both empty and containing the RNA genome, and on full capsids of a salt-stable mutant and empty capsids of the subE mutant. Full capsids resisted indentation more than empty capsids, but all of the capsids were highly elastic. There was an initial reversible linear regime that persisted up to indentations varying between 20% and 30% of the diameter and applied forces of 0.6-1.0 nN; it was followed by a steep drop in force that is associated with irreversible deformation. A single point mutation in the capsid protein increased the capsid stiffness. The experiments are compared with calculations by finite element analysis of the deformation of a homogeneous elastic thick shell. These calculations capture the features of the reversible indentation region and allow Young's moduli and relative strengths to be estimated for the empty capsids.

Bromovirus↗
PubMed2006

High-frequency stress relaxation in semiflexible polymer solutions and networks.

We measure the linear viscoelasticity of sterically entangled and chemically cross-linked networks of actin filaments over more than five decades of frequency. The high-frequency response reveals rich dynamics unique to semiflexible polymers, including a previously unobserved relaxation due to rapid axial tension propagation. For high molecular weight, and for cross-linked gels, we obtain quantitative agreement with predicted shear moduli in both amplitude and frequency dependence.

Actin Cytoskeleton↗
PubMed2006

Structural and mechanical study of a self-assembling protein nanotube.

We report a structural characterization of self-assembling nanostructures. Using atomic force microscopy (AFM), we discovered that partially hydrolyzed alpha-lactalbumin organizes in a 10-start helix forming tubes with diameters of only 21 nm. We probed the mechanical strength of these nanotubes by locally indenting them with an AFM tip. To extract the material properties of the nanotubes, we modeled the experiment using finite element methods. Our study shows that artificial helical protein self-assembly can yield very stable, strong structures that can function either as a model system for artificial self-assembly or as a nanostructure with potential for practical applications.

Computer Simulation↗
PubMed2005

Rapid chiral assembly of rigid DNA building blocks for molecular nanofabrication.

Practical components for three-dimensional molecular nanofabrication must be simple to produce, stereopure, rigid, and adaptable. We report a family of DNA tetrahedra, less than 10 nanometers on a side, that can self-assemble in seconds with near-quantitative yield of one diastereomer. They can be connected by programmable DNA linkers. Their triangulated architecture confers structural stability; by compressing a DNA tetrahedron with an atomic force microscope, we have measured the axial compressibility of DNA and observed the buckling of the double helix under high loads.

Base Pairing↗
PubMed2005

Short-time inertial response of viscoelastic fluids: observation of vortex propagation.

We probe the response of viscous and viscoelastic fluids on micrometer and microsecond length and time scales using two optically trapped beads. In this way we resolve the flow field, which exhibits clear effects of fluid inertia. Specifically, we resolve the short-time vortex flow and the corresponding evolution of this vortex, which propagates diffusively for simple liquids. For viscoelastic fluids, this propagation is shown to be faster than diffusive and the displacement correlations reflect the frequency-dependent shear modulus of the medium.

Journal Article↗
PubMed2005

High-frequency microrheology of wormlike micelles.

We have measured the frequency-dependent shear modulus of entangled solutions of wormlike micelles by high-frequency microrheology and have compared the results with those from macrorheology experiments done on the same samples. Using optical microrheology based on laser interferometry we have measured loss and storage moduli over six decades in frequency up to about 100 kHz. We present data over a decade in concentration in the entangled regime and find good agreement between micro- and macrorheology, thus validating recently developed microrheology techniques. By collapsing data for different concentrations, we furthermore determine both the concentration scaling of the plateau modulus and a power-law exponent of the complex shear modulus at high frequencies.

Journal Article↗
PubMed2004

Bacteriophage capsids: tough nanoshells with complex elastic properties.

The shell of bacteriophages protects the viral DNA during host-to-host transfer and serves as a high-pressure container storing energy for DNA injection into a host bacterium. Here, we probe the mechanical properties of nanometer-sized bacteriophage phi 29 shells by applying point forces. We show that empty shells withstand nanonewton forces while being indented up to 30% of their height. The elastic response varies across the surface, reflecting the arrangement of shell proteins. The measured Young's modulus (approximately 1.8 GPa) is comparable with that of hard plastic. We also observe fatigue and breakage of capsids after probing them repetitively. These results illustrate the mechanoprotection that viral shells provide and also suggest design principles for nanotechnology.

Bacteriophages↗
PubMed2003

Deformation and collapse of microtubules on the nanometer scale.

We probe the local mechanical properties of microtubules at the nanometer scale by radial indentation with a scanning force microscope tip. We find a linear elastic regime that can be described by both thin-shell theory and finite element methods, in which microtubules are modeled as hollow tubes. We also find a nonlinear regime and catastrophic collapse of the microtubules under large loads. The main physics of protein shells at the nanometer scale shows simultaneously aspects of continuum elasticity in their linear response, as well as molecular graininess in their nonlinear behavior.

Animals↗
PubMed2001

Instability of myelin tubes under dehydration: deswelling of layered cylindrical structures.

We report experimental observations of an undulational instability of myelin figures. Motivated by this, we examine theoretically the deformation and possible instability of concentric, cylindrical, multilamellar membrane structures. Under conditions of osmotic stress (swelling or dehydration), we find a stable, deformed state in which the layer deformation is given by deltaR infinity r(square root[B(A)/(hB)]), where B(A) is the area compression modulus, B is the interlayer compression modulus, and h is the repeat distance of layers. Also, above a finite threshold of dehydration (or osmotic stress), we find that the system becomes unstable to undulations, first with a characteristic wavelength of order square root[xi(d)0], where xi is the standard smectic penetration depth and d0 is the thickness of dehydrated region.

Biophysical Phenomena↗
PubMed2000

Imaging microtubules and kinesin decorated microtubules using tapping mode atomic force microscopy in fluids.

The atomic force microscope has been used to investigate microtubules and kinesin decorated microtubules in aqueous solution adsorbed onto a solid substrate. The netto negatively charged microtubules did not adsorb to negatively charged solid surfaces but to glass covalently coated with the highly positively charged silane trimethoxysilylpropyldiethylenetriamine (DETA) or a lipid bilayer of 1,2-dipalmitoyl-3-dimethylammoniumpropane. Using electron beam deposited tips for microtubules adsorbed on DETA, single protofilaments could be observed showing that the resolution is up to 5 nm. Under conditions where the silane coated surfaces are hydrophobic, microtubules opened, presumably at the seam, whose stability is lower than that of the bonds between the other protofilaments. This led to a "sheet" with a width of about 100 nm firmly attached to the surface. Microtubules decorated with a stoichiometric low amount of kinesin molecules in the presence of the non-hydrolyzable ATP-analog 5'-adenylylimidodiphosphate could also be adsorbed onto silane-coated glass. Imaging was very stable and the molecules did not show any scan-induced deformation even after hundreds of scans with a scan frequency of 100 Hz.

Animals↗
PubMed2000

Working strokes by single molecules of the kinesin-related microtubule motor ncd.

The ncd protein is a dimeric, ATP-powered motor that belongs to the kinesin family of microtubule motor proteins. Here we resolve single mechanochemical cycles of recombinant, dimeric, full-length ncd, using optical-tweezers-based instrumentation and a three-bead, suspended-microtubule assay. Under conditions of limiting ATP, isolated and transient microtubule-binding events exhibit exponentially distributed and ATP-concentration-dependent lifetimes. These events do not involve consecutive steps along the microtubule, quantitatively confirming that ncd is non-processive. At low loads, a single motor molecule produces ATP-triggered working strokes of about 9 nm, which occur at the ends of binding events.

Adenosine Triphosphate↗
PubMed2000

Results of the American Association of Bioanalysts national proficiency testing programme in andrology.

Proficiency testing samples for antisperm antibodies (ASAB), sperm count, morphology and vitality were mailed to participating laboratories. The majority participating utilized Immunobead ASAB procedures (81 versus 14% mixed antiglobulin reaction and 5% 'other'), and there was 95.6 +/- 1.2% agreement on the presence or absence of ASAB. The majority of laboratories utilized manual (79%) versus computer assisted semen analysis (CASA; 15%) methods. Approximately 64% used the haemocytometer and 26% used the Makler counting chambers for manual counts. Coefficients of variation (CV) in sperm counts ranged from 24 to 138%, with CASA displaying lower overall CV (53 +/- 8%) than manual methods (80 +/- 9%). A wide variation in the reports of percent normal morphology was noted (CVs calculated from arc sin transformed means ranged from 15 to 93%). Participants using American Society of Clinical Pathologists (ASCP) criteria reported sperm morphology values that were clustered in the 'normal' range (11 out of 12 samples), while those using strict criteria were clustered in the 'abnormal' range (10 out of 12 samples). Good agreement was observed in sperm vitality (overall mean CV = 18%). These data highlight the urgent need for improvement in overall quality of andrology testing and indicate that practical proficiency testing programmes can be made available on a large scale.

Data Collection↗
PubMed1998

Highly processive motility is not a general feature of the kinesins.

Evidence is presented that the kinesin-related ncd protein is not as processive as kinesin. In low surface density motility experiments, a dimeric ncd fusion protein behaved mechanistically more similar to non-processive myosins than to the highly processive kinesin. First, there was a critical microtubule length for motility; only microtubules longer than this critical length moved in low density ncd surfaces, which suggested that multiple ncd proteins must cooperate to move microtubules in the surface assay. Under similar conditions, native kinesin demonstrated no critical microtubule length, consistent with the behavior of a highly processive motor. Second, addition of methylcellulose to decrease microtubule diffusion decreased the critical microtubule length for motility. Also, the rates of microtubule motility were microtubule length dependent in methylcellulose; short microtubules, that interacted with fewer ncd proteins, moved more slowly than long microtubules that interacted with more ncd proteins. In contrast, short microtubules, that interacted with one or a few kinesin proteins, moved on average slightly faster than long microtubules that interacted with multiple kinesins. We conclude that a degree of processivity as high as that of kinesin, where a single dimer can move over distances on the order of one micrometer, may not be a general mechanistic feature of the kinesin superfamily.

Adenosine Triphosphate↗
PubMed1998

Two-dimensional tracking of ncd motility by back focal plane interferometry.

A technique for detecting the displacement of micron-sized optically trapped probes using far-field interference is introduced, theoretically explained, and used to study the motility of the ncd motor protein. Bead motions in the focal plane relative to the optical trap were detected by measuring laser intensity shifts in the back-focal plane of the microscope condenser by projection on a quadrant diode. This detection method is two-dimensional, largely independent of the position of the trap in the field of view and has approximately 10-micros time resolution. The high resolution makes it possible to apply spectral analysis to measure dynamic parameters such as local viscosity and attachment compliance. A simple quantitative theory for back-focal-plane detection was derived that shows that the laser intensity shifts are caused primarily by a far-field interference effect. The theory predicts the detector response to bead displacement, without adjustable parameters, with good accuracy. To demonstrate the potential of the method, the ATP-dependent motility of ncd, a kinesin-related motor protein, was observed with an in vitro bead assay. A fusion protein consisting of truncated ncd (amino acids 195-685) fused with glutathione-S-transferase was adsorbed to silica beads, and the axial and lateral motions of the beads along the microtubule surface were observed with high spatial and temporal resolution. The average axial velocity of the ncd-coated beads was 230 +/- 30 nm/s (average +/- SD). Spectral analysis of bead motion showed the increase in viscous drag near the surface; we also found that any elastic constraints of the moving motors are much smaller than the constraints due to binding in the presence of the nonhydrolyzable nucleotide adenylylimidodiphosphate.

Calibration↗
PubMed1996

Protein tracking and detection of protein motion using atomic force microscopy.

Height fluctuations over three different proteins, immunoglobulin G, urease, and microtubules, have been measured using an atomic force microscope (AFM) operating in fluid tapping mode. This was achieved by using a protein-tracking system, where the AFM tip was periodically repositioned above a single protein molecule (or structure) as thermal drifting occurred. Height (z-piezo signal) data were taken in 1 - or 2-s time slices with the tip over the molecule and compared to data taken on the support. The measured fluctuations were consistently higher when the tip was positioned over the protein, as opposed to the support the protein was adsorbed on. Similar measurements over patches of an amphiphile, where the noise was identical to that on the support, suggest that the noise increase is due to some intrinsic property of proteins and is not a result of different tip-sample interactions over soft samples. The orientation of the adsorbed proteins in these preliminary studies was not known; thus it was not possible to make correlations between the observed motion and specific protein structure or protein function beyond noting that the observed height fluctuations were greater for an antibody (anti-bovine IgG) and an enzyme (urease) than for microtubules.

Animals↗